Amplifier Capacitance

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

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 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?

the turn on/off thump in your NAD amplifier is a totally different issue compared to what Atmasphere is talking about.
The thump that you are hearing during on/off is the inrush current charging the capacitors. this (huge) inrush current will create a spike on the supply rails of the power amp which in turn will create a voltage spike on the power amp output terminals (speaker binding posts). This in turn will create a thump in your speakers. The issue with the NAD is that it never had any output protection circuitry to avoid 'thumping' the speakers (if the NAD had output protection circuitry that was malfunctioning, that amp would not play any music). This is indeed a design flaw. Back in the 1980s, I & some extended family members did own integrated amps that always thumped the speakers. I guess back then output protection circuits were not always put into cheaper amplifiers - just the way it was back then!
The thump that Atmasphere is talking about is when a power supply cap fails (bad part/age/heat, etc) & the power supply is now faulty. Then, the power amp can motorboat/thump which is a sign of low freq instability.
Two different issues but quite similar symptoms.
The thing to note about the motorboating is that the amp will thump every time it's playing low freq content.
The turn on/off thump occurs just once at turn on & once at turn off - no thumping while playing program material.

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.

Ralph, let me see if understand what you wrote above. I'm getting confused because w.r.t. the amplifier you are using frequency & w.r.t. the power supply you are using timing constant. Freq & timing constant are reciprocals of each other.

So, what you are saying is that for motorboating to occur the time constant of the amplifier has to be higher ("If the amplifier has a -3 db point that is a frequency lower than...") than the time constant of the power supply. Correct?
So, basically, the bandwidth of the power supply should always be lower than the bandwidth of the power amp. Correct?