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Amplifier Capacitance

Okay, naive question of the day: except for cost and perhaps size limitations inside the casing, why don't manufacturers go bonzo large on capacitance? For instance, I'm thinking of replacing the caps in this damaged Hafler DH220 I've got and, while I probably won't find any that fit, I started to wonder why limited myself to two 18-19k cans? Why not 50k, 80k, even 100k if I could fit it? (I doubt any of those would, but you get the drift.).

Now, in an exisiny design there is the voltage rating to concern yourself with, I believe. So this limits your choices. But in a newly developed amp the designer has more leeway, correct? So again, why not go bonzo? Capacitance seems like once of those things that should really make a big diffence in amp performance, no? In fact, shouldn't it particularly help offset a somewhat weaker power supply as well?

Okay, my naïveté is showing, better zip up....
aewhistory
kijanki's avatar
kijanki
3,575 posts
 
why not increase the voltage rating, to improve the longevity, ESR, and inductance characteristics instead?

The price of higher voltage, low ESR, low inductance caps is perhaps too high. I checked once site that sells Hypex class D kits. Power supply module was by far the most expensive because of BHC slit foil low inductance electrolytic caps. People try to remedy inductance of large caps by placing small non-inductive caps in parallel. It lowers caps reactance at high frequencies but also creates parallel resonance circuit that will ring under rapid current draw. There is a reason design engineer avoided it.
atmasphere's avatar
atmasphere
7,068 posts
 
Bombaywalla, sorry for my tardy response; obviously you got it right.
atmasphere's avatar
atmasphere
7,068 posts
 
Kijanki, I thought you might find this interesting. Years ago I worked on a popular tube preamp. I found that it was a copy of the Marantz 7 circuit, sans tone controls.

This preamp tended to have a problem- which was if you played a bassy track too loud, it could thump and do some weird stuff- especially if you were able to watch the woofer cones.

Anyway, it turned out that the timing constants of the preamp section went lower than that of its power supply. I spoke to the designer (a Brit) who was not at all happy to hear of my diagnosis, despite admitting to me seconds before that the bass instability was a problem.

The take-away is this is not an issue limited to amplifiers...
kijanki's avatar
kijanki
3,575 posts
 
Ralph, that's very interesting. I just looked at Jeff Rowland Capri preamp - the upper limit is 350kHz but bottom is only 10Hz. It could be direct coupled with servo (integrator) in the feedback set to 10Hz. In any case, he did it for a reason. My small Rowland 102, as well as big 625, have both 5Hz bottom -3dB but all his preamps have bottom at 10Hz. He also uses SMPS in preamps. I know he does in Capri, not sure about others. He might feel that limiting rumble or power supply effects should be done as early as possible. Additional 5Hz bandwidth in the amp improves slope, adding second pole, without affecting much overall 10Hz bottom limit.
atmasphere's avatar
atmasphere
7,068 posts
 
That's interesting. I would have thought the Roland stuff to go lower. We set our poles at 2 Hz so there would be no phase shift at 20Hz. We use servos too- in fact its part of our patent. It was a trick making them be stable with cutoff frequencies that low. One thing is sure- you can't use a servo to define the LF -3db point (the result is that every component in the servo circuit will have an audible artifact, and that is something a servo should never do).

If the rumble is going to be there, whether you have 2 Hz or 5 Hz cutoff is not a lot of difference as long as you have good LF stability. If there is instability rumble will drive you crazy!
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