2. amps-preamps
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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
4,452 posts
 
Class AB - there is not fixed "time constant" since discharge time depends on loudness/load. We could take discharge time at max volume and compare it to reciprocal of -3dB bottom frequency of the amplifier but it wouldn't make difference to me since I don't listen at full volume. We could argue that smaller modulation at lower volumes will be "proportional" to level of the signal but once we draw less than max current, power supply voltage modulation at low frequencies becomes compensated by amplifier (since it is regulated).

Class A - does not apply since amplifier draws the same current even at DC output.
aewhistory OP
70 posts
 
I'm still working on some of this conversation, but could someone explain "time constant" to me? I'm assuming this is something like what it sounds, but I'm not sure. It sounds almost as if amplifier power supplies work like CPUs in the sense that they have clear cycles and clocks that designers must take into account to keep everything running. I don't know if the analogy holds, but it sounds similar to me.

I'm starting to realize just how much I DON'T know about power supplies. Wow. And I am the knowledgable one among my friends. Eeeeek!
kijanki's avatar
kijanki
4,452 posts
 
Aewhistory, Time constant is amount of time it takes to bring voltage to 63.2% of desired value. For instance applying 10 volts to 1000uF capacitor thru 10ohm resistor will result in 6.32V on capacitor after time equal 10ohm x 1000uF = 10ms. Same would apply to discharging from 10V to 3.68V (10V-6.32V). This time is called time constant RC.

Instead of saying "Time necessary to charge capacitor" we say "time constant". It is shorter and more precise. In our case it just means amount of time to have significantly lower supply voltage (discharge supply capacitors) when playing very low frequencies very loud. We want this voltage steady since any variations might compromise amp's operation (output affected by supply voltage changes).
We are looking at small changes in supply voltage (not 63.2%) but using terms like "time constant" or "-3dB frequency" just to have some reference point. From that we can, if necessary, recalculate exact percentage changes at particular frequency.
aewhistory OP
70 posts
 
Kijanki, thanks for that explanation, it was excellent. Do you teach electrical engineering by any chance? You, bombaywalla, and Atmasphere have done a magnificent job explaining very esoteric (to me) subjects. This is difficult to do, especially when speaking to a layman. I've taught college history now for 12 years and it can be daunting getting people to relate to human history, so I've always wondered how people in the sciences make their subjects more approachable and relatable. Discussing matters here certainly gives me an idea how this can be done as I've never encountered a group with the combination of technical savvy and possessing the ability/willingness to explain as here at Audiogon.
kijanki's avatar
kijanki
4,452 posts
 
Teaching? No, Almarg would be much better at this. He explains things with much more clarity. History was always difficult for me.
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