Hum on mixer...

  • Thread starter Thread starter WykedPT
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rob aylestone said:
I suppose we've lost the old system where chassis ground and electrical ground may or may not be connected to the audio ground. Tony Waldron, the designer of many house price Cadac audio mixers was frequently at odds with audio device manufacturers who routinely connected the audio ground inside a box to the chassis and electrical ground creating so many ground paths that fault finding was a total pain - and the faults were often one device where noise currents got bonded to the audio ground, and the piece of kit performed flawlessly, until another piece of kit with the same system was connected. Both devices were silent, but the combination produced severe noise elsewhere, with that bit of perfect equipment getting the blame!

It can be interesting if you have time to meter between a single piece of kit's chassis, power ground and audio ground on the connectors. Some have no connection others are joined all toget
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"It can be interesting if you have time to meter between a single piece of kit's chassis, power ground and audio ground on the connectors. Some have no connection others are joined all together." Some circuits Rob take audio/supply ground to chassis via back to back diodes, 100R and 100nF all in parallel although such a technique might be peculiar to valve guitar amps? And in that case the jacks almost always have the sleeves insulated from chassis unlike the majority of mixers say which have metal jacks bolted to front and back panels.

However the internal signal and power grounds are arranged it is important that the incoming mains earth goes directly to chassis secured by a nut,bolt and star washer used for NO other purpose, e.g. securing a transformer. Worth a varder to check with S/H or very old equipment.

I understand that the AES recommendation for XLR inputs is that pin one be taken solidly to chassis by the shortest possible route?
Fine, even with phantom power to microphones but there is always the possibility of a mic input being connected to something else these days? Mind you, isolating DC-DC converters are readily available these days and pretty cheap.

There are some rules and best practices for avoiding/fixing ground loops but every situation is its own little puzzle and some 'cut&try' is often required!

Dave.
 
Thanks Rob.. All the I/O connections between my mixer and ADAT are 1/4 inch. I'm not having any problems with mics. It's the connections between mixer and ADAT. Like I stated above I get no hum between the outs to the ADAT. The hum occurs coming from the outs on the ADAT to the tape returns on the mixer. I'm leaning towards a bad ground in the outs of the ADAT. Haven't been able to get to a tech as I have been snowed in. Also as above, if I put a ground to no ground adaptor on the power cable the hum goes away...Kind of strange.
 
Sorry - my fault for digressing into grounding issues in general. In your example, it's quite possible that the adat is not faulty at all, and while connecting it produces a hum - the problem could be the recorder is just exposing a fault elsewhere. Ground loops, for example, rely on there being two or more ground circuits - the direct one via the cable and another, possibly by the mains wiring or crazy paths like remote controls, or midi cable that kind of thing. I used to have an xlr M to F and a 1/4" M to F with no ground connected at all. They got used loads of times. Curing the hum - but often they should not have worked, but somehow, they did, using grounds accidentally created somewhere else.
 
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