Hi all,
I was strolling thru my pics of my 604 tweeter repair I did a few years ago and noticed they have the loading caps installed, and in the last week I have been getting ready to setup my A7-500's and found in my research that sometimes the 802 and 808 drivers didn't ship with the phenolic loading caps, and some people have removed them for home use.
So my question is, does the same thinking apply to the 604s? Remove the loading cap and install felt in the back cover of the driver, for low power home use?
Anyone done it and can comment on any changes to the sound?
Cheers and happy new year.
The loading caps were there to limit the excursion of the HF diaphragm, so as to protect it from abuse, which was becoming more and more common at the time (and still is - people overdrive their sound systems because they are "into" ear-bleed causing sound levels). I take them out and replace them with a piece of thick felt to eliminate reflections off of the inside of the rear cover (fiberglas works well for this, too).
Todd W. White, Owner & Webmaster
Altec Lansing's (unofficial) Homepage
Dropbox
Loading Cap .pdf
A partly related question: I have seen loading caps both with rubber gasket and without (the former way more common). Since you always want to acoustically separate different drivers fom each other, especially bass from tweeter, I find a gasket a reasonable part here, either to the loading cap or to the rear cover.
I have never seen a gasket attached to the rear cover.
And if the loading cap is removed it feels like the risk for sound pressure in the box to affect the hf diaphgram is increased.
Anyone here that has some thought about this?
@quaramon
I don't believe that for regular home HiFi the pressure ( within a box ) is an issue ( perhaps it might be at H.T. levels ).
FWIW, the 900 series ( 902, 908, 909 ) drivers have a rubber boot that surrounds the ferrite magnet, which then terminates ( wraps over ) the first few mm of the top plate.
- The back cap is then bolted down on top of this rubber ( which then acts as a gasket, of course ).
Its clear to me that Altec was addressing the pressure issue ( for SR/PA work ) with this design.
Last edited by Earl K; January 21st, 2026 at 11:17 PM.
I believe you!Originally Posted by Earl K
You are right, now when you mention it I remember a pair of 902 I had with that rubber surround.FWIW, the 900 series (902, 908, 909) drivers have a rubber boot that surrounds the ferrite magnet, which then terminates ( wraps over ) the first few mm of the top plate.
- The back cap is then bolted down on to of this rubber ( which acts as a gasket of course ).
We'll see, I might put a thin rubber gasket under the rear cover when I put together my 620. Non destructive and feels good......
You will have to cut down the screw length. Or just get shorter SS screws at the HW store.
Typically 1/4” felt.
It would be interesting to do a DATS freq sweep of impedance with and without the loading cap. With and without the felt etc.
I did some of that but didn’t save the data. And I don’t have any loading caps.
Enjoying Altec Speakers since 1972
Ron,
JeffM has already done these studies.
Here are his pics showing the effects on impedance and freq. response ( Loading Cap; with = RED or without = BLUE, ).
Last edited by Earl K; January 23rd, 2026 at 12:17 PM.
Once I get setup again, I now have a "reference" condesor mic and can make the sweeps with REW. My previous mic was an AKG C414EB.
I am also going to pop off the loading caps on the 604 Big Reds. And to answer the question about pressure on the diaphragm without the cap, the back cover should preclude that occurance.
Cheers and thanks for all the input.
PS I'm still not getting email notifications of replies, not sure why. Todd?
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