hum in audiostatic, depends on polarity mains
When I connected one of my audiostatics to the mains I heard a sudden hum I never heard before. The other esl was completely silent. I turned off the amplifier but the hum continued. Obviously it had to do with the esl itself, something in the high voltage generator. I disconnected the audiostatic from the mains and the hum immediately dissapeared. Then I put the audiostatic on again with reversed polarity of the mains; the hum dissapeared. Switching again from polarity the hum returned. I connected the esl in the polarity with no hum, enjoined the music.At the end of listening I changed polarity again but then no hum.
I haven't opened it yet.
What could have caused this strange behaviour?
Ps the highvoltage section of audiostatic (old model) consist of a simple ladder circuitry. There are no transformers at the input; the 220 volt is directly fed into the ladder circuitry instead. There is a resistor between the center tap of the audiotrannies and the 'zero' of the ladder circuitry.
The symptom of hum without audio input sure sounds like the result of leakage loading down the HV supply and increasing ripple.
I know you said you turned off the amplifier, but did you leave the speaker wires connected?
If so, does your amplifier use a safety ground, such that the negative speaker wire has a low resistance path to the mains ground?
Perhaps a leakage path had developed on the PC board somewhere between the speaker input traces and the HV supply.
With both speaker cables disconnected the problem still continues.
When time allows I will check out the HV circuitry. Maybe a diode failure?
Diode or capacitor failure in the multiplier chain could certainly cause an audible hum.
But, with the speaker cables disconnected, I can't think of any failure mode that would manifest with one polarity of mains connection and not the other.
A very odd problem you have....I am interested to learn what you find when you get a chance to check out the HV supply.
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