View Full Version : Loading cap removal

08-26-2006, 04:44 PM
Can someone tell me the why's and wherefores of removing the loading caps and installing felt pads on the Altec 902-8b HF drivers?

louped garouv
08-29-2006, 08:40 AM
generally speaking (regarding ALtec drivers), removing the loading cap, replacing with shorter screws, adding felts -- is used to get higher frequency extension and supposed smother response curve, at the expense of power handling capacity.... hth david

Steve Schell
08-29-2006, 11:29 AM
As I understand it, loading caps have been used to help control diaphragm excursions with high power inputs at lower frequencies. A diaphragm's suspension, especially if made from aluminum, can work harden then fail after many large excursions. The loading cap presents an acoustic load to the diaphragm that helps to limit these excursions and extend diaphragm life.

To the best of my knowledge, the loading cap first appeared in 1945 in the high frequency section of the 604 Duplex, really an Altec 802. I'm not sure whether the standalone 802 driver of the time used the loading cap. This was the first appearance of the tangential compliance in Altec drivers, and it allowed for greater low frequency excursions. It is probably not coincidental that the loading cap appeared at the same time. Since then, Altec most often fitted the loading cap to drivers intended for high level use, and omitted it and substituted the felt in the rear cover in drivers intended for consumer use. The felt pad in the rear cover dates back to the original small format driver, the Lansing 801 field coil driver of 1937, and is present in most of these early drivers.

When using any of these drivers for typical hi fi use with adequate protection from the crossover, I feel that the loading cap is unnecessary and that the drivers sound better without it. I have also found that the drivers sound better still, with greater clarity and less confusion in the sound, with the back cover removed entirely. This is a subjective thing, and somewhat controversial, and measurements won't show any difference with the cover on or off. This can only be done when the driver is not located in a bass cabinet, and is in a situation where kids or cats won't have access to the naked diaphragm!

Some people think that the volume of air trapped by the back cover is a designed part of the driver, like the sealed rear chamber used for reactance annulling in a bass horn. I don't think this is the case in most drivers though. Early examples of the Western Electric 555W vented the rear of the diaphragm through a mesh screen, and Wente's patent on the original compression driver with circumferential phasing plug, as used in the Fletcher System, indicated screened vents in the rear cover. Lansing's first compression drivers for the Shearer Horn System were based on Wente's experimental driver, and his rear covers had a series of vent holes around the perimeter. My guess is that the pioneers eventually found that sealing the rear covers eliminated a source of contamination to the driver and made no difference in the response curves. Still, I have found many times that removing the cover eliminates a subtle layer of sonic hash from the driver's sound. YMMV of course.

louped garouv
08-29-2006, 11:34 AM
and now we know.....

The Rest of the Story

I love your posts Steve, thanks for sharing the knowledge....

08-29-2006, 11:40 AM
May we have a pic of a "loading cap," please? This is something more than the normal rear cap, no?

louped garouv
08-29-2006, 11:47 AM
i dont have a photo, but they are little black plactic discs that are mounted to the diaphragm ring of the altec drivers, inside the normal back cover that would have the model sticker on it.......

**edit: what is strange to me is that the spec sheet for the 808-8B in the library does not show a pic of the loading cap, but in the few pairs that I have seen, they always had loading caps and symbiotic diaphragms, not the plain AL ones that appear to be in the spec sheet's pictures....

if you remove the loading cap, put the felt in, and change the diaphragms to AL, you have essentially changed the driver to 802G spec from 808B

08-29-2006, 10:00 PM
May we have a pic of a "loading cap," please? This is something more than the normal rear cap, no?

Image courtesy GPA

(Note; non-ferrous mtg. screws need to be shorter if disposing of cap)

08-29-2006, 10:15 PM
While I'm clear on the cap's purpose, I'm not convinced of the oft-repeated effects. Soon as I can clear a couple of projects, I'll post some measured comparisons.

While running without either the loading cap or the rear cover may not show differences on FR, I suspect it will on sens/output.

Though likely not the intent to reactance-annull, it CAN be acheived, though not with either standard config (on 511 or 811). It ain't too far off with the cap.

It's all about Vb.

Steve Schell
08-29-2006, 10:43 PM
Moldyoldy, please do publish the results if you do make some measurements. The only ones I have done with rear cover on/off were some on axis frequency reponse sweeps using LMS. I would run a sweep with the cover on, remove the cover and run a second sweep. LMS makes it easy to overlay the plots. I could find no differences greater than might occur when running two sweeps under identical conditions.

Perhaps an impulse measurement would be more revealing; this is only a guess. My theory has been that a variety of reflections and standing waves could occur inside the cover which would be in the driver's pass band and which could reflect back through the diaphragm into the horn. Anyway, I have convinced myself on a few occasions that the sound was a bit more clear and less harsh with the cover off.

I have used one compression driver where I believe the designer's intent was to put the enclosed rear volume to work. Jonas Renkus designed the Emilar EC-600 driver in about 1980. It uses a 6" (!) aluminum diaphragm and has a 3" exit. It was intended for high level voice announce and midrange duty. The diaphragm clamping ring is secured by a cylinder cast into the inside of the rear cover, and the trapped air volume is very small. I experimented with using the EC-600 on a long 60Hz. bass horn, and the output below 100Hz. was very disappointing until I removed the rear cover. The bass response increased dramatically, but then there was nothing to hold the diaphragm to the top plate!

Steve Gonzales
08-29-2006, 11:31 PM
Ahhh.... Another dose of the good stuff! .You always have such interesting information to share. Thanks Bud.

08-30-2006, 09:46 PM
Steve definately gets our attention with his vast knowledge of lansing history. He should write a few books.

08-30-2006, 10:44 PM
Steve definately gets our attention with his vast knowledge of lansing history. He should write a few books.

What a splendid idea!

There's probably a least a book's worth of great info in his accumulated posts.

Makes good reading.

Steve Schell
08-31-2006, 12:37 AM
Thanks for the kind words guys. Not sure if I'll ever be organized enough to write a book, but I have mightily enjoyed learning about audio history over the past decade and giving a little recognition to some of the Great Ones that preceded us.

One observation that keeps hitting me on the head over and over again as I continue to learn is how very much Jim Lansing accomplished in his relatively short (24 year) career in the loudspeaker business. His work resulted in the mainstays of the Altec and JBL product lines for decades after his death, and the designs continue to be echoed in factories all around the world.

Glen, you have added a great deal with your serial number study, and your analysis of the N-1000 network. Here is another example of Lansing's broad knowledge and creativity. He was a working locomotive all through his career, and never more than when he was trying to get JBL off the ground in the late 1940s. I have done quite a bit of speaker work the past few years, and it seems to take me forever to get anything finished. Then I look at the never ending stream of surviving late '40s JBL pieces that keep turning up, much of it from Jim's hand. He built and finished dozens of cabinets, designed and built hundreds of drivers and networks, and on and on- all in three years! Holy cow, he got a lot done. As a friend of mine who has also studied Lansing's work said "He worked quickly and tirelessly."

John Edwards told us that he kept a motel room rented for Jim around the corner from the the Fletcher Drive factory during the week, but most often he wouldn't use it, instead working long into the night and perhaps catching a few winks on a sofa or shop table. More of John's recollections of Jim Lansing are located here:


Gary L
08-31-2006, 07:32 AM
It will be very interesting to see the test results when you get a chance to do them.
I have a set of 806 drivers that have both loading caps and rear covers but no felt pads inside the covers. I am sure I could barrow the pads from other drivers and do some experimenting but the test equipment will be a much better analysis of this.

The more I use the speakers with the 806/loading caps and rear covers the more I feel the sound is too busy as compared to others with just rear covers and felt pads. Never tried any of my drivers with caps and covers completely removed but this could also be interesting if not too dangerous. I never know when you guys mention these ideas just how much power I can be safe with in the home setting so I fear doing many of these experiments. Burning up diaphrams is not my idea of fun but I would like to try some of these ideas to see if I can even hear a difference.


02-17-2011, 09:08 PM
I just replaced two 6 uF loading caps on 902 drivers that had indifferent output. The one cap I measured read .016 uF on my Fluke. With the loading caps removed, the drivers now sound very good.


[I have been reading for a long time--thanks, guys.
This is the first time I had something helpful to add].