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MJ Dijkstra 2nd October 2012 11:40 AM

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.

bolserst 2nd October 2012 01:38 PM

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.

MJ Dijkstra 11th October 2012 07:37 AM


With both speaker cables disconnected the problem still continues.
When time allows I will check out the HV circuitry. Maybe a diode failure?

bolserst 11th October 2012 08:20 PM

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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