In article , Clive Arthur
scribeth thus
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.

Unlikely but...

I once had to repair a small mixer - a simple noisy pot replacement was
needed - but I noticed a distinct buzz from the headphones. Taking
scope probe in hand I set about trying to find the fault, but it
disappeared whenever I tried to hook a probe on.

Long story short - it was a very old mixer using Germanium transistors,
at least one of which had some paint missing. Every time I bent over it
to hook a probe on I put the now light-sensitive transistor in shadow
from the fluorescent lights. Under normal use, with the covers in
place, this couldn't happen.

Cheers

Bin there seem that even in more recent Varactor controlled and
modulated VHF transmitters...

--
Tony Sayer


Man is least himself when he talks in his own person.

Give him a keyboard, and he will reveal himself.