By the way the buzz gradually fades up to a peak then stays there, I'm guessing a cap is charging up somewhere.
Don't want to mess with the Behringer since it's new, but am going to try all the options for screen grounding within the amp. At the moment, the screens go to amp ground on each card. Not connected to ground at the XLR. I'll try running a ground from the star ground point to the screen ground at the XLR. The XLR body is already grounded via the chassis.
a rule of professional electronics is "First - check the power supply"
You might need a 'scope to do this properly - correct volts is necessarily the answer.
Progress.
Ran a ground inside the amp from the star ground point to the screen pin on the XLR7. Buzz now constant - and makes no difference if you get close or touch anything.
X/0 was bolted to the speaker stand. Stand made of ally Speedframe, and the joints (corners etc) are plastic covered so may well not make a good electrical connection. Amp is just sitting on crossbars and has rubber feet, so insulated.
Unbolting the X/O and just leaving it lying on top of the amp, clean.
Insulating the mounting between X/O and stand is going to need thought, though. At the moment it's just a basic rack mount.
If anodized, or even just the oxide layer may make for poor electrical contact
Have you considered getting some clamp-on ferrites to remove common mode signals? If there are earth currents flowing it can only help.
They are best for high frequencies, but the fact you hear a buzz rather than hum I'm wondering if suppression within the switch mode power supply is placing hf currents into the earth path.
In article snipped-for-privacy@davenoise.co.uk>, Dave Plowman (News) snipped-for-privacy@davenoise.co.uk> scribeth thus
What happens when you just switch the unit on and have Nothing connected to the XLR input?
What happens if you put either a short or a low value resistor say 100 ohms across each balanced input separate ones on each input mind.
Could be. I've used rivnuts into the speedframe. But still not sure why it would need a good ground connection to the stand?
Wonder why walking up close to it makes the buzz go away. Presumably that is entirely mechanical, for some reason your ears don?t hear the buzz when they are in that location relative to the speakers.
That's what I'd be looking for. Monitor the cuurent draw, does it follow the buzz level? The buzz is a symptom of the PSU running out of steam, do the driver chips/transistors get warm/hot?
As it takes a while to appear and does so slowly I'd guess that it's only just unstable and requires a signal to kick it off.
as I said - check the power supply.
More likely to just be some mechanical quirk which sees the buzz more audible when the ears are further from the speakers.
bad decoupling cap feeding high impedance source that picks up hum from the body once the cap has stopped being a cap.
The buzz is well below peak programme level. On a PPM. I'd guess at it peaking 2 with the actual mod peaking 6. (At the levels I'm mostly driving everything at) So it is sort of masked by anything continuously loud. But very obvious on quiet bits. But if you wind up the drive to the speakers, the buzz remains constant.
Not too keen on breaking into a brand new unit to measure current draw.
Did you actually read my post? The buzz stops as you approach the device. But perhaps your body is out of phase. ;-)
Been doing some checks. Each section of the stand (made from Speedframe) does seem to be electrically insulated from others - due to the plastic sleeves in the joints. Even although the joints are made of steel.
So what we effectively had was two bits of approx 6" long 1" square tube bolted to the mounting brackets of the X/O, and making a reasonable electrical connection. Forming if you want an 'H' shape. Wonder if this was acting like an aerial? Picking up gawd knows what, of course.
oscillation of
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? Though ISTR this is an actve tri-amped speaker system?
Which indicates it's not incoming to that amp.
Fair enough, scope? AC Volt meter? With no programme there shouldn't be any "signal" from, DC to light, across the speakers. You should be able to measure the buzz when present, if it happens with the meter connected but fairly small not amp supply rails that I'd expect with the thing "hooting". B-)
They are all of a foot long. ;-) Speakon 8 and car trailer multi core cable.
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.
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.
I thought I'd seen it all given how many bits of gear I'd plugged up at work over the years. ;-)
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.
I used to have a Behringer analogue mixer which had poor RF protection on the mic inputs. Had to add ferrite beads and a few pFs. So perhaps that's a design feature of Behringer kit.
In article snipped-for-privacy@davenoise.co.uk>, Dave Plowman (News) snipped-for-privacy@davenoise.co.uk> scribeth thus
Now thats is odd., Assuming it used the THAT and SMM series of balanced line receivers they should only see the input differential voltage unless that differential system isn't working i.e. the common mode side is up s**te creek.
Electronic balance is fine but it must be isolated from its supply limes and other odd references around.
Maybe one input side is getting connected to the metal earthwork?.
Bring it up here Dave got the equipment to suss that one out;!.
output?
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.
So following the parasitic oscilation theory. Once kicked off it carries on.
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.
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 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.
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