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).