about the voltage doubler : i measured frequency at pin 7 of LT1054 : 21,25 kHz. Is that a "low" frequency? I've read 1054 has a nominal frequency of 25 kHz.
That looks OK to me certainly within the board specs of the datasheet.
The low frequency units are around 5kHz and 10kHz, which is within the audio band and can be heard as a whine o whistle.
The reason why the LT1054 is better for audio is that 25kHz is above audio.
*However* with an analog delay the analog delay chips switch the waveform internally and they rely to some extent on the power supple being clean. So what I think is happening in your case is that process is demodulating the power supply ripple down into the audio band. It's basically the same processes used to demodulate AM radio signals into the audio band.
I'll make some internet search because I really need to understand how voltage doubler works. I must admit that i don't understand what's going on here. More specifically : how the "switching thing" works?
Normally the chips produce a negative voltage but with the added diodes you also get doubling.
From this circuit,
http://moosapotamus.net/files/ADA-MN3007-rev5-Jan2010-Documentation-rev20120225.pdfSteps:
1) pin 2 is switched to ground, cap C39 gets charged to +9V (less diode and switch drop) through diode D8.
2) pin 2 is switched to +9V, that makes the cap (C39) voltage add to the +9V rail. The voltage on the positive
side of cap C39 is now at 9V + 9V (less drops) which is a bit less than 18V. At the same time
D8 becomes reversed bias and not long has an effect, and D11 becomes forward biased and the
18V (less diode and switch drops) at the positive terminal of C39 gets dumped into C40 via D11.
So C40 gets charge to just under 18V. The diode drops and switch drops at each of the steps
drops the voltage a bit from the full doubled voltage.
While in step 1, the charge in C40 holds the output at 18V. The load will cause discharge C40 and that causes
ripple on the output voltage. This is exactly the same way you get ripple on a rectified and filtered DC supply
which uses a transformer + diodes + filters caps.