Amplifier Capacitance

Thank you for a marvelous reply! This explains quite a lot to me, especially using a combo of larger and smaller caps for nimbleness as well as keeping a large reserve. I forgot that I had seen this before and wondered why.

I still have some questions though. How does the amp draw power from the PS and caps? In other words, are the caps charged then remain as a reserve while the PS powers the amplification? Then the caps are simply drawn upon during the difficult moments that exceed the PS capabilities? OR is it something more like a cycle of the PS charging the caps and the caps supplying the power for amplification. As such, as power is pulled from the caps it is replaced by the PS and during the intense moments the amp can use as much power as is available in the caps while the PS recharges the caps? Or am I completely off?

I am also wondering why my idea for "bonzo" caps would be harmful for the PS, but I have an idea or two why this might be the case. I had assumed that extra large caps would allow a modest PS to deal nicely with heavy transients. OTOH, having expended that charge, this modest PS would have to recharge my "bonzo" caps AND continue to supply amplification power. This is why it would cause damage isn't it? It would essentially push the PS to it's limits far too often?

I'm an engineering newb (actually, newb would be generous) but I've been trying to learn more lately, so I apologize if some of these questions seem remarkably stupid. Of course, it is nice being 'remarkable'. ;)

Thanks, Aaron

Aewhistory, Look at this chart: http://www.powerint.com/en/community/papers-circuit-ideas-puzzlers/circuit-ideas/careful-rectifier-diode-choice-simplifies-and-

Top graph shows amp's supply voltage. Capacitor is charged from transformer thru rectifier only in short moments of time when voltage goes up (bottom graph is charging current). Picture is greatly exaggerated - in reality line representing voltage is almost straight (very small ripple) and capacitor charging happens in very short high current "spikes". Amplifier current demand from capacitors might be constant (class A) or vary a lot with the music (class AB). Very large electrolytic capacitors are characterized by capacitance (opposition to change in voltage), inductance (opposition to change in current) and ESR (effective series resistance) that represents pure resistance. Obviously we want a lot of capacitance to store energy but we don't want inductance since it is opposing rapid current changes. Best solution to lower inductance would be to use less inductive capacitors (expensive) or to use more of small capacitors in parallel (capacitance increases, inductance decreases, ESR decreases).

Bombaywalla mentioned two problems with a lot of capacitance - rush current and over-stressing power supply. Initial current will be higher and last longer to charge larger capacitance resulting in blown fuse or damaged rectifier. Rush current could be limited by soft start circuit - basically a temporary current limiter but amplifier has to be designed for that.
Second part is a little more difficult to explain. Imagine perfect capacitor with a lot of capacitance, no inductance and no ESR. What will be the shape of the voltage on the upper graph? - almost straight line with very, very small ripple. Charging time of capacitors will now be very short (only when voltage goes up) while charging current spikes will have higher amplitude (to deliver same average power) limited only by transformer and power line. This large current spikes might damage rectifier or overheat transformer. Again, it is a little more complicated with transformer since average amp's power is technically the same. The problem is that core of transformer will be heated with high frequency component (iron losses) of narrow spikes, while copper windings will be heated (copper losses) more since, in spite of the same average value, RMS value of current (representing heat) will be much higher.
There is also possibility that ripple current (charging current) peaks might now be too high for caps you selected. Anything can be done (carefully), but linear power supply is not that simple to design properly.

excellent reply Kijanki.

Aewhistory, if you read my reply & Kijanki's you should have all your questions answered. :-)

There is one missing element.

That is that the power supply has a timing constant. That is to say, there is a certain time period that will elapse if the power transformer is unplugged, where the voltage will sag to a certain point while the supply is under load.

Then there is a timing constant in the amplifier itself. This is the -3db point of the amplifier.

If the amplifier has a -3 db point that is a frequency lower than that of the timing constant of the power supply, then the amplifier can modulate the supply, which results in IM distortion amongst other things.

This is why an amplifier should never be direct-coupled from input to output! Otherwise, the amount of capacitance needed to get the timing constant of the supply low enough goes towards infinity.

This is why an amplifier can 'motorboat' (repeated thump) if a filter capacitor fails in the supply- the timing constant has become so high that the amplifier exhibits low frequency instability.

I apologize if I went a little too esoteric here, but there is obviously more to it than just inrush currents and the like. If something is not clear let me know.

This is more than I could have hoped for--in a good way--and I am extremely grateful! It will take me a little while to digest this, but there is a point mentioned by Atmasphere that I might be able to relate to. I recently bought an NAD 2600 with a disclosed possible flaw--a "thump" when it turns on and off. I got it for about $100, so it think it was a reasonable price. Amp plays great, but that on/off sound isn't so nice. So I had posted about this to confirm this was an issue and not just "the way this amp is" and someone confirmed this was a flaw. So my question is: is this the 'motorboat' thump to which you are referring Atmasphere, or is this a different problem?

It is nice to be able to start to diagnose these problems. This is one of the reasons I've started to try to learn about amps. I'll never be a deisgner by any stretch of the imagination, but for years I've enjoyed working with my hands building computers and doing odd repairs on things like motherboards, replacing caps, etc. So I can do it but there is a tremendous difference between "take out part, replace with same thing" and "take out part, replace with something different". At least to me it is way different. Anyway, I've got three amps that require attention that I may venture a repair on: two I bought that way for cheap, cheap (an Adcom GFA-5400 & an NAD-2600) and a Hafler DH220 that was bought working but damaged in shipping (I didn't actually want three projects, but owell, c'est la vie).

Okay, back to digesting this info.... Aaron