When is used used up?

When its FRIED!!

I have owned many old mac amps, and when I acquire them, I send them to Mac for general servicing. I have NEVER had them do anymore than replace a burned out lamp. Ever! And I am talking about amps and preamps that were 15 to 30plus years old. Each one met spec. The same is true when I bought my harman kardon Citation 17 preamp and 16A amplifier. I sent them to hk on Long Island and they sent them back with a data sheet. They far exceeded factory specifications. It erks me a little when someone says 'oh I replaced the caps in this or that and now the unit sounds awesome' (caps are so easy to change and this seems to be the rage today) Capacitors do NOT DEGRADE over time, they FAIL. Which means if your amp is playing, your caps are fine.

So, I think that if a unit was manufactured using quality parts, you should be ok, at least that has been my experience.
Norman

"Capacitors do NOT DEGRADE over time, they FAIL."

- I don't know where you got this from, but even capacitor manufacturers admit that electrolytic capacitors dry out (and rate them). At room temperature they will last for 30-50 years but temperature accelerates process greatly (each 10degC cuts life by 50%). ESR of capacitor increases over time and, in presence of big currents, causes internal self heating (just few degrees) and so forth.

Power supply capacitor is in series with a speaker (circuit closes thru power supply) and any increase in ESR will show as worse bass control (lower DF) and loss of dynamics. We get slowly used to "new sound" but difference after replacement can be huge.

There are electrolytic caps that are better quality, lower ESR, lower inductance (like slit foil caps) and higher temperature ratings but they are expensive. Better (high end)manufacturers most likely use better parts.

"Power supply capacitor is in series with a speaker (circuit closes thru power supply) and any increase in ESR will show as worse bass control (lower DF) and loss of dynamics."

I would love to read the white paper on this.. if you have one please let me know. Yes, caps can dry out and when they do the short or fail. Until the point of failure, it is my understanding that they are performing/suppling necessary and proper capacitance and therefore should have no adverse effect on the circuit.

Norman

Norman - Not likely to find White Papers on basic stuff like that.

Amp's output current flows from power supply capacitor thru it's Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL) then thru output transistors and speaker to come back to power supply capacitor.

Amplifier attempts to regulate/mantain required voltage level but power supply cap's ESL will make it less than perfect at higher frequencies while it's ESR if high enough will cause high voltage drops and eventually amp will run short of voltage.

ESR of capacitor is constantly increasing because of drying effect up to point that after long time self heating (big current x ESR) will cause thermal runaway effect (ESR increases with temperature) and capacitor might even explode. To prevent catastrophic explosion all larger electrolytic caps have built-in fuse (rubber plug).

Quality of sound depends on quality of power supply caps (ESL, ESR, leakage etc). Best caps have extremely low ESR and ESL. Sometimes people bypass electrolytic caps with much lower inductance film caps (to speed-up response) but this creates parallel resonance circuit with ESL (and therefore ringing).