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.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

"N_Cook" wrote in message
...
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.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/





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.

N_Cook wrote:
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.

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)

In article ,
"N_Cook" wrote:

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.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

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.

--
Andrew Erickson

"He is no fool who gives what he cannot keep to gain that which he cannot
lose." -- Jim Elliot

Ron(UK) wrote in message
...
N_Cook wrote:
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.

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.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

"Gareth Magennis" wrote in message
...

"N_Cook" wrote in message
...
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.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/





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

"N_Cook" wrote in message
...
Ron(UK) wrote in message
...
N_Cook wrote:
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.

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.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/



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

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

Arfa

Arfa Daily wrote in message
...

"N_Cook" wrote in message
...
Ron(UK) wrote in message
...
N_Cook wrote:

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


http://www.maplin.co.uk/Search.aspx?...0transform er
&source=15&SD=Y

Arfa



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


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N_Cook wrote:
Ron(UK) wrote in message
...
N_Cook wrote:
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.
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.



http://www.behringer.com/DI400P/index.cfm?lang=ENG

or if you want to build some yourself,

http://www.maplin.co.uk/Module.aspx?...rmers&doy=17m6

Ron

"N_Cook" wrote in message
...
Arfa Daily wrote in message
...

"N_Cook" wrote in message
...
Ron(UK) wrote in message
...
N_Cook wrote:

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


http://www.maplin.co.uk/Search.aspx?...0transform er
&source=15&SD=Y

Arfa



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


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/


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