Oak Lodge Consulting Ltd

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JBL L300 Summit Renovation

Introduction

This article describes my search for a pair of JBL L300 speakers, and my subsequent renovations and improvements to achieve a truly outstanding level of performance.

The Situation

About 25 years ago I was living in Munich, Germany. Germany is a country where large loudspeakers are still popular. During a visit to a Munich Hi-Fi specialist I had a demonstration of various speakers, including Audiostatic, Quad Electrostatics, and a pair of JBL L300 Summits. The JBLs were out of my price range, but what I heard I can still remember. They were amazing, in terms of breadth, scale and sheer impact. Coincidentally there was a review of them by Martin Colloms that month that concluded,Quite simply, they delivered the good1s. I agreed with that, and then some. Since then I have gone through many other speakers, including Kef 105 Monitors, Klipschorns, IMF Transmission Lines, Spendor BCIII and several home-brews. The Klipsches were near to what I was seeking, but they seemed to trade detail for clout and ultimately I found them tiring. Then there was a local development. A nearby store started trading used high-end audio systems. I quickly went through amplifiers from Accuphase, Krell, Lumley and finally I settled on Nelson Passs Aleph 5 power amp and matching pre-amp. This combination provided outstanding detail and focus, and also kept the house warm as a bonus. The Pass-amps continue to be my reference amplifiers. I started to look into selecting speakers. A pair of Spendor BCIII speakers arrived. They provided excellent detail, imaging and clarity, but insufficient excitement or involvement. (They now reside in Brussels with my son.) Remembering the sound of the L300s (from a quarter of a century before), I tried to locate a pair in the UK, without any luck. JBL over here (outside of professional systems) are now best known for their in-car equipment. Then on holiday in France I happened across a French Hi-Fi magazine. In the small ads in the back pages there were several L300 ads, with prices at around 1500 a pair. After an incredible amount of arranging via telephone, fax, and my son in Brussels, I selected a Parisian dealer advertising a pair of JBL L300 Summits in perfect condition and with re-coned bass-units. (The dealer is best kept nameless in this charade). I sent off the check. It was cashed immediately. The speakers finally arrived just over three months later in a homemade packing case. Picture P1 shows how the speakers looked originally (this is from JBLs original advertising). This article describes what I did to try to recapture that sound of 25 years ago.

The Starting Point

To say that these speakers were not perfect is an understatement. The wood was scratched. What should have been tinted glass tops were cheap brown plastic. The wire connectors were split. The wheels were missing. However, this is all easy stuff to fix. Worse though was the complex front grille arrangement. One grille looked as if someone had stomped all over it. The repair made by the previous owner was to glue and staple the wooden frame together. Unfortunately the glue had dripped through into the cloth, staining the brown cloth a dirty white. Apart from that, on connecting the speakers they sounded just terrible, sharp, aggressive and with a massive, boomy and loose bass. I found that the level settings for the mid and high frequencies were both set full on. Even reducing them to center did not remove the edge from the sound. I did wonder about the hearing of the previous owner! The bass units had been reconed though.

I have been unable to track down this review. I should be grateful if anyone out there can point me at a copy of it.
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In comparison with my recent Spendors, the sound had taken a great leap backwards. The detail and depth had vanished, and the imaging was very poor. There was simply no comparison, even though they cost three times the price of the Spendors. This was clearly going to be a more extensive project than I had expected. My initial thought was to sell them, but I doubted if they were saleable (except perhaps to people like me!). So, I made a plan to try to get the speakers back to my remembered quality of a quarter of a century previously. It worked. Heres the plan.

The Plan

I decided to broadly separate the work into the Cosmetics and the Electricals. The Cosmetics would address everything to do with the appearance, and the Electricals would address everything to do with the sound-generating components. Before going too far, I wanted to research the speakers, in terms of components, cross-over design, cabinet design and so on. I soon found, via the Internet, like-minded individuals who were happy to provide good background information, and Ken Manson (who I contacted via a JBL enthusiast group), even sent me an original JBL color brochure of the speakers. The Audioheritage (www.audioheritage.org ) website was a goldmine of information about the speakers, and I managed to get a copy of the cross-over design from Wizards Electronics Repair Center for just $ 3.00! See Schematic 1. An exploratory opening up of the speakers was very reassuring. There were no screws into wood. It was all metal bolts into metal sockets. Moreover the design was beautifully modular. Access to all components is either by removing the bass unit or by removing the access panel at the rear. Apart from the level controls (which are bolted behind a JBL identification strip and which would be ruined by their removal), I can now strip one of these speakers in about 15 minutes, and without fear of damaging fittings. This kind of cheered me up after a pretty depressing start.

The Cosmetics

First to be fixed, due to the weight, was the wheels, or rather the absence of wheels. There were holes where they had been fitted, so I had to avoid these, as my castors had slightly larger base. I really could not handle spikes on 75 kgs. speakers, so wheels it had to be. The wheels helped a lot. Next, the cabinet was repaired with small amounts of plastic wood, and repeatedly oiled (Danish Oil). The result was a beautiful deep glow from the walnut veneer. Judicious use of matt black paint was used to repair the various minor scuffmarks. A visit to the glass specialist produced a perfectly fitting smoked glass top to replace the cheap plastic used by my Parisian friend. Much more difficult was the grille damage. The grille is very visible and any irregularity looks very bad. I decided that I had to rebuild the entire grille of one speaker. The grille is complex, some three inches deep, with a front and rear-frame (so as to clear the mid-range horn diffuser), see picture P2, and tapered front to back. The grille cloth is extremely stretchy and open-weave. I could not find, and still have not found, a source for this cloth, so I needed to be able to reuse anything I removed. The original grille frame was made from particleboard, which is fragile for frame purposes. I decided to use MDF instead, and used the frame of the good speaker as a model. I rebated a recess about 1 wide and 1/8 deep all around, so as to allow the cloth to be stapled in place. After the cloth had been removed from the damaged frame I attempted to reduce the staining from the dripped glue. I shifted most of it but not all. In the process however, the cloth shrank dramatically. It still fits the new grille, but the tension is too high for comfort and I continue to look for this quality of cloth as a back up. It has been in place for about three months so perhaps there is no cause for alarm. Fitting took a while, but the rebuilt grille is indistinguishable from the other and the work was certainly worthwhile.

By far the most critical part of the grill rebuild is the tapering from front to back. The tapering is controlled by the four corner pieces, which were undamaged and able to be reused. Without these the work would be quite complex. You can see a photo of the damaged frame on the floor outside of my garage in picture P3. It was totally useless. The tension of the cloth finally caused it to collapse after just a few days. Finally, I contacted JBL in the UK to get appropriate JBL badges for the speakers. I eventually reached one of their enthusiasts who remembered these speakers and promptly dispatched two large JBL badges. Now the speakers looked (more than) acceptable in a domestic environment. But they still sounded terrible.

The Electricals

I had several stages planned for the sound improvement. One was to bi-wire the speakers. I have used this in the past and found that on high power levels it makes a significant improvement to the sound. On high efficiency speakers like the Klipschorns or the L300s the improvement is unlikely to be much. However, since I had made a bi-wire cable (consisting of Monster cable for the bass and solid-silver for the rest) for the Spendors, I wanted to use it for the L300s too. The bi wiring, for compatibility reasons, involved the use of Neutrik Speakon 4-way connectors, to replace the damaged bare-wire connectors of the L300s. The internal wiring would also be upgraded, with heavy-duty silver-plated wire for the bass and a continuation of the solid silver for the mid and high units. Obviously all soldering had to be with silverloaded solder. Finally, I intended to replace the smaller electrolytics with Musicap equivalents and to bypass the larger electrolytics. This did not quite work out as planned.

Stripping the Speaker

To do this work, the speaker should be disconnected and positioned on its back, with connecting wires disconnected. The bass unit is removed first (and placed somewhere where it will not be trodden on). The bass unit should be held with 8 Allen bolts. I found that my speakers had just four loose bolts, possibly accounting for some of the boom. These bolts go into T-sockets behind the front-panel (which is not removable). The small rear panel must be removed for access to the mid-range unit. The rubber seal around this panel had dried out and needed to be replaced. I used a double synthetic-rubber seal in its place. Removing this panel exposes the rear of the mid-range unit and its connections. Note that there is a kind of wooden key fitted to this back-panel. It appears to offer some support for the mid-range unit. The result is that there is only one way of fitting the back-panel. The treble unit is held via 4 external bolts. If you undo all of them the unit will fall inside. As it is very heavy this is not a good idea. The best approach is to hold the back of the unit with one hand at the same time as undoing the bolts from the front. You have to find a way of getting your arm past the middlefrequency horn and up to the rear of the treble-unit. I never needed to remove the midrange horn, but I have long arms! Picture P4 shows the bass and treble units after they have been removed. The crossover is located on the floor of the speaker and is held in place by four bolts, the wires to the speaker inputs and the wire to the variable inductors which are mounted on the inside front panel. I never did remove these, because to do so would cause damage to the JBL model information transfer. Conveniently the cables connecting the variable inductors to the crossover are long enough (after their ties have been cut) to allow you to work on the crossover outside of the cabinet. You can see the general working arrangement in picture P3.

At this point I had a speaker cabinet containing just the mid-range unit and the inductors. The cross over can be held about a foot away from the cabinet. I found that the inner-floor of the speakers (both of them) had no absorbent covering. However, it is clear from the original JBL brochures that they should have had, so I purchased some wadding and added around 1 of wadding. I only did this after all the other work was finished, to keep out of the way of all the wiring. As I took apart the speaker I checked the connections against those documented in the crossover. They were identical. I mention this because sometimes manufacturers make undocumented alterations. However, here JBL had been completely accurate.

The Speaker Connectors

This part was the only disappointing part of the whole construction. The two spindly connectors were mounted on a fiberboard panel. The connections on the inside of the panel were made via two deteriorating spade connectors. The wire was of reasonable quality, but nothing special. The problem I had was that Speakon sockets need a thicker mount for their screw fittings. I decided to preserve the fiberboard, but to back it with MDF panel, drilled for the Speakon connectors. The technique was to cut the appropriate size hole in the fiberboard and to do the same in the MDF panel. Then I aligned them from the inside and used the now vacant holes in the fiberboard from the original connectors to screw the MDF panel to the fiberboard. The Speakon connector was kept in place as a guide for this tricky alignment operation.

General Re-wiring

I simply cut my silver wire to the same lengths as the original wires and replaced them one by one, preserving the color coding (as described in the cross-over diagram). However, although this was straightforward, you need to read about the crossover itself (below) before undertaking this task.

The Crossover.

Unlike previous crossovers on which I have worked, this was all wired with a point-to-point wiring. So, no printed-circuit board! The crossover was approximately 10 by 4 by 6. The top was a removable (4 screws) metal grille, mounted on 4 aluminum standoff posts. Immediately under the grille were the pointto-point wiring, capacitors and heavy-duty resistors. But not all components! It turned out that there was a further, lower, layer that contained all of the heavy-duty components like inductors and the larger capacitors. The wires for these were brought through the top layer at several points. Unfortunately the wires had been pulled through to keep them short, so there was no way of getting to them without cutting all of the wires. Worse (for me, but better for the sound), I could see through the gap that all of these heavy components had been potted in a heavy resin and were completely immovable. Obviously this will reduce the effect of vibration, but without destroying the unit, I had no way to remove these components, and I really wanted to reuse the original crossover as far as possible. (Postscript: I have since been told that gentle heating will allow this resin to be poured out, allowing access to the components. The resin can then be reused. I never tried this at the time). My upgrade plan was to replace all capacitors, and to use silver wire wherever possible. There was an intermediate stage, where I reused the existing large capacitors, but added high-speed bypass capacitors to them. I tested this on one speaker, but I found that the full capacitor replacement was superior (but not enormously so, so if you want to save money anywhere in this project, I would suggest adding bypasses to the larger capacitors, rather than replacing them, depending of course on their condition).

The smaller capacitors were all replaced with Musicap equivalents (see picture P5). This was very simple as they were all top mounted as you can see. The solution for the larger capacitors was not very elegant, but worked. I built three fiberboard panels of approximately 3 by 4, each holding modern, nonelectrolytic equivalent values, all with 5% tolerance. The 16.5mF capacitor I built from three paralleled capacitors. I found that by using very long side cutters, I could cut the wires to the large resin-bound capacitors just above their cases and tease these wires through to the top. I could then replace these wires with silver wiring directly to my fiberboard modules, which I mounted on the outside of the crossover box. You can see how this looks in picture P6. The Post-It notes on the wires are my notes about where each wire should go. I was also careful to preserve the orientation of my new capacitors with the originals, by keeping them parallel. The only remaining task was to split the crossover to allow the bi-wiring. At this point the number of wires hanging from the crossover was getting difficult to manage, and I did not want to hard-solder four more long wires from the unit through to the back of the loudspeaker cabinet, so I decided to use shorter wires from the back of the Speakon connector, now fitted to the back panel of the speaker, and to mount multiway connection posts directly onto the crossover board. There is plenty of space for this, and apart from drilling the board for these mounting points, you only need to trim the ventilation grille to allow the extra entry points for the cables. This gives a more flexible set up, at the expense of a couple of extra metal-to-metal connection points in the loudspeaker-amplifier pathway. There are many spare connection tags on the tag-strips, so separating out the bass-unit component of the crossover from the rest was quite straightforward. I found that the point-to-point wiring makes things harder to follow than with a printed circuit, so I labeled components and tag points with labels from the crossover schematic. I also took digital photographs to help when repeating the work on the second speaker.

Re-Assembly

Stage 1: Stage 2: Stage 3: Stage 4: Stage 5: Stage 6: Re-mount the crossover in the cabinet Connect the tweeter, before replacing it inside the cabinet. Put in the (missing) wadding on speaker floor, around the crossover Connect the mid-range unit. Replace the rear-access-panel, replacing the dried out seals as suggested above With the speaker on its back, re-connect the bass unit and drop / bolt it into the cabinet

Finally, set the speaker upright. This should get you to a working speaker, bi-wired, and with the latest capacitor and wiring technology. Picture P7 shows one of the finished speakers near the equipment rack.

Results

In the past when I have made successful equipment upgrades I have noticed that I listen more often, that I hear instruments and voices which were not previously obvious, and so on. Well, I heard all of this and more. Firstly, the big problem of edginess has completely gone. More detail is being revealed than ever before. The bass boom has also gone, although I would be hard-pushed to say which change actually did it. The integration of bass with the rest of the music is tighter and overall the system sounds faster and crisper. Compared with the Spendor BCIII, the imaging of the L300 is not so precise, but the excitement and involvement is much better. The bass is solid and detailed, but not intrusive at all. I frequently hear instruments of which was not previously aware playing away in the deep bass. Similarly on close harmony, the voices easily separate out instead of blurring together. I am suspicious that my son might read this, so I will not go on about the Spendors and their failings in case he wants an exchange.
I suppose the question arises as to whether I have achieved the sound I heard in Munich over 25 years ago. To be perfectly honest, I have no idea. But I am very happy with the sound that I have and I hope to stay with this particular set up for a very long time. This is probably the most significant upgrade I have made, excluding the Pass amps themselves. If you have the chance to work on a pair of these excellent loudspeakers I recommend that you try the above changes for yourself. If you want help or encouragement, contact me on george@oaklodgeconsulting.co.uk
This article was published in AudioXpress, a US-based magazine for audio enthusiasts who like to get their hands dirty.
The finished result! My Nelson Pass amp and Pre-amp are on the right, with a Sony SACD-1 player and (very narrow) a Technics tuner. Nowadays, these speakers are long gone to greener pastures. That is, I sold them! I have since performed a minor renovation on a pair of Yamaha NS1000M speakers and several major renovations on Quad ESL 57s. The differences in the sound are just amazing but thats another story.

About the Reviewer

George Brooke is a Technical Trainer / Consultant as well as being a PRINCE2 Approved Trainer. As Head of Software Development at Siemens Nixdorf he managed solutions development for the Financial, Retail, Hotel and Energy markets for UK/European departments and customers worldwide, from concept through to delivery and roll-out. He runs a training and consultancy company based in Cambridge UK focusing on OO design with UML, programming and implementation techniques and IT project management. Contact him at George Brooke