Weird sounds on power-up

[Daikini]

My stereomour made some noise on startup when it was stock, just until the tubes heated and the LED's in the C4S lit up, and  then it was quiet.

I've been upgrading the power supply components a piece at a time, and the power-up sounds changed subtly upon replacement of the resistors with chokes.

When I replaced the 100 uF electrolytic caps with film caps (33 uF Claritycap TCs), it started making a strange sound.  It's a fast, low "whup whup whup whup," kind of like a helicopter noise.  I can hear it with the sub off, but with the sub on, there's a lot of port noise, and the cone is visibly travelling quite a bit.  I wouldn't say it's very loud, but it ain't quiet either.  It doesn't start immediately, and it lasts a few seconds, and then stops abruptly when the LEDs light, and then after that the amp is as quiet as ever.  Music sounds great, more clarity than with the electrolytic.

Does anyone have any idea what this is?  Should I be concerned about it damaging the speakers?  Is there a fix for it?

Thanks!

[Paul Joppa]

Not positive, but I think you are getting a relaxation oscillation during the time when the 2A3 is conducting but the driver is not. The 2A3 grid is then coupled to the power supply through the C4S and coupling cap, and the ringing of the power supply components provides the necessary resonance. A larger capacitor or a smaller choke will reduce the ringing. Or of course a resistor, as originally designed. Then, when the driver conducts, the C4S is no longer saturated and isolates the 2A3 grid from the power supply rail.

This is possibly a good time to point out that amps are more than the sum of the parts, so changing (upgrading?) a part may have unintended consequences.

[Daikini]

I think I have some 22 uF Solens with the proper voltage rating that I can parallel with the 33 uFs to increase the capacitance, if I can find a place to put them.  It's getting pretty crowded in there! 

I'm using Triad C7X chokes.  When you say smaller choke, you mean less henries, right?

Other than making a weird noise at startup, this doesn't seem to negatively affect performance once the driver is conducting.  What are the symptoms of a ringing power supply and how might it negatively affect the performance of the amp?

[Paul Joppa]

... What are the symptoms of a ringing power supply and how might it negatively affect the performance of the amp?

Good question, much "debated" on the forums - especially DIY Tubes at the Audio Asylum. You'd have to wade through quite a bit of acrimony to get the few nuggets of opinion, not to mention the very very few actual bits of data. Waste of time, IMHO. I don't recall any agreement on exactly what it is, or whether it was negative, positive, or neutral. I don't recall ever seeing anything relevant to parallel feed amps, either. The only useful discussion was on how to use PSUD-II to identify aspects of power supply transient response.

[Daikini]

The extra 22 uF stopped the sound.

I see what you mean about the threads on this topic on said forum. People really get heated about this stuff. I'm not quite to the point that I'm ready to design power supplies from scratch, but it seems there's a lot more to it than I had considered.  More than one way to get the desired voltage, each with its own compromises.

In any case, I don't think I can stuff any more chokes or giant caps under the hood of my stereomour, and it's sounding very nice. Time to stop messing with it and just listen