There is an old rule in DIY electronics. Your wonderful device works fine
when its a spiders web of bits joined by wire on a workbench, but as soon as
you put it into a case things start to not work quite right.

At the old TV company I worked at they designed a little rf transformer
wound on a bit of ferrite bead. It did what it was supposed to on paper and
on the breadboard version of the circuit, However when fitted on the pcb and
the case put over it, a pattern came on the screen. Eventually I happened to
put my finger on the transformer when it was open while testing it and up
came the patterning. It soon transpired that a stage was very slightly
unstable and the capacitance of the case or of a finger moved the frequency
to a resonant one and off it went.
Mind you I have a bass bin here which picked up Radio China International
till .1 uf capacitors were fitted to all the psu lines inside the cabinet.
Its totally unscreened inside and hence all sorts of crap gets in, still
some problems from switch clicks.
Brian

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In article ,
Bob Minchin wrote:
On 13/04/2019 13:00, Dave Plowman (News) wrote:
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.

Dave, try turning off all the lights and see if it improves.
I once had a job when the lights were modulating some of the
semiconductors particularly glass envelope diodes and putting 100Hz side
bands on the signals passing through (100Hz as this is the rate old
style mains lamps turn on and off when driven with 50Hz).

Both amp and crossover are fully enclosed in ally cases. Putting a hand
near then from any direction gets rid of the buzz. I'm thinking to do with
body capacitance. Somehow.

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*Geeks shall inherit the earth *

Dave Plowman London SW
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