Normally one can tell its the phone. Its a very obvious sound.
Also some electrolytic capacitors need a normal capacitor across them to
act correctly I used to find if driving sensitive electronics.
I had an old Sinclair like power amp that was a wonderful radar pulse
receiver and needed this mod to fix it but it was not hand sensitive it was
just wide open from dc to light kind of thing.
It might be interesting to drive it into power resistive loads and see if
you can measure the hum on any or all of the outputs. You could do that now
but its very fiddly. It might at least localise it, but if it is drawing
extra current when it hums I still think of parasitic oscillations issues.
You can easily blow up tweeters this way if you get a feedback loop going!
Brian

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"Graham." wrote in message
...
Three way speaker driven by three power amplifiers all balanced input.
Crossover Behringer CX3400 analogue type which sounded not bad at all.

Wanting to experiment further, got a Behringer DCX2496 which is a direct
substitute, but digital, so allows a choice of crossover curves, delays to
each unit, and EQ too.

First impression, setting it to the same as the analogue one, was no
audible difference, which is good.

But slowly, when left on, it developed a buzz from the speaker(s) Loud
enough to be annoying.

Approach the speaker and the buzz drops in level and get close enough and
it goes. So you can play it with a hand rather like a Theramin. ;-)

Thinking it faulty, got another. Same thing. Swapped back to the analogue
one - fine.

Mains grounds and XLR grounds all check out OK.

Difficult to tell if the buzz is from all three speaker units, as when you
get your ear close enough to each one, it stops.

If you power up with no signal, no buzz (at least for over an hour). With
signal, it starts after a few minutes.

Any educated guesses? I've never come across anything like this before.

Have you tried turning off your phone, hearing aid, cardiac pacemaker,
(in that order)


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Graham.

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