Electronics funny.

[The Natural Philosopher]

No Dave, I never read your posts.

You've been killfiled for years

[Fredxx]

I also think this is an earthing issue, exacerbated by the new unit's switch mode power supply.

Hence my suggestion in an earlier post to use cable ferrite chokes that help to eliminate HF earth and common mode issues.

However the behaviour of connecting an input that causes a 'buzz' that remains after disconnecting the input does sound more sinister, such as an instability. I looked up Behringer and their equipment gets mixed reviews as well as being the wrong end of a few legal issues with pirating designs.

[tony sayer]

On another group a poster there says he's used lots of these units without any buzz or hum problems at all..

[tony sayer]

See the other post to Dave L re this,

[Dave Plowman (News)]

Absolutly not.

But if it were a problem with the amp input side, why was the analogue X/O clean?

[Jac Brown]

Can't be that given that two identical copies have the buzz problem.

[Jac Brown]

Very likely.

The hash from some switch mode power supply somewhere.

[Dave Plowman (News)]

Found some Tufnol lying around, so made the new brackets. Absolutely no difference. Seems it was the position between the two units causing the buzz - not connection to the stand. Dropping the X/O out of its mounting on to the top of the amp - only a couple of inches - sorted the buzz. Not easy to reposition either of them.

So in desperation, removed the mains earth from the X/O. Result, clean as a whistle with it back in its original position with the ally brackets too.

Looked inside. Mains earth is a short length of wire from the IEC inlet to the chassis. Signal ground is the PCB bolted to the chassis. (I'd already tried disconnecting the XLR screens without success.)

Amp is much the same. IEC in and IEC unswitched out are grounded to the same point on the chassis. Signal grounds to a different point on the chassis.

Ground loops (to me) are usually a hum rather than buzz, so still don't know what is going on.

And still don't know why the old X/O which has the same grounding arrangements was OK. As I said, the only real difference is the old had an analogue PS, while the new a SMPS.

I'm happy to leave things with no mains earth to the X/O, and rely on the XLR screens for a safety ground. It's not going to be unplugged etc when in use.

[Bill]

In message [e-mail]>, "Dave Plowman (News)" [e-mail]> writes

Based on my experience with SMPS powered laptops in a pro situation, this always sounded to me like shash of one sort or another from the power supply, possibly induced into an earth loop.

It ought really to be queried with Behringer.

[tony sayer]

In article [e-mail]>, Dave Plowman (News) [e-mail]> scribeth thus

Dave, this just shouldn't be a problem with pro equipment least pro in this sense being balanced in's and i presume out?.

We have a lot of pro audio gear over time and never have we had this happen and that does inc some equipment from Behringer.

You shouldn't have to rely on the XLR screens as a safety ground their not there for that the main safety earth is, least thats how it works and has worked for many years in my past experience.

The metal chassis should be earthed and for safety reasons via the mains provided earth the XLR cable screens should be connected to that as well internally, Unless its a big mixing desk studio installation where a separate "tech" earth is provided or called for to ground audio cables and that will be to reduce the possibility of picked up noise.

I reckon it way well be that this unit is duff somewhere, it was a new ones was it not and not a second-hand one where someone might have been at the insides perhaps?...

[Clive Arthur]

<snip>

I posted here a while back about a two core (ie unearthed) mains IEC C13 cable which I threw away. I found another one yesterday.

Cheers

[Dave Plowman (News)]

Indeed. Hence me posting about it. ;-)

Same here. When I worked for Thames, we'd buy the latest semi-pro gizmo from the liked of Behringer just to assess it. Mainly outboards for use in the music recording studio. Things like digital reverbs started out in that sort of market before being available from the more respected pro sources. And they all simply plugged in and worked.

Oh, indeed. But without knowing what else to try, I'm going to keep it as is. The fuse in the Behringer is 1 amp, and the XLR screens and connectors will handle that as a fault current quite happily. I'd not do it where others were likely to have to fiddle with it.

Thing is, it was the same with two units... But not the older Behringer X/O. Which is what confused me, and the reason for this thread.

[Fredxx]

Which is what I expected for the reasons stated, hence the suggestion of placing a ferrite in the mains cable.

Running without an earth is not ideal. Perhaps use a low current RCD might mitigate the danger.

[Bill]

In message [e-mail]>, "Dave Plowman (News)" [e-mail]> writes

At the risk of sounding nannying, it should at least be reported to Behringer. If two of these are faulty, it points at least to a bad batch.

I'm sure it is safe in your use, but other, less experienced, people will presumably buy these and who knows what earths will be left off to cure the buzz.

[Fredxx]

More likely a design flaw.

We don't how the earth is connected internally. The earth inside could be exceptionally quiet and the buzz caused from associated equipment and it's associated earthing.

Earthing arrangement can be a nightmare with no obvious solution and the inappropriate apportioning of blame.

I hate the idea of the removal of an earth. It reminds me of days when some would leave the earth off an oscilloscope to improve noise performance or to look/compare at signals at raised voltages.

[Bill]

In message <q9cs8g$fbf$ [e-mail], Fredxx [e-mail] writes

Very early in my "career" I was called on to make sure the Scottish Variety Orchestra was "safe". The guitarist's amp had no earth, no grommet where the remaining two cores went through the metalwork and the outer sleeve was cut through. I was the only engineer on site. We were not allowed to fix problems in musician's equipment and it was before the days of safety isolating transformers. The whole orchestra waited while he went home to fetch another amp. I got a huge bollocking because of the cost, and just one small message of praise and support.

[Dave Plowman (News)]

I can't be certain the Behringer unit is faulty. The amplifiers are home made, and it could be some interaction I've not fathomed out.

It's not a common problem with this unit. Others seem to have found it fine.

[Dave Plowman (News)]

Indeed - my problem. I do know how the earths are connected. In both the X/O and amp they go straight to the chassis via just about the shortest route.

Tell me about it. ;-)

Yup.

[Dave Plowman (News)]

Many years ago, a friend bought a reasonably expensive Pioneer separates Hi-Fi system. Turntable, amp, tuner, cassette deck. With all the mains going to the same 4 way trailing socket and all the phonos connected, it hummed. making up phono leads with no screen connection at one end - and trying every combination of connected and non connected ones didn't work. Got rid of the hum, but ended up with buzz. Only answer was to remove all the mains grounds except the amp one.

Quad at that time only grounded the pre-amp. Everything else had two core mains leads. Using 110v mains plugs. ;-)

[tony sayer]

In article [e-mail]>, Bill [e-mail] scribeth thus

Yes quite a few accounts over the years of the lead singer/guitarist writhing around when he grabbed the mic stand...

Other had wrapped around his fretboard..