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RE: Where does the current come from?

All good points...

1. "...large value caps as the last element do not give me a sound I would call musical."

I believe that's very possibly a result of hysteresis, a common characteristic of electrolytic capacitors. Regardless of the specific cause, there's a straightforward test to determine whether the last capacitor is performing correctly. Simply sweep the amp over frequency at a reasonable power level, say 1/3 - 1/2 its rated power, and monitor the last filter cap with a high sensitivity oscilloscope. Any significant audio energy present at that cap represents imperfect bypassing, leaving open the possibility of coloration. Conversely (and this is the point I really want to stress), if the audio frequency voltage across that cap is zero (or in practice, only a very tiny fraction of the voltage swing at the anode of the output stage), the cap will not color the sound. That's true even if it's 50 years old, exhibits terrible leakage, ESR and hysteresis, and has only 0.1 uF of capacitance remaining. If no audio signal appears across the last filter cap, it is physically impossible for that cap to be the cause of amplifier coloration.

2. The change of inductance over frequency (for all the reasons you mentioned) is understood. The important question is whether sufficient rejection is maintained. I believe this isn't generally a problem, if for no other reason than the decreasing XC of the last filter cap with increasing frequency. If the overall effectiveness is in doubt, one solution would be the addition of a smaller value inductor in series with the primary choke. Adding a small value "HF grade" cap in parallel with the last filter cap might also be beneficial.





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