Computer feeds into stage PA query

[N_Cook]

Someone wanting a large mixer amp repairing, not seen yet , as left the
owners do do a bit more exploration, as on phone prompting, the problem only
occurs with a feed from a MAC based system, not when instrument feeds only.

Reported as a whirring , booming noise, but not mains hum, at bass
frequencies but noticeable at all levels
Told them to borrow someone elses pc, or transfer to CD/tape etc to rule in
or out , firstly. Assuming a perfectly good pc system and a properly working
mixer/pa, does this sort of intrusion sound familiar to those familiar with
PC feeds, perhaps beats between SMPS oscillator and mains, badly mismatched
impedances on feed hookup, ground loop/lift problem, just a few thoughts.

 

[N_Cook]

thoughts.

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)

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.

 

[Arfa Daily]

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.

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

 

[Arfa Daily]

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.

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

http://www.maplin.co.uk/Search.aspx?criteria=audio isolation transformer&source=15&SD=Y

Arfa

 

[N_Cook]

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

http://www.maplin.co.uk/Search.aspx?criteria=audio isolation transformer
&source=15&SD=Y

Arfa

I was confusing with these sort of microphone transformers
http://www.partridgeelectronics.co.uk/TR050.jpg

 

[Arfa Daily]

I was confusing with these sort of microphone transformers
http://www.partridgeelectronics.co.uk/TR050.jpg

I guess if it's a theatre or studio or something, and this turned out to be
the solution, a 'proper' transformer might be justified, but for repair and
basic test purposes, any little cheapo signal tranny will do. I can't
remember exactly which ones I used in my test box. They were either from
Maplin or Farnell and almost certainly 600 ohm 1 : 1+1. Obviously, in a lot
of cases, this represents a poor match to the input impedance of the amp
being tested, and I'm sure that if you started measuring the frequency
response of the amp that the box is feeding, this mismatch would be able to
be seen, but subjectively, *I* can't hear any difference by the box being in
line, or not.

It's a very useful piece of kit in my workshop, used pretty much every day.
I put two adjustable 'ranges' on it. One that goes from zero to a few mV to
represent guitar pickup and mic output levels, and one that goes up to a
couple of volts for 'line' levels. It has a switch to 'mono' the two
channels. I put loop-'thru on it and also it has a battery powered op-amp
based level booster, output socket switched for power in the same way that
many external effects are, to give an output of 8v or so p-p for testing the
output power of slave heads that need to see this sort of drive level from
desks or slaving outputs on other amps. I think I've got the schematic for
it scratched out on the back of a fag packet somewhere, if you're
interested.

Arfa