Model 6176 Channel Strip
Universal Audio Part Number 65-00053
Universal Audio, Inc. Customer Service & Tech Support: 1-877-MY-AUDIO Business, Sales & Marketing: 1-866-440-1176 www.uaudio.com
Notice _____________________________________________________________
This manual provides general information, preparation for use, installation and operating instructions for the Universal Audio 6176. The information contained in this manual is subject to change without notice. Universal Audio, Inc. makes no warranties of any kind with regard to this manual, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose. Universal Audio, Inc. shall not be liable for errors contained herein or direct, indirect, special, incidental, or consequential damages in connection with the furnishing, performance, or use of this material. Copyright 2009 Universal Audio, Inc. All rights reserved. This manual and any associated software, artwork, product designs, and design concepts are subject to copyright protection. No part of this document may be reproduced, in any form, without prior written permission of Universal Audio, Inc. Trademarks 6176, 1176LN, 2-1176, LA-610, LA-2A, 2-LA-2, LA-3A, 2-610, 710, 4110, 8110, SOLO/110, SOLO/610, 2192, DCS Remote Preamp, UAD and the Universal Audio, Inc. logo are trademarks of Universal Audio, Inc. Other company and product names mentioned herein are trademarks of their respective companies
Contents of This Box This package should contain: One 6176 Tube Preamplifier / Compressor 6176 Operating Instructions IEC Power Cable Registration Card
A Letter From Bill Putnam, Jr. _____________________________________________________________
Thank you for purchasing the 6176 Channel Strip. The 6176 combines a modified channel of our 2-610 vacuum tube microphone preamplifier with our 1176LN all discrete solid state FET-based limiter / compressor. The 2-610 was inspired by the microphone preamp section of the original 610 console designed by my father, M.T. Bill Putnam, in 1960. The 610 was a rotary-control console and it was also the first console of a modular design. Although technologically simple compared to modern consoles, the 610 possessed a warmth and character that kept it in demand for decades. A prominent component of my fathers United/Western Studios in Los Angeles, the 610 was used on many classic recordings by Frank Sinatra and Sarah Vaughan, as well as the Beach Boys Pet Sounds and the Doors LA Woman. My father also designed the 1176 limiting amplifier, back in 1966. There were many versions of the 1176 produced throughout the years. We patterned our 2001 reissue on the D/E versions, prized by engineers the world over and widely believed to be the best-sounding models. These versions were characterized by their custom transformer input stage, low noise (LN) circuitry, and Class A (1108 style) output stage. (Later versions replaced the Class A output stage with a push-pull Class-AB output stage, and eventually replaced the transformer input with a differential op-amp circuit.) The limiting / compression section in the 6176 captures that highly recognizable blackface 1176LN sound. Most of us at Universal Audio are musicians and/or recording engineers. We love the recording process, and we really get inspired when tracks are beautifully recorded. Our design goal for the 6176 was to build an integrated mic preamp / compressor that we would be delighted to use ourselves one that would capture the original character of the 610 and 1176LN to create a device that induces that a-ha feeling you get when hearing music recorded in its most natural, inspired form. Youll find that the controls of the 6176 are simple and essential: we added only those features required for practical use without needless duplication of functionality found elsewhere in most studios. The transformers and tubes received much of our R&D attention. We opted for a transformer design that features double-sized alloy cores with custom windings. Our tubes are carefully selected and tested individually. To us, this extra effort is well worth the time and cost because the end result is an outstanding yet easy-to-use product that combines a legendary mic preamp with a legendary limiter/compressor, making for a sound that is truly extraordinary. Developing the 6176 as well as Universal Audios entire line of quality audio products designed to meet the needs of the modern recording studio while retaining the character of classic vintage equipmenthas been a very special experience for me and for all who have been involved. While, on the surface, the rebuilding of UA has been a business endeavor, it's really been so much more than that: in equal parts a sentimental and technical adventure. We thank you, and we thank my father, Bill Putnam. Sincerely,

Bill Putnam, Jr.

Important Safety Instructions _____________________________________________________________
Before using this unit, be sure to carefully read the applicable items of these operating instructions and the safety suggestions. Afterwards, keep them handy for future reference. Take special care to follow the warnings indicated on the unit, as well as in the operating instructions. 1. Water and Moisture - Do not use the unit near any source of water or in excessively moist environments. 2. Object and Liquid Entry - Care should be taken so that objects do not fall, and liquids are not spilled, into the enclosure through openings. 3. Ventilation - When installing the unit in a rack or any other location, be sure there is adequate ventilation. Improper ventilation will cause overheating, and can damage the unit. 4. Heat - The unit should be situated away from heat sources, or other equipment that produce heat. 5. Power Sources - The unit should be connected to a power supply only of the type described in the operating instructions, or as marked on the unit. 6. Power Cord Protection - AC power supply cords should be routed so that they are not likely to be walked on or pinched by items placed upon or against them. Pay particular attention to cords at plugs, convenience receptacles, and the point where they exit from the unit. Never take hold of the plug or cord if your hand is wet. Always grasp the plug body when connecting or disconnecting it. 7. Grounding of the Plug - This unit is equipped with a 3-wire grounding type plug, a plug having a third (grounding) pin. This plug will only fit into a grounding-type power outlet. This is a safety feature. If you are unable to insert the plug into the outlet, contact your electrician to replace your obsolete outlet. Do not defeat the purpose of the grounding-type plug. 8. Cleaning - Follow these general rules when cleaning the outside of your 6176: a. Turn the power Off and unplug the unit b. Gently wipe with a clean lint-free cloth c. If necessary, moisten the cloth using lukewarm or distilled water, making sure not to oversaturate it as liquid could drip inside the case and cause damage to your 6176 d. Use a dry lint-free cloth to remove any remaining moisture e. Do not use aerosol sprays, solvents, or abrasives 9. Nonuse Periods - The AC power supply cord of the unit should be unplugged from the AC outlet when left unused for a long period of time. 10. Damage Requiring Service - The unit should be serviced by a qualified service personnel when: a. The AC power supply cord or the plug has been damaged: or b. Objects have fallen or liquid has been spilled into the unit; or c. The unit has been exposed to rain; or d. The unit does not operate normally or exhibits a marked change in performance; or e. The unit has been dropped, or the enclosure damaged. 11. Servicing - The user should not attempt to service the unit beyond that described in the operating instructions. All other servicing should be referred to qualified service personnel.

Table of Contents _____________________________________________________________
Notice.....ii A Letter From Bill Putnam, Jr....iii Important Safety Instructions... iv Two Page, Two Minute Guide To Getting Started... 1 Front Panel.... 3 Rear Panel..... 9 Interconnections.... 10 Insiders Secrets.... 11 The Technical Stuff.... 18 Glossary of Terms.... 35 Recall Sheet.... 39 Specifications..... 40 Additional Resources / Product Registration / Warranty / Service & Support. 41
The Two Page, Two Minute Guide To Getting Started _____________________________________________________________
No one likes to read owners manuals. We know that. We also know that you know what youre doingwhy else would you have bought our product? So were going to try to make this as easy on you as possible. Hence this two-page spread, which we estimate will take you approximately two minutes to read. It will tell you everything you need to know to get your Universal Audio 6176 up and running, without bogging you down with details. Of course, even the most expert of us has to crack a manual every once in awhile. As the saying goes, as a last resort, read the instructions. Youll find those details youre cravinga full description of all front and rear panel controls, interconnection diagrams, insiders secrets, history, theory, maintenance information, block diagrams, specifications, even a glossary of termsin the pages that follow. Manual conventions: Means that this is an especially useful tip Means that this is an especially important bit of information And when we need to direct you to a page or section elsewhere in the manual, well use the universal signs for rewind ( ) or fast forward ( ). Getting Started With Your 6176: Step 1: Decide where the 6176 is to be physically placed and place it there. The 6176 is housed in a standard two-rackspace 19" chassis, and so we recommend that it be securely mounted in a rack if possible. Step 2: Mute your monitors and then, using balanced cables with XLR connectors, connect the 6176s rear panel Preamplifier and Compressor line outputs to the appropriate inputs on your patch bay, mixer, or DAW. (Note: If you only have one available patch point or channel input, connect only the Compressor line output.) Step 3: Set the Polarity switch on the left side of the front panel to its down position, ensuring that the output signal will be in phase with the input signal. If using a microphone, set the Pad switch (on the left side of the front panel) and the Phantom Power switch (in the center of the front panel) to their down positions so that the pad is not engaged and no phantom power is being applied. Step 4: Set the JOIN/SPLIT switch in the center of the front panel to its up (JOIN) position. This ensures that the 6176 will act as a channel strip, with its preamp output internally routed to its limiter/compressor. (Note: In JOIN mode, neither the Preamp line output nor the Compressor line input is functional.) Step 5: Connect a source signal (mic, line, or Make only one type of connection (Mic, instrument) to the 6176s rear panel balanced XLR Line, or Hi-Z) to the 6176 Preamp. Preamp mic or line input, or to the front panel unbalanced " jack Hi-Z input. ( see page 10 for an interconnection diagram) Step 6: Make sure the Power switch in the center of the front panel is off (down) and then connect the supplied IEC power cable to the rear panel AC power connector. Step 7: On the left side of the front panel, turn the Level control to its minimum (fully counterclockwise) position and set the Gain control to 0 (center position). 1

Step 8: On the right side of the front panel, set the Ratio switch to BP (fully counterclockwise) in order to temporarily bypass the 6176 limiter / compressor section. Set the Meter switch to PRE so that the meter displays the output level of the signal leaving the 6176 preamp section. Step 9: Power on the 6176. The purple LED above the power switch will light up. Because the 6176 is a tube device, it needs several minutes to achieve stable operating temperature. During warm-up, audio quality may vary slightly. Step 10: If a microphone requiring 48 volts of phantom power is connected to the 6176, turn on the +48V switch (up position). Step 11: Unmute your speakers and begin monitoring the 6176 Compressor line output. Use the 6176 front-panel Input Select switch to select the connected source and then apply input signal while slowly increasing the Level control to approximately 7. You should now be hearing signal. Step 12: While watching the 6176 meter, set the preamp Gain switch (on the left side of the front panel) so that sufficient signal strength is achieved. Experiment with differing degrees of preamp Gain and Level to hear the various amounts of coloration the 6176 can impart to your signal. For the cleanest, most uncolored signal, set the Gain switch to its lowest usable setting with Level set between 7 and 10. If you are using a microphone and you hear distortion even at the lowest Gain level, employ the -15 dB pad to reduce the input level. If you are using a connected microphone or HiZ instrument input, experiment also with various impedance settings to achieve optimum tonality for your source signal. Step 13: Experiment with the effect of the 6176s high and low shelving filters on your source signal. Step 14: Now its time to activate the limiter/compressor section of the 6176 on the right side of the front panel. Begin by setting the Input and Output knobs to approximately 5 (12 oclock position) for unity gain. Then set the Ratio to 1 (or, alternatively, turn the Attack knob past its fully counterclockwise position so that it clicks). In this mode (a compression ratio of 1:1), signal passes through the limiter/compressor section, thus adding color, but with no gain reduction. Compare the difference in tonality between ratios of BP and 1. Step 15: To hear the effect of the 6176 gain reduction circuitry, For more information, refer select different Ratios of 4, 8, 12, 20, and ALL. (ALL mode to the Front Panel and duplicates the overdrive that occurs when all four ratio buttons Rear Panel sections on on an original 1176 are pushed in simultaneously) and varying pages 3 9. the Input level and Attack and Release times. Note that, unlike many other devices, the 6176 attack and release times are faster when their associated knobs are turned clockwise, and slower when they are turned counterclockwise. Set the Meter switch to GR in order to view the amount of gain reduction being applied to the signal. Note that as you increase the amount of gain reduction (by raising the Input level and/or selecting higher ratios), the overall signal may be attenuated. If necessary, you can make up the difference by turning the Output knob clockwise. Change the Meter switch to COMP in order to view the final output level. A meter reading of 0 corresponds to an output level of +4dBm at the 6176 output. Step 16: Set the JOIN/SPLIT switch in the center of the front panel to its down (SPLIT) position. In this mode, the internal connection between the output of the 6176 preamp section and the input of the limiter/compressor section is broken, so that each section functions as an independent device, each with its own dedicated input(s) and output.

Interconnections _____________________________________________________________
When operating in JOIN mode, the rear panel Preamp LINE OUT and the Limiter/Compressor LINE IN jacks are automatically disconnected.
For most applications, we recommend keeping the 6176 preamp Level control set between 7 and 10. Adjustments can then be made to the preamp Gain, Impedance, and Filter controls, as well as the various limiter/compressor controls, to achieve the optimum sound for your signal source. (If you are using the 6176 in JOIN mode, begin by setting the limiter/compressor Input and Output controls to approximately 5 for unity gain.)
Insiders Secrets _____________________________________________________________
6176 Versus 2-610 / 1176LN Why go for the 6176 over a 2-610 and an 1176LN chained together? One factor, of course, is price: the 6176 costs significantly less than a combination of a 2-610 and an 1176LN. Another factor is space: the 6176 combines both signal processors in a single chassis, thus saving potentially valuable real estate space in your rack. Dont forget that, when operating in SPLIT mode, the two sections can be accessed independently and can process entirely different signals. But just as important is sound. Indeed, the beauty of the 6176 is that it rides gain just like an 1176LN and it sounds quite a bit like one, yet it still delivers its own sonic signature. It features an all-new MOSFET regulated power supply, which not only tightens up bass response but also reduces selfinduced noise, making the 6176 a quieter, more hi-fi version of the original 1176LN. For many years, engineers have had to make the choice between an 1176LN or an LA-2A (another vintage compressor, now manufactured by Universal Audio, available as a standalone unit or in combination with a channel of the 2-610 preamp, in our LA-610 model), depending on the tone they want, not necessarily on how they differ as compressors. The 6176 offers another choice, plus its augmented with tube input circuitry and pre-compression EQ! Vocals, Vocals, Vocals The preamp section of the 6176 utilizes a channel of our popular 2-610 stereo mic preamp, favored by many engineers for vocal recording. In his December 2001 review for MIX magazine, Michael Cooper raved about the 2-610s abilities in this regard, writing, The 2-610 is the richest, fattest and sweetest mic preamp Ive ever heard on vocals. Bigger than life and possessing astounding depth, the sound made all other mic preamps Ive used sound somewhat 2-D by comparison. The bottom end was big and tight, mids incredibly clear, yet warm as hot fudge, and the sweet highs ultra-smooth.

The 6176 has a powerful and articulate
mid-range lens and the compression has always been one of my favorites producer Brian Ahern
Producer Brian Ahern (Emmylou Harris, Johnny Cash) points out that most voices and instruments are defined by their mid-range frequency content. He goes on to say that The 6176 has a powerful and articulate mid-range lens and the compression has always been one of my favorites.
Barry Rudolph, in his June, 2003 review of the 6176 for MIX magazine, pointed to the equalization controls as key for recording vocals, saying The EQ is wonderful for opening up the top end (10 kHz) on a vocal mic. the 70Hz LF shelf is smooth and fine for rolling off mic proximity effects or subsonic noise. He adds, For vocal recording, I adjusted the unit for the cleanest sound by backing down the gain selector and keeping the [preamps] output-level knob nearly full-up. Using a vintage Neumann M49 mic, I found that by boosting 1.5 dB at both 10 kHz and 100 Hz, the EQ corrected that mics occasional tendency to sound nasal. I got a big vocal sound with very good dynamic range and a warmth that helped out when my female singer sang at full voice and near the top of her range.
And reviewer George Shilling, writing for Resolution magazine in their May/June 2003 issue, said, The [6176 preamp] section is everything I remember about the 2-610a big warm, clear character, enhancing beautifully. Doing a last-minute vocal on a long project where several vintage channels of the grey-blue variety had been used for the bulk of recording, the vocals suddenly came to life with an extra sparkle when using the 6176 as a recording channel. It was noticed immediately by all present, after finding the optimal of the two input impedance settings. The ability to juggle the two gain controls for a variable amount of [tube] drive is a bonus, enabling the user to vary the tone subtly. The compressor section of the 6176, of course, is based on For vocals there really isnt the 1176, long known by engineers to be an essential tool in recording vocals. Industry legend Andy Johns (Led a better compressor Zeppelin, Rolling Stones) says flatly, For vocals there engineer Andy Johns really isnt a better compressor. Bruce Swedien is another legendary engineer who is a die-hard 1176 fan. I love them on vocals, he says. All of the Michael Jackson and James Ingram vocals that everyone has heard so much were done with at least one of those 1176s. I couldnt part with them for anything. They sound fabulous. Added reviewer Hugh Robjohns, writing about the Universal Audio 1176LN reissue in Sound on Sound magazine in June, 2001: The 1176LN is judged by many to be unsurpassed as a vocal compressor, and I would certainly agree that it can be extremely effective. It can be surprisingly transparent when used fairly gently on a 4:1 ratio, a setting whose warm, [tube]-like quality can be sublime on softer voices. Yet it can also accommodate the raunchiest hard compression demands too, which can be fantastic on strong, belted-out rock vocals. And reviewer Trevor Curwen, writing for The Mix in August 2000, reported that When recording vocals, the 1176[LN] was. used with a low ratio, resulting in a very natural, smooth sound and even performance being captured. Strapping the compressor across the vocal when mixing, and adding just a little more squeeze, gave it the presence it needed to sit consistently in the mix, with a nice top end to the sound. Producer/engineer Mike Shipley (Def Leppard, Shania Twain) says, I grew up using 1176sin England they were the compressor of choice. Theyre especially good for vocals. most anything else I can do without, but I cant be without at least a pair of 1176s and an LA-2A. The 1176 absolutely adds a bright character to a sound, and you can set the attack so its got a nice bite to it. I usually use them on 4:1 [ratio], with quite a lot of gain reduction. I like how variable the attack and release is; theres a sound on the attack and release which I dont think you can get with any other compressor. I listen for how it affects the vocal, and depending on the song I set the attack or releasefaster attack if I want a bit more bite. Producer/Engineer Mike Clink (Guns N Roses, Sammy Hagar) agrees. I find that I actually use 1176s more now than I ever did, he says. I like them because they bring out the brightness and presence of a soundthey give it an energy. It seems like when Im mixing I end up using an 1176 on the vocals every time. Jim Scott, who won a Grammy for Best Engineered Album for Tom Pettys Wildflowers, says I use 1176s real conservatively and they still do amazing things. I always use them on vocals. Im always on the 4:1 [ratio], and the Dr. Pepper [input/output settings]you know, 10 oclock, 2 oclock, and it does everything I need. They have an equalizer kind of effect, adding a coloration thats bright and clear. Not only do they give you a little more impact from the compression, they also sort of clear

Remember, the limiter/compressor section in the 6176 is program-dependent. Thats an important feature that allows it to be used in a musical, percussive way. Lets say you have a medium tempo, 4/4 rock beatan excellent scenario for using ALL mode. In this application, youd probably have a lot of input level, a slowish attack (so that the transients sneak through), and a quick release. The sonic result is extraordinary. First, the kick drum causes a great concussion, which is enhanced by the unique ALL mode distortion. As it does so, the other frequencies suck in, followed by an exaggerated release and recovery, and then the rest of the drum kit sound returns. all in rather dramatic fashion. Electric Guitar and Bass Theres something very special about the mix of tube preamplification and electric guitar and bass. This is an area where the 6176 positively shineslittle wonder, considering its lineage in the 2-610 and 1176LN. Myles Boisen stated flatly in his review of the 2-610 that the unit was an absolute smash hit for guitar and electric-bass recording at my studio. One technique that both Boisen and Cooper employed with great success was to split the electric guitar or bass signal and then plug one side into the Hi-Z input and the other into an external tube direct (DI) box whose output was connected to the the other channels mic input. Cooper also found that cranking the [preamp] Gain to +10 gave a slight bark that was perfect for country lead guitar fills. Even if you dont want to compress a signal, the use of 1:1 mode in the limiter/compressor section allows the 6176 to act as a tone box, adding both solid-state amplification and tube overdrive that can make an electric guitar sound positively monstrous. Andy Johns remembers using a pair of vintage 1176s on Jimmy Pages guitar on the song Black Dog for the multiplatinum Led Zeppelin IV album, connecting them in series (with the output of one feeding the input of another), but with one of them having all ratio buttons out (the equivalent of setting a 1:1 ratio on the 6176). Black Dog has a direct Gibson Les Paul Sunburst 52, he recalls, going right into the mic amps on the mixer, which is going through two 1176s, and it sounds like some guy in the Albert Hall with a bunch of Marshalls. I couldnt have done it without the 1176s. There is not another compressor that will do that, because [it allows you to] take out the compression [circuitry]. The use of 1:1 mode allows the 6176 to act as a tone box, adding both solid-state amplification and tube overdrive that can make an electric guitar sound positively monstrous. In his review for The Mix, Trevor Curwin used an 1176LN reissue extensively on electric guitar, both in the recording and mixing stages, and reported excellent results: Used on a 4:1 ratio when recording some electric guitars through a miked amp, it didnt take much to get a great sounding result. Just using around 3 dB of gain reduction added a very useful character to the sound. There is something about an original 1176 that adds a certain presence and bite that can be especially pleasing on electric guitar, and this new unit had that very same character about it.

Versatility Of course, no preamp or compressor, no matter how well designed, is perfect for all applications or for all microphones. Fortunately, the 6176 is designed to work with a wide variety of microphones and signal sources, and we think youll find that it acts as the perfect sonic complement for most of them. Reviewer Myles Boisen observed in his February 2002 review of the 2-610 for Electronic Musician magazine that the unit can work magic on amplified instruments, electronic keyboards, strings, horns, and percussion (including wood and metal percussion, alto and tenor saxophones, and trumpet) and in ambient-miking applications. I also highly recommend it for use with ribbon and other lowimpedance microphones. Barry Rudolph reported that Recording any Recording any instrument or vocal with instrument or vocal with the 6176 immediately the 6176 immediately places that sound places that sound source on a proper and wide source on a proper and wide stage stage. The 6176s personality includes tight Barry Rudolph, MIX magazine and clean low frequencies (if you run the unit clean) with a very forward and thick-sounding midrange coloration thats augmented by the bright sound of the [limiter/compressor] section. Percussion instruments benefit from slight preamp overload, reducing spikes, while electric guitars fatten up very well even without the [limiter/compressor] switched in. Using an external EQ and/or compressor after the preamp stage will get you anywhere else youd like, but it is hard to resist not using the unit as is for all recordings! Controlled Distortion The 6176 is more than just a preamp and limiter/compressor; its unique characteristics make it a tone shaper as well. One of its features is ultra-fast attack and release times, and used correctly (or incorrectly, depending on the way you look at it), you can use it to add distortion to any otherwise pristine audio track. Running most sources through a distortion device can cause the signal to lose some of its definition as you increase the effect. Also, distortion devices tend to add a significant amount of noise. But with the 6176, you can compress your signal and add distortion without losing definition, and while only minimally adding noise. Since the attack and release can happen so fast, set at their fastest values, they impart minute level fluctuations over the audio. The result is a special kind of distortion not available through any other means. This distortion can be adjusted to taste by altering the attack and release times, and by the compression ratio. Of course, you can also adjust the Input control to set how often the source will go into this distorted compression. Probably the most distorted sound youll get out of the 6176 is in ALL mode, with attack and release set to their fastest times. By simply backing off on the Input, Attack or Release controls, you can lessen the effect. If youre after a compressed grand piano sound with a little edge to it, try routing the signal through a 6176 with Gain of +5 or +10, a little EQ boost at 4.5k, a compression ratio of 8:1, a moderate attack time (around the 12 oclock position), and a very short release time. Using the Input knob, dial in 6 dB or so of gain reduction (youll need to make up the corresponding amount with the Output knob) and your piano will take on some unique sonic characteristics youve probably never heard before.

Mixing Applications The line-level input of the the 6176 allows it to be used in mixing as well as tracking. Even if no equalization is used, and even if the limiter/compressor section is set to Bypass, the signal continues to pass through the transformers, the tubes, and the polarity-reverse circuits, making it extremely useful for coloration of tracks. As Myles Boisen said in his review of the 2-610, Any recording can potentially benefit from line-level tube-stage processing. Whats more, the fact that many of the front panel controls of the 6176 are stepped makes it easy to reproduce settings. Cooper found that with its Gain switch set to +10, [the 2-610] fattened up kick drum and bass tracks very nicely, and Boisen reported that he was able to successfully use the 2610 to salvage overhead drum-mic tracks that had phase problems and a thin sound. Of course, raising the Gain and decreasing the preamp Level adds more tube coloration to the signal being processed. At the +10 Gain value, Cooper reported in his review of the 2-610, it was possible to get deliciously nasty distortion on line-level tracks. Using an extremely fast attack time (which enables the 6176 to control peak levels as well as sustained tones) allows it to effectively tighten up individual drum tracks in the mix stage.
As reviewer Hugh Robjohns points out, the original 1176LN had a slightly bright characteractually more of a subtle spectral tilt than an obvious high-frequency lift which generally helps tracks to cut through in a mix without you needing to even reach for EQ. Throughout the years, engineers have variously referred to this characteristic sound as edge, growl, present and urgent. Generally speaking, the higher the Input level, the more these descriptive terms come into play. The 6176 has a similar sonic footprint, though with improved bass response. Youll find that its operation is most transparent when doing gain reduction of 4 dB or less. This will serve to subtly reign in dynamic variations in the audio while still adding its characteristic tone. Judicious amounts of limiting or compression can also help get every syllable of a lead vocal intelligible, even in a dense backing track, and can also help backing vocals to sit correctly. In addition, the extremely fast attack time offered by the 6176 limiter / compressor (which enable it to control peak levels as well as sustained tones) allows it to effectively tighten up individual drum tracks in the mix stage. Live Applications Although the 6176 was designed primarily for use in recording, it can also serve as a powerful addition to a live sound rig, especially in FOH (Front Of House) applications. We know of several professional electric bass players who use the 2-610 as their onstage preamp, plugging their instrument directly into its Hi-Z input and then routing the 2-610 output to their power amp. The same trick works even better with the 6176, where you have the benefit of compressing or limiting the signal before sending it to the power amplifier.

The Technical Stuff _____________________________________________________________ Compressor / Limiter Basics
The function of a compressor is to automatically reduce the level of peaks in an audio signal so that the overall dynamic rangethat is, the difference between the loudest sections and the softest onesis reduced, or compressed, thus making it easier to hear every nuance of the music. Compression is sometimes referred to as peak reduction or gain reduction, because a compressor (or limiter, when acting more severely) rides gain on a signal much like a recording engineer does by hand when he manually raises and lowers the faders of a mixing console. Its circuitry automatically adjusts level in response to changes in the input signal: in other words, it keeps the volume up during softer sections and brings it down when the signal gets louder. The amount of gain reduction is typically given in dB and is defined as the amount by which the signal level is reduced by the compressor. Compression or limiting enables even the quietest sections to be made significantly louder while the overall peak level of the material is increased only minimally. The dynamic range of human hearing (that is, the difference between the very softest passages we can discern and the very loudest ones we can tolerate) is considered to be approximately 120 dB. Early recording media such as analog tape and vinyl offered much less dynamic range, so compression was a virtual necessity, raising the overall level of the material (making it hotter) without peak levels causing distortion. While many of today's digital recording media approach or even exceed 120 dB of available dynamic range, quiet passages of recorded music can still be lost in the ambient noise floor of the listening area, which, in an average home, is 35 to 45 dB. Despite the increased dynamic range, compression is especially important when recording digitally, for two reasons: One, it helps ensure that the signal is encoded at the highest possible level, where more bits are being used so that better signal definition is achieved. Secondly, it helps prevent a particularly harsh type of distortion known as clippingsomething that, ironically, only occurs in digital recording, due to the inherent limitations of digital technology. During recording, compression is customarily used to minimize the volume fluctuations that occur when a singer or instrumentalist performs with too great a dynamic range for the accompanying music. It can also help to tame acoustic imbalances within an instrument itselffor example, when certain notes of a bass guitar resonate more loudly than others, or when a trumpet plays louder in some registers than in others. Properly applied compression will make a performance sound more consistent throughout. It can tighten up mixes by melding dense backing tracks into a cohesive whole, can make vocals more intelligible, and can add punch and snap to percussion instruments like kick drum and snare drum, making them more present without necessarily being louder. It can also impart tonal coloration, making a signal warmer and fatter. Compression can even serve as a musical tool, enhancing the sustain of held guitar notes or keyboard pads, or providing a snappier attack to horn stabs or string pizzicato.

great, results in virtually no increase in output level. Note that the 6176 has been designed so that selecting higher ratios also raises the threshold level. As an aside, an expander is the opposite of a compressor: a device which increases the dynamic range of a signal. For example, a 10 dB change in the input signal might result in a 20 dB change in the output signal, thus expanding the dynamic range. Knee A compressor's knee determines whether the device will reach maximum gain reduction quickly or slowly. A gradual transition (soft knee) from no response to full gain reduction will provide a gentler, smoother sound, while a more rapid transition (hard knee) will give an abrupt slam to the signal. The 6176 utilizes soft knee compression and limiting, which is generally preferred for most musical applications; hard knee compression or limiting is more often used in applications where instrumentation (such as broadcast transmitter towers) must be protected from transient signal overloads. Attack and Release The main key to the sonic imprint of any limiter or compressor lies in its attack and release times; these are the parameters which most affect how tight or how open the sound will be after gain reduction. The attack time describes the amount of time it takes the limiter/compressor circuitry to react to and reduce the gain of the incoming signal, usually given in thousandths of a second (milliseconds) or even millionths of a second (microseconds). A fast attack kicks in almost immediately and catches transient signals of very brief duration (such as the beater hit of a kick drum or the pluck of a string), reducing their level and thus softening the sound. A slow attack time allows transients to pass through unscathed before compression begins on the rest of the signal. The release time is the time it takes for the signal to then return to its initial (pre-compressed) level. If the release time is too short, pumping and breathing artifacts can occur, due to the rapid rise of background noise as the gain is restored. If the release time is too long, however, a loud section of the program may cause gain reduction that persists through a soft section, making the soft section inaudible. In the 6176, both the attack and release times are user-selectable. Attack time can be set to between 20 microseconds and 800 microseconds; release time can be set to between 50 milliseconds and 1100 milliseconds (1.1 seconds). Unlike many other devices, however, the 6176 Attack and Release times get faster, not slower, as their corresponding knobs are turned up (clockwise).
Output (Makeup Gain) Finally, an output control is employed to make up for the gain reduction applied by the gain reduction circuitry; on the 6176, this is the function of the Output knob. Makeup gain is generally set so that the compressed signal is raised to the point at which it matches the level of the unprocessed input signal (for example, if a signal is being reduced in level by approximately -6 dB, the output makeup gain should be set to +6 dB). As you are adjusting a limiter or compressor, a switchable meter such as the one provided by the 6176 can be helpful in order to view the strength of the incoming signal (displayed when the meter is set to PRE), the strength of the outgoing signal (displayed when the meter is set to COMP), or the difference in levels between the original input signal and the gain-reduced output signal (displayed when the meter is set to GR). When in GR mode, the 6176 meter will read 0 dB when there is no incoming signal or when no compression is being applied.

dBV - Short for decibels as referenced to voltage, without regard for impedance; thus, one volt equals one dBV. DI - Short for Direct Inject, a recording technique whereby the signal from a high-impedance instrument such as electric guitar or bass is routed to a mixer or tape recorder input by means of a DI box, which raises the signal to the correct voltage level at the right impedance. Dynamic microphone - A type of microphone that generates signal with the use of a very thin, light diaphragm which moves in response to sound pressure. That motion in turn causes a voice coil which is suspended in a magnetic field to move, generating a small electric current. Dynamic mics are generally less expensive than condenser or ribbon mics and do not require external power to operate. Dynamic range - The difference between the loudest sections of a piece of music and the softest ones. The dynamic range of human hearing (that is, the difference between the very softest passages we can discern and the very loudest ones we can tolerate) is considered to be approximately 120 dB. Modern digital recording devices are able to match (or even exceed) that range. EQ - Short for Equalization, a circuit that allows selected frequency areas in an audio signal to be cut or boosted. FET An acronym for Field Effect Transistor, a type of solid-state semiconductor. Gain reduction - A synonym for compression or limiting. Hi-Z - Short for High Impedance. The 6176s Hi-Z input allows direct connection of an instrument such as electric guitar or bass via a standard unbalanced " jack. High shelving filter - An equalizer circuit that cuts or boosts signal above a specified frequency, as opposed to boosting or cutting on both sides of the frequency, which is what happens with a typical peak/dip EQ. Impedance - A description of a circuits resistance to a signal, as measured in ohms or thousands of ohms (K ohms). The symbol for ohm is. Knee - A compressor's knee determines whether the device will reach maximum gain reduction quickly or slowly. A gradual transition is called "soft knee, while a more rapid transition is called hard knee. Soft knee compression and limiting is generally more desirable for musical applications. Limiter - A compressor that operates at high compression ratios of 10:1 or higher. Limiting - A more severe form of compression, where a high compression ratio (of 10:1 or higher) is being used. Limiting Amplifier - A synonym for a limiter/compressor. Line level - Refers to the voltages used by audio devices such as mixers, signal processors, tape recorders, and DAWs. Professional audio systems typically utilize line level signals of +4 dBM (which translates to 1.23 volts), while consumer and semiprofessional audio equipment typically utilize line level signals of 10 dBV (which translates to 0.316 volts).
Low shelving filter - An equalizer circuit that cuts or boosts signal below a specified frequency, as opposed to boosting or cutting on both sides of the frequency. Makeup gain - A control that allows the overall output signal to be increased in order to compensate (make up) for the gain reduction applied by the compressor. Mic level - Refers to the very low level signal output from microphones, typically around 2 millivolts (2 thousandths of a volt). Mic preamp - The output level of microphones is very low and therefore requires specially designed mic preamplifiers to raise (amplify) their level to that needed by a mixing console, tape recorder, or digital audio workstation (DAW). MOSFET An acronym for Metal Oxide Semiconductor Field Effect Transistor, an advanced type of solid-state FET. Negative feedback - Not just something to fear on Ebay. Negative feedback is a design technique whereby a portion of the preamplifiers output signal is reversed in phase and then mixed with the input signal. This serves to partially cancel the input signal, thus reducing gain. A benefit of negative feedback is that it both flattens and extends frequency response, as well as reducing overall distortion. Noise floor - Unwanted random sound (noise) added by an electronic device. Patch bay - A passive, central routing station for audio signals. In most recording studios, the linelevel inputs and outputs of all devices are connected to a patch bay, making it an easy matter to reroute signal with the use of patch cords. Patch cord - A short audio cable with connectors on each end, typically used to interconnect components wired to a patch bay. Peak reduction - A synonym for compression or limiting. Ratio - see Compression Ratio Release time - The time it takes for a signal to return to its initial (pre-compressed) level. If the release time is too fast, "pumping" and "breathing" artifacts can occur, due to the rapid rise of background noise as the gain is restored. If the release time is too slow, however, a loud section of the program may cause gain reduction that persists through a soft section, making the soft section inaudible.

Ribbon microphone - A type of microphone that works by loosely suspending a small element (usually a corrugated strip of metal) in a strong magnetic field. This "ribbon" is moved by the motion of air molecules and in doing so it cuts across the magnetic lines of flux, causing an electrical signal to be generated. Ribbon microphones tend to be delicate and somewhat expensive, but often have very flat frequency response. Threshold - A term used to describe the level at which a compressor starts to work. Below the threshold point, the volume of a signal is unchanged; above it, the volume is reduced. In the 6176, threshold is determined by the setting of the Input and Ratio controls. Transformer - An electronic component consisting of two or more coils of wire wound on a common core of magnetically permeable material. Audio transformers operate on audible signal and are designed to step voltages up and down and to send signal between microphones and line-level devices such as mixing consoles, recorders, and DAWs. Transient - A relatively high volume pitchless sound impulse of extremely brief duration, such as a pop. Consonants in singing and speech, and the attacks of musical instruments (particularly percussive instruments), are examples of transients. XLR - A standard three-pin connector used by many audio devices, with pin 1 typically connected to the shield of the cabling, thus providing ground. Pins 2 and 3 are used to carry audio signal, normally in a balanced (out of phase) configuration.
Recall Sheet _____________________________________________________________
Specifications _____________________________________________________________
Preamp: Microphone Input Impedance Balanced Line Input Impedance Hi-Z Input Impedance Maximum Microphone Input Level Maximum Output Level Internal Output Impedance Recommended Minimum Load Frequency Response Maximum Gain Signal-to-Noise Ratio Limiting Amplifier: Input Impedance Output Load Impedance Frequency Response Gain Distortion Selectable, 15k (ohms) or (floating) 20 Hz to 20 kHz 1 dB 40 dB, 1 dB > 0.5% T.H.D. from 50 Hz 15 kHz with limiting, at 1.1 seconds release setting. Output of +22 dBm with no greater than 0.5% T.H.D. > 75 dB Adjustable, from 20 to 800 milliseconds Adjustable, from 50 milliseconds to 1.1 seconds Optional, using 1176SA stereo interconnect accessory Selectable, 500 (ohms) or 2k 20k Selectable between 2.2 M or 47 k +18 dBu (2k input impedance and 15 dB Pad in) +20 dBm 20 Hz to 20 kHz +0, -1 dB 65 dB (500 input impedance) > 90 dB (@ maximum gain)
Signal-to-Noise Ratio Attack Time Release Time Stereo Interconnection General: Tube Complement Power Requirements Power Connector Fuse Power Indicator Light Dimensions Weight

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Table of Contents __________________________________________________________
A Letter From Bill Putnam, Jr....ii iii Important Safety Instructions.... Two Page, Two Minute Guide To Getting Started...2 Front Panel..... 4 Rear Panel.... 7 Interconnections..... 8 Insiders Secrets.... 9
The Technical Stuff..... 15 History of the 2-1176.... 15 Compressor / Limiter Basics.... 17 About All-Button Mode.... 20 About Class A.... 20 Operating the 2-1176 in Link Mode... 20 Making A Custom Insert Cable... 21 Maintenance Information.... 23 Meter Calibration.... 23 Changing Voltage...23 Changing Fuses....24 2-1176 Circuit Details....25 Glossary of Terms..... 30 Recall Sheet..... 33 Specifications..... 34 Additional Resources / Product Registration / Warranty / Service & Support... 35
The Two Page, Two Minute Guide To Getting Started __________________________________________________________
No one likes to read owners manuals. We know that. We also know that you know what youre doingwhy else would you have bought our product? So were going to try to make this as easy on you as possible. Hence this two-page spread, which we estimate will take you approximately two minutes to read. It will tell you everything you need to know to get your Universal Audio 2-1176 up and running, without bogging you down with details. Of course, even the most expert of us has to crack a manual every once in awhile. As the saying goes, as a last resort, read the instructions. Youll find those details youre cravinga full description of all front and rear panel controls, interconnection diagrams, insiders secrets, history, theory, maintenance information, block diagrams, specifications, even a glossary of termsin the pages that follow. Manual conventions:
Means that this is an especially useful tip Means that this is an especially important bit of information
And when we need to direct you to a page or section elsewhere in the manual, well use the universal signs for rewind () or fast forward (). Getting Started With Your 2-1176: Step 1: Decide where the 2-1176 is to be physically placed and place it there. The 2-1176 is housed in a standard two-rackspace 19" chassis, and so we recommend that it be securely mounted in a rack if possible. Step 2: Make sure the front-panel power switch is in its down (OFF) position, and then connect the supplied IEC power cable to the rear panel AC power connector. On the rear panel, set the Input-Z switch for both channels to their up (15K) position unless you are interfacing with vintage equipment, in which case the corresponding Input-Z switch should be placed in its down (600) position. Step 3: Mute your monitors and then, using balanced cables with XLR connectors, make connections to the 2-1176 rear panel XLR line inputs and outputs. Most often, these connections will be to a patch bay or to and from a channel or bus insert on a mixer. ( see page 8 for an interconnection diagram) Step 4: Set the LINK/DUAL switch in the center of the front panel to its down (DUAL) position and set the BYPASS/COMP switch in the center of the front panel to its down (COMP) position. Step 5: Set both Meter switches to their rightmost (+4) position. This ensures that both meters displays final output level. Step 6: Set the Input and Output knobs for both channels to approximately 5 (their 12 oclock position) for unity gain.

Step 7: Set the Attack and Release knobs for both channels fully counterclockwise (to their SLOW position). Step 8: Set the Ratio knobs for both channels to their 4 position, thus selecting moderate compression (at a 4:1 ratio) for both channels. Step 9: Power on the 2-1176. The purple LED above the power switch will light up. Step 10: Unmute your speakers and begin monitoring the 2-1176 left channel output only. At the source, raise the level of the left channel input signal until the left 2-1176 meter shows optimum signal strength (around 0 VU, with occasional excursions into the red, but with no audible distortion). Step 11: Now its time to hear the effect of compression on your signal. Set the Meter switch for the left channel to its leftmost (GR) position; the left channel meter will reflect the amount of gain reduction. As you increase the amount of compression (by raising the level of the Input knob), the overall signal will be attenuated; turn the Output knob clockwise to make up the difference. Set the Meter switch back to its rightmost (+4) position in order to view the output level of the left channel of the 2-1176 and ensure that you are applying the correct amount of makeup gain. You can compare the uncompressed signal with the compressed signal with the use of the BYPASS/COMP switch in the center of the front panel. (Note that the Input and Output knobs for each channel remain active even in BYPASS mode.) Step 12: Experiment by selecting different Ratios, including ALL (All-Button mode, equivalent to pressing in all four Ratio buttons on an 1176LN simultaneously). Note that selecting a Ratio of 1 (1:1) results in no gain reduction, but that the color of the 2-1176 channel is still imparted to the signal. (This has the same effect as setting the 2-1176 to BYPASS mode.) Also try varying the Input level and Attack and Release times for different compression and limiting characteristics. Note that, unlike many other devices, the 2-1176 attack and release times are faster when their associated knobs are turned clockwise, and slower when they are turned counterclockwise. Set the left channels Meter switch to GR in order to view the amount of gain reduction being applied to the signal. Note that as you increase the amount of gain reduction (by raising the Input level and/or selecting higher ratios), the overall signal may be attenuated. If necessary, you can make up the difference by turning the Output knob clockwise. Return the Meter switch to its rightmost (+4) setting in order to view the final output level. A meter reading of 0 corresponds to an output level of +4 dBm at that channels output. Step 13: Repeat steps 10 - 12 above, this time routing signal to the right channel only and adjusting the 2-1176s right channel controls as desired. Note that, in DUAL mode, the two channels operate completely independently of one another, giving you two discrete compressor/limiters. Step 14: To hear the effect of the two compressors stereo linked, set the LINK/DUAL switch in the center of the front panel to its up (LINK) position, and set the Input and Output knobs in both channels identically. Note that, in Link mode, the left channel Attack and Release knobs act as master controls for both channels. ( see page 20 for more information about operating the 2-1176 in Link mode)

Setting the Ratio to 1 (1:1) disables compression altogether for that channel; however,
signal continues to pass through the channels circuitry. This is used to add the color of the 2-1176 without any actual gain reduction. (The same effect is achieved when the 21176 is placed in BYPASS mode. ( see #10 on the following page for more information) 4 - Selects a 4:1 ratio (moderate compression). 8 - Selects an 8:1 ratio (severe compression). 12 - Selects a 12:1 ratio (mild limiting). 20 - Selects a 20:1 ratio (hard limiting). All - Implements the All-button or 4 button mode (sometimes also known as British Mode), in which the channel of the 2-1176 duplicates the overdrive that occurs when all four ratio buttons on an original 1176 are pushed in simultaneously. ( see page 20 for more information) -5-
Engineers typically use the ALL Ratio on drums or on ambience or room mics. It can also be
used to dirty up a bass or guitar sound, or for putting vocals in your face. ( see page 20 for more information)
When a Ratio of ALL is selected, distortion increases radically due to a lag time on the
attack of initial transients and there are constant changes in the attack and release times, as well as a change in the bias points. Consequently, the meter will go wild, often resting at maximum.
(8) LINK / DUAL switch - Sets the 2-1176 to either dual mono or stereo linked operation. When set to the down (DUAL) position, the 2-1176 operates as two separate compressors, with the two channels operating completely independent of one another. When set to the up (LINK) position, the two channels of the 2-1176 act as a single stereo compressor, with equal gain reduction on both sides, even when there is transient activity only on one channel. ( see page 20 for more information)
When operating the 2-1176 in LINK mode, the Input and Output knobs should normally be
set to the same position in both channels. ( For more information, see page 20.)
affect both channels and the right channels Attack and Release controls become non-functional. ( For more information, see page 20.) (9) Power - Turns the 2-1176 power on or off. When powered on, the purple LED immediately above the switch lights up. (10) BYPASS / COMP switch - Allows a comparison between the compressed signal and the noncompressed signal. However, regardless of the position of this switch, signal continues to pass through the 2-1176 circuitry (thus picking up the signature 2-1176 sound) and the Input and Output knobs remain active. When in the up (BYPASS) position, a ratio of 1 (1:1, or no compression) is applied to both channels of the 2-1176 regardless of the setting of their Ratio switches. When in the down (COMP) position, the selected Ratio for each channel is applied.

The BYPASS / COMP switch setting always affects both channels of the 2-1176,
regardless of the LINK / DUAL switch setting.
In order to obtain a specific amount of limiting or compression for each channel of the
2-1176, begin by setting both the Input and Output knobs to approximately 5 (their 12 oclock positions) for unity gain. Set the Ratio as desired, then set the Attack and Release controls to approximately 5 (their 12 oclock positions) so that some gain reduction is enabled. Set the Meter switch to GR so that the meter shows the amount of gain reduction, then slowly turn the Input control up until the desired amount of gain reduction is achieved. Finally, adjust the Attack and Release times until they are suitable for the program material and make up any gain necessary by raising the Output knob (set the Meter switch to +4 in order to have the meter display the final output level).
Rear Panel __________________________________________________________
NOTE: The 2-1176 provides two channels, each of which have identical rear panel controls. (1) Line INPUT - Connect line-level input signal for each channel to these balanced XLR connectors. Pin 2 is wired positive (hot). This signal will normally be arriving via a patchbay from a channel or bus insert send. (2) Line OUTPUT - Balanced XLR connectors carrying the line-level output signal of each 2-1176 channel. This signal will normally be routed via a patchbay to a channel or bus insert return. (3) Input-Z switch - When in the up position, the input impedance for that channel will be 15K ohms, suitable for interfacing the 2-1176 with most modern devices. When in the down position, the input impedance for that channel will be 600 ohms, typical of older vintage gear. In some situations you may perceive a brighter tone and slightly greater depth by using the 15K ohm position, even when interconnected with vintage equipment. (4) AC Power Connector - Connect a standard, detachable IEC power cable (supplied) here.
Interconnections __________________________________________________________

Try using the 2-1176 as a bus
insert effect. or even as an Aux send effect!
Darwin Grosse took a slightly different approach. I set up my mixer with a subgroup bus containing the 2-1176 as an insert effect, he wrote, then selectively sent a variety of instruments and vocal tracks to that bus (in addition to the main bus). This proved to be the perfect use for All Button

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mode. Adding a bit of slam-compressed audio to the mix produced a brighter mix without highfrequency overload. It also really helped both drums and guitars maintain a strong place in the mix without overwhelming vocals and bass. For even more control, I also hooked up the 2-1176 as an aux send effect, again in All Button mode. This worked even better, since I was able to control the relative mix of instruments sent to the compressor, and (with a little tuning of the 1176s gain stages) get an even more balanced, brighter and punchier mix. These are techniques that are often overlooked by project studio users, but are great ways to improve troublesome mixes. The 2-1176 shone in this application. Grosse also experimented with the use of the 2-1176 as a mastering tool. I took a recently mastered recording of a solo guitarist and gave it the 1176 treatment, he wrote. Immediately, I noticed that the various tracks seemed to coalesce into a whole, with the sound of the 1176 limiters helping pull the various tracks (which had been recorded in several different locations) together. I recorded the result through a nice set of converters, re-marked the tracks and passed it on to the artist. A big thumbs-up was the result. Even if you dont opt to compress or limit the final mix, its often worth trying routing the signal through a 2-1176 in BYPASS mode (equivalent to selecting Ratios of 1 [1:1] for both channels). Even though the gain reduction circuitry is disconnected in Bypass mode, the signal continues to pass through the 2-1176 transformers, thus picking up their signature sound. Ducking Applications When the 2-1176 is placed in stereo Link mode, the Input knobs in each channel serve to mix the two incoming signals to the side-chain circuit of both channels. In most musical applications, you will want the left and right signals to affect the compressor equally, so that a transient occurring in one channel will serve to increase compression or limiting in both channels the same way. Thus, for normal operation in most musical and recording applications, it is important that the Input and Output knobs for both channels be set to the same value. However, one byproduct of this circuit design is that it allows you to make the gain reduction circuitry more sensitive to one channel than the other. If you therefore increase the amount of Input in one channel relative to the other, the compressor will be more sensitive to transients occurring in the that channel. This can sometimes be useful in broadcast applications, where you might want to duck a musical bed, for example, whenever an announcer is speaking. It can also serve as a creative tool in musical applications: for example, you might want to have one instrument automatically lowered in level whenever another instrument comes in, and then return to its original level when the second instrument stops playing. Or you can create dramatic ambience effects by using a transient (say, a snare or kick drum hit) fed into one channel of the 2-1176 to raise and lower the level of a room mic track fed into the other channel. Your only limit (pardon the pun) is your imagination!

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The Technical Stuff __________________________________________________________ History of the 1176
The original Universal Audio 1176, designed by Bill Putnam, was a major breakthrough in limiter technology the first true peak limiter with all transistor circuitry offering superior performance and a signature sound. Evolved from the popular Universal Audio 175 and 176 vacuum tube limiters, the 1176 retained the proven qualities of these industry leaders, and set the standard for all limiters to follow. In fact, the 1176 may well be the most loved limiter/compressor in history. Its trademark lightning-quick attack and release times and the tone of its Class A output amplifier have enhanced countless recordings for more than forty years. As is evident from entries and schematics in his design notebook, Putnam experimented extensively at the time with the then newly developed Field Effect Transistor (FET) in various configurations and eventually found a way of using it as the gain-controlling element of a compressor/limiter. The original version of the 1176, released in 1967, was denoted the 1176A, but was revised to the model AB only a few months later, with improvements in stability and slightly reduced noise. The following year saw revision B, with further minor changes to the preamplifier circuit. These models all featured a brushed aluminum faceplate with a blue meter section.

1176 Revision B

Revision C, released in September 1970, saw two major changes. One, the unit now sported a black faceplate instead of silver, and, two, it was now designated an 1176LN, with the LN standing for low noise. This model featured the first major modification to the 1176 circuit, designed by Brad Plunkett in an effort to reduce noise, hence the birth of the 1176LN. Numerous design improvements followed, resulting in at least 13 revisions of the 1176. Plunketts LN circuitry was originally encased within an epoxy module, but a subsequent redesign fully integrated these improvements with the main circuit board, resulting in revision D.

1176LN Revision D

Revision E was introduced in the early 1970s and was the first to accommodate European 220V mains power with a voltage selector on the rear panel. Of all the revisions, model D and model E are considered to have superior sound and are thus the most sought-after versions by audio engineers.

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The Technical Stuff __________________________________________________________
Another significant redesign occurred in 1973. The revision F output stage was modified to provide higher output current capability by using a push-pull circuit design borrowed from Universal Audio's new 1109 preamplifier. This new output stage replaced the original Class A circuit borrowed from the 1108 preamp. The meter drive circuit was also updated, with an operational amplifier instead of the previous discrete circuit. The classic transformer front end of the 1176 met its demise with the model G, in which an electronically balanced input stage replaced it. The final update, the model H, simply marked a return to a silver faceplate and the addition of a blue UREI logo. It was at this time that the short-lived 1178 Dual Peak Limiter made its appearance. While in theory this was a stereo version of the 1176LN, it had a number of shortcomings. For one thing, it had only one set of attack and release controls and only one set of ratio select pushbuttons, so even though it had a Stereo/Mono switch and therefore could operate in dual mono, this mode had limited value since it lacked independent controls for those three important parameters. The 2-1176 is therefore not based on the 1178, but instead on the present-day Universal Audio 6176 Channel Strip, which combines classic design with modern circuitry.

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Output (Makeup Gain) Finally, output controls are employed to make up for the gain reduction applied by the gain reduction circuitry; on the 2-1176, this is the function of each channels Output knob. Makeup gain is generally set so that the compressed signal is raised to the point at which it matches the level of the unprocessed input signal (for example, if a signal is being reduced in level by approximately -6 dB, the output makeup gain should be set to +6 dB). As you are adjusting a limiter or compressor, switchable meters such as those provided for each channel of the 2-1176 can be helpful in order to view the strength of the outgoing signal (displayed when the meter is set to +4), or the difference in levels between the original input signal and the gainreduced output signal (displayed when the meter is set to GR). When in GR mode, the channels meter will read 0 dB when there is no incoming signal or when no compression is being applied.

About All-Button Mode

One of the most unique features of the 2-1176 is the ability to select a compression Ratio of ALL (equivalent to pressing all four Ratio buttons in simultaneously on an 1176LN) to create extreme amounts of compression or limiting. In this All-Button mode (sometimes also known as British Mode because of its extensive use on many seminal British recordings of the 60s and 70s), distortion increases radically due to a lag time on the attack of initial transients (a phenomenon which might be described as a "reverse look-ahead"). The ratio goes to somewhere between 12:1 and 20:1, and the bias points change all over the circuit, thus changing the attack and release times as well. The unique and constantly shifting compression curve that results yields a trademark overdriven tone that can only be found in this family of limiter/compressors.

About Class A

Most electronic devices can be designed in such a way as to minimize a particularly unpleasant form of distortion called crossover distortion. However, the active components in Class A electronic devices such as the 2-1176 draw current and work throughout the full signal cycle, thus eliminating crossover distortion altogether.
Operating The 2-1176 In Link Mode
The circuitry in the two channels of the 2-1176 have been carefully matched to allow smooth and musical operation when stereo linked. When operating the 2-1176 in Link mode ( see #8 on page 6), the Input knobs in each channel serve to mix the two incoming signals to the side-chain circuit of both channels. In most musical applications, you will want the compressor to equally affect the left and right signals, so that a transient occurring in one channel will serve to increase compression or limiting in both channels the same way. Thus, for normal operation in most musical and recording applications, it is important that the Input knobs (as well as the Output, Attack, and Release knobs) for both channels be set to the same value. However, because there can be minor variations in the tolerance of any potentiometer, we recommend that you calibrate the two Input controls as follows:

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The Technical Stuff
_______________________________________________________________________ 1. While in Link mode, send stereo signal to the 2-1176 but monitor the left channel only. 2. In the left channel of the 2-1176 only, set the Ratio switch as desired and adjust the Input and Output knobs until the desired amount of compression or limiting is heard. 3. Duplicate those same settings in the right channel of the 2-1176 and then begin monitoring both channels, in stereo. 4. Set the 2-1176 to Dual mode and mute the stereo source. Instead run a steady-state test signal through both channels. 5. Set the Meter switch for both channels to +4, then adjust the lower of the two Input knobs so that the output on both meters is identical. 6. Finally, disconnect the test signal and restore the original stereo signal. One byproduct of this circuit design is that it allows you to make the gain reduction circuitry more sensitive to one channel than the other. Thus, for example, if you set the Input knob for the left channel considerably higher (turned more clockwise) than the right channel, the compressor will be more sensitive to transients occurring in the left channel. This can sometimes be useful in broadcast applications, where you might want to duck a musical bedfor example, whenever an announcer is speaking. (see page 14 for more information.)
When operating the 2-1176 in Link mode, both channels should normally be set to the
same Ratios as well as to similar Input and Output levels.
When operating the 2-1176 in Link mode, the left channels Attack and Release controls become master controls and the right channels Attack and Release controls become non-functional.
Making A Custom Insert Cable
In order to ensure unity gain, the input and output to a compressor are normally derived from a mixer channel or bus insert send and return. However, most mixing consoles provide such inserts on unbalanced TRS (Tip/Ring/Sleeve) connectors, with the tip carrying the send and the ring carrying the return, with the sleeve serving as common ground. Premade insert Y-cables that provide a single TRS plug on one end and two XLR connectors on the other end are readily available for this purpose. However, it can be considerably more cost-effective to make your own custom insert cablesomething which requires only basic soldering skills and a few inexpensive parts.

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To make such a cable, first acquire the following components: (1) Female XLR cable connector (1) Male XLR cable connector (1) 1/4" TRS cable connector A suitable length of light gauge unbalanced microphone cable A short piece of 22 gauge bus wire Then follow these steps to assemble the cable: 1. Cut two lengths of audio cable suitable to reach from the 2-1176 to the insert point on your mixer. Light gauge cable should be used in order to allow the two cables to comfortably fit inside a TRS jack. Use a short piece of bus wire to tie pins 1 (cold) and 3 (ground) together on each XLR connector. Only solder pin 3, as you will also need to solder an audio lead into pin 1. 2. On both XLR connectors, solder the hot (white) lead into pin 2, and solder the cold (black) lead into pin 1, as well as the other end of the bus wire (as described in the previous step). 3. Solder the hot (white) lead from the cable connected to the male XLR to the tip connection point of the TRS jack. Solder the hot (white) lead from the cable connected to the female XLR to the ring connection point of the TRS jack. As shown in the photograph below, combine the cold lead and the shielding together at the ground point.
4. Your finished cable should look like the photograph below. Be sure to check continuity with a voltmeter or test light before use, to ensure proper grounding and signal flow.

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The Technical Stuff __________________________________________________________ Maintenance Information
Meter Calibration The 2-1176 meters may occasionally need to be calibrated. This is accomplished by adjusting the Zero Set potentiometer, located through a small hole on the front panel between each channels Input and Output knobs.
The procedure for adjusting the meter is as follows: 1. Power on the 2-1176. 2. Set the Meter switch for the channel being calibrated to its +4 position. 3. Set the Input control for the channel being calibrated fully off (turn the knob fully counterclockwise). 4. Use a small screwdriver to slowly adjust that channels Zero Set potentiometer so that its meter reads 0 dB. Watch how the meter settles before completing the calibration. Changing Voltage The 2-1176 can operate from 100 V to 240 VAC / 50-60 Hz. The internal power supply is self-sensing and will automatically work throughout this voltage range.

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Changing Fuses The AC power fuse is located on the power supply circuit board inside the 2-1176, directly behind the AC power connector, accessed by removing the top panel of the 2-1176. Always remove the power cord before checking or changing the fuse! The required fuse is a T2A 250V, 5mm x 20mm.
Never substitute a different fuse type!

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The Technical Stuff __________________________________________________________ 2-1176 Circuit Details

Figure 4 - Using an FET as a voltage-variable resistor. The combination of R5 and Q1 acts as a voltage divider which controls the gain. Figure 4 shows how the FETs resistance determines the gain of this section. Resistor R5 and the FET essentially comprise a voltage divider circuit. The lower the FETs resistance, the less gain this stage will have. The FET acts like a variable resistor, where the resistance is determined by the control voltage that is applied to it. Note that the greater the voltage applied to the gate of the FET, the less resistance, hence large signals cause the FET to reduce the gain. Larger input signals result in a higher voltage from the gain control circuit, which will lower the gain, hence reducing the signal level. This is the basis of the limiting action. Note that the 1176 is a feedback style compressor since the sidechain circuit samples the signal level after the gain reduction. LN Circuitry The LN circuit, which appeared in revisions C and later, was designed to reduce the distortion that the FET introduced in the gain reduction stage. FETs are inherently nonlinear devices, and any non linear device will introduce signal distortion. The LN circuitry was designed to ensure that the FET stayed as much within a linear region as possible, thus reducing unwanted distortions. Much of what is now known about the operation and design guidelines of FETs was very new at the time the 1176 was designed. Initially, the decision was made to try to keep the LN circuit a secret and file for a patent. In order to accomplish this it was decided to build the LN circuit in a separate module. This module was then attached to the circuit board. The first revision to have the LN circuitry was revision C. This was accomplished by attaching an LN module to the revision B circuit board. This module turned out to be a problem to manufacture and the decision was made to revise the circuit board to accommodate the LN circuitry without the module. This then became revision D1.

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Output Amplifier The output amplifier is a Darlington pair followed by a class A stage based on a 2N3053 transistor. The 1176 output stage was essentially the same as the Universal Audio 1108 pre-amplifier. The output transformer is a custom transformer designed by Bill Putnam Sr. Aside from offering output impedance matching, the transformer forms an integral part of the feedback network used to stabilize the output stage. Note that later revisions (F and beyond) used a push-pull (class AB) output stage. (Revision E was essentially the same as revision D. Revision E added 220 Volt operation as well as a 10 M resistor across the ratio switch to avoid pops while changing between compression ratios.) Gain Reduction Control Circuit

Figure 5 - Gain Reduction Control Circuit

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As shown in Figure 5, this circuit controls the amount of compression as well as the attack and release times of the limiter. The input to this circuit is taken from the output of the preamplifier section, just before the volume control potentiometer (R23). The compression Ratio switches determine the level of the signal which is sent to the sidechain. This determines the amount of limiting or compression. Transistor Q7 acts as a phase inverter which is followed by Q8 , an emitter follower. This signal then feeds CR3 as well as Q9 and Q10, which comprise another phase inverter / emitter follower combination. This is then fed to CR2. Note that the signal applied to CR2 is 180 out of phase with the signal at CR3. Since they are out of phase, the combination of CR2 and CR3 act as a full-wave rectifier. The output of the rectifiers are then filtered by C22 which smooths the signal. This DC voltage is proportional to the level of the input signal. A bias level is applied to the diodes by the Ratio switches. This controls the threshold of limiting, and is adjusted for the correct value as determined by the currently selected compression ratio selection. R55 controls the compressors attack time by regulating how fast C22 is charged. Likewise, R56 determines the compressors release time by controlling the rate at which C22 discharges. The output of this stage is then applied to the gate of the FET in the gain reduction circuit, which in turn controls the gain in the manner previously described.

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Glossary of Terms __________________________________________________________
Ambient noise - Low-level noise created by environmental factors such as fans, air conditioners, heaters, wind noise, etc. Attack time - Describes the amount of time it takes compressor circuitry to react to and reduce the gain of incoming signal. A compressor set to a fast attack time kicks in almost immediately and catches transient signals of very brief duration, reducing their level and thus "softening" the sound. A slow attack time allows transients to pass through unscathed before compression begins on the rest of the signal. The 2-1176 attack time ranges from 20 to 800 microseconds. Auxiliary (Aux) send - A mixer bus output designed to combine and send multiple signals to an external processor or monitor system. Balanced - Audio cabling that uses two twisted conductors enclosed in a single shield, thus allowing relatively long cable runs with minimal signal loss and reduced induced noise such as hum. Bus - The point in an audio mixer where various signals are blended together. Bus insert - An insert across a mixer bus. Channel - A functional path in an audio circuit. A mixer provides multiple channels, each with its own dedicated input(s) and several outputs, such as buses, auxiliary sends, etc. Channel insert - An insert across a mixer channel. Class A - A design technique used in electronic devices such that their active components are drawing current and working throughout the full signal cycle, thus yielding a more linear response. This increased linearity results in fewer harmonics generated, hence lower distortion in the output signal. Clipping - A particularly harsh form of audio distortion, caused when the loudness of an incoming signal exceeds an audio recording devices capability to represent its amplitude. When that happens, the peaks of the signal simply get clipped off, thus drastically changing the waveform. When clipping occurs in a digital recording device, the result is an especially unpleasant sound. Compression - The process of automatically reducing the level of peaks in an audio signal so that the overall dynamic rangethat is, the difference between the loudest sections and the softest onesis reduced, or compressed. Compression is sometimes described as gain reduction or peak reduction. Compression ratio - A term that describes the amount of increase required in the incoming signal in order to cause a 1 dB increase in output. A ratio of 4:1, for example, means that any time there is an increase of 4 decibels in the loudness of the input signal, there will only be a 1 dB increase in output signal. When compression ratios of 12:1 or higher are being used, the device is instead said to be limiting. DAW - An acronym for Digital Audio Workstationthat is, any device that can record, play back, edit, and process digital audio. dB - Short for decibel, a logarithmic unit of measure used to determine, among other things, power ratios, voltage gain, and sound pressure levels.

Glossary of Terms

_______________________________________________________________________ Peak reduction - A synonym for compression or limiting. Program dependent - Refers to a parameter that varies according to the characteristics of the incoming signal. To some degree, the amount of 2-1176 gain reduction is program dependent. Ratio - see Compression Ratio Release time - The time it takes for a signal to return to its initial (pre-compressed) level. If the release time is too fast, "pumping" and "breathing" artifacts can occur, due to the rapid rise of background noise as the gain is restored. If the release time is too slow, however, a loud section of the program may cause gain reduction that persists through a soft section, making the soft section inaudible. The 2-1176 release time ranges from 50 milliseconds to 1.1 seconds. Terminal Strip - An insulated stamped strip of tin-plated loops of copper, used for multiple electrical or audio interconnections. Sometimes called a barrier strip. Threshold - A term used to describe the level at which a compressor starts to work. Below the threshold point, the volume of a signal is unchanged; above it, the volume is reduced. In the 2-1176, threshold is determined by the setting of the Input and Ratio controls. Transformer - An electronic component consisting of two or more coils of wire wound on a common core of magnetically permeable material. Audio transformers operate on audible signal and are designed to step voltages up and down and to send signal between microphones and line-level devices such as mixing consoles, recorders, and DAWs. Transient - A relatively high volume pitchless sound impulse of extremely brief duration, such as a pop. Consonants in singing and speech, and the attacks of musical instruments (particularly percussive instruments), are examples of transients. TRS - Short for Tip/Ring/Sleeve, a standard quarter-inch jack connector, with the tip and ring carrying audio signal and the sleeve attached to the shield of the cabling, thus providing ground. When used for mixer channel or bus inserts, the tip and ring will typically carry send and return signals, respectively. When used for balanced connections, the tip and ring will carry the same audio signal, with one signal out of phase with the other. TS - Short for Tip/Sleeve, a standard quarter-inch jack connector, with the tip carrying audio signal and the sleeve attached to the shield of the cabling, thus providing ground. Unity gain - No boost or attenuation of the incoming signal. When set to unity gain, a devices output signal will be at exactly the same strength as its input signal. XLR - A standard three-pin connector used by many audio devices, with pin 1 typically connected to the shield of the cabling, thus providing ground. Pins 2 and 3 are used to carry audio signal, normally in a balanced (out of phase) configuration.

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Recall Sheet __________________________________________________________

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Specifications
_______________________________________________________________________ Input Impedance Output Load Impedance Frequency Response Gain Distortion 15 k, 600 selectable 600 (floating) 20 Hz to 20 kHz 1 dB 40 dB, 1 dB > 0.5% T.H.D. from 50 Hz 15 kHz with limiting, at 1.1 seconds release setting. Output of +20 dBm with no greater than 0.5% T.H.D. > 85 dB Adjustable, from 20 to 800 microseconds Adjustable, from 50 milliseconds to 1.1 seconds Left channel Attack and Release links to right channel in LINK mode XLR Gain Reduction (dB) and Output (dB) 100V - 240V, auto-sensing power supply Detachable IEC power cable T2A 250V, 5mm x 20mm 19" W x 3.5" H x 12.25" D (two rack unit) 9.6 lb. (with box, 14.8 lb.)
Signal-to-Noise Ratio Attack Time Release Time Stereo Operation
External Connections Meter Power Requirements Power Connector Fuse Dimensions Weight

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Additional Resources/Product Registration/Warranty/Service & Support __________________________________________________________ Additional Resources
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Product Registration

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Warranty

The warranty for all Universal Audio hardware is one year from date of purchase, parts and labor.

Service & Support

Even gear as well designed and tested as ours will sometimes fail. In those rare instances, our goal here at UA is to get you up and running again as soon as possible. The first thing to do if youre having trouble with your device is to check for any loose or faulty external cables, bad patchbay connections, grounding trouble from a power strip and all inputs/outputs (mic/line/Hi-Z, etc.). If your problem persists, call tech support at 877-MY-UAUDIO, or send an email to hardwaresupport@uaudio.com, and we will help you troubleshoot your system. (Canadian and overseas customers should contact their local distributor.) When calling for help, please have the product serial number available and have your unit set up in front of you, turned on and exhibiting the problem. If it is determined your product requires repair, you will be told where to ship it and issued a Return Merchandise Authorization number (RMA). This number must be displayed on the outside of your shipping box (use the original packing materials if at all possible). Most repairs take approximately 2 - 4 days, and we will match the shipping method you used to get it to us. (In other words, if you shipped it to us UPS ground, we will ship it back to you UPS ground; if you overnight it to us, we will ship it back to you overnight). You pay the shipping costs to us; we ship it back to you free of charge. Qualified service under warranty is, of course, also free of charge. For gear no longer under warranty, tech bench costs are $75 per hour plus parts.