On Mon, 15 Apr 2019 16:41:45 +0100, Dave Plowman (News) wrote:

Buzz is a symptom possibly caused by the thing oscillating flat
out
ultrasonically or in the low RF range... Bung a scope on the
output?

Try shorter or different speaker leads?

They are all of a foot long. ;-) Speakon 8 and car trailer multi core
cable.

Though ISTR this is an actve tri-amped speaker system?

Yup. And didn't buzz with the previous X/O which is a direct replacement
Exactly the same physical size, and even the mains in and XLRs in
roughly the same place.

Behringer are "budget" and make down to a price, corners get cut.
Most of the time they get away with the cut corners but not always.
It's the silly little things like decoupling capacitors and the like
that get cut.

But if you wind up the drive to the speakers, the buzz remains
constant.

Which indicates it's not incoming to that amp.

Quite. Although it does seem to be started by the input signal. Unplug
the XLR to the X/O input, no buzz - at least for as long as I've tried
it. With signal starts in a couple of minutes after power up. And
continues if you then unplug the input.

So following the parasitic oscilation theory. Once kicked off it
carries on.

Not too keen on breaking into a brand new unit to measure current

draw.

Fair enough, scope? AC Volt meter?

Just re-read the thread and it's the crossover you are changing, the
power amps remain the same. Do that amps get hot/draw more with the
buzz present, it could be their power supply(s) that are suffering
the strain rather than the one in the cross over.

I thought I'd seen it all given how many bits of gear I'd plugged up at
work over the years. ;-)

Parasitic oscillation in "pro" audio kit doesn't happen very often. I
think I've only encountered once, maybe twice in 40+ years. Normally
shows itself by a PPM being end stopped but nothing audible.

If I unplug the X/O from the amp with the buzz going, the buzz stops.
That's not to say it isn't some form of interaction.

If the crossover is hooting, unplugging it from the amps disconnects
that signal, the amps stop drawing current and the amp PSUs can
return to normal, sans buzz.

--
Cheers
Dave.