Computer feeds into stage PA query

Computers are notorious for making noise in sound systems, usually it is in fact the SMPS, both in the PC and/or the monitor.

As it happens, we had a guy come in just last week to the club I work in, to record the event on a laptop. I almost knew there would be noise problems and indeed there was, a broad spectrum of hums and buzzes. We have a stereo transformer isolation box for just such occasions, and inserting this inline with the audio signal cables cured the noise completely, and a clean recording resulted to boot. It is not a fault with the mixer. (You must isolate the earths and the signals, no direct connection to the PC at all).

Gareth.

Gareth.

Yes, it`s power supply noise from the pc, feeding the signal via a decent transformered di box sometimes works - it`s often a case of suck it and see.

Ron(UK)

As others have said, it's most likely electrical noise from the Mac's power supply finding its way into the audio path, either via ground or directly from the audio signal. Some of the G5 PowerMacs (and maybe other models) are a bit notorious for power supply noise issues in cases like this, and simple isolation may not entirely solve the problem as in some cases I believe the noise is present in the analog audio output signal itself.

In addition to trying various isolation measures--always a good idea--it might be worth looking into using a professional-quality add-on sound card or interface, or connecting the computer through its optical digital audio output (if it has one built-in) and an external A/D converter. The latter option has an advantage in allowing complete electrical isolation.

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Anyone know what the bare minimum is for trying out just one channel ground isolation via a transformer, 1:1 pulse transformer perhaps ?. Not sure I could lay my hands on one of those mu-metal cased audio single/balanced transformers and I seem to remember they are quite expensive.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I'll go along with that. I use my workshop computer as a source of music for testing repaired amps, and at first I had all manner of problems. Investigation revealed that the two outputs from the soundcard have significant, and different, DC offsets on them. I eventually overcame this, and made the whole system more versatile into the bargain, by flinging together an interface box, using transformer isolation. This has the additional advantage that by wiring the tranformers in the appropriate way, you also get a balanced output for testing balanced inputs.

Arfa

Choice of a few at your local Maplin store, none too expensive.

Arfa

I was confusing with these sort of microphone transformers

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

or if you want to build some yourself,

Ron