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Wiring question on power cord replacement

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Anonymous
October 27, 2004 5:22:09 PM

Archived from groups: rec.audio.tech (More info?)

I’m replacing the unshielded power cord on a microphone
preamp. Besides having an insulated ground wire there is
a raw wire lead. That raw lead is attached to the plug
ground, of course, but in addition to the foil wire wrap and
in turn to the stranded wire shielding. What is the purpose
of the raw wire lead and what should be done with it? I was
under the impression that for maximum RF/EMF rejection
the coax shield/foil only drained into the common ground of
the plug, not to the equipment itself.
Anonymous
October 28, 2004 7:22:43 AM

Archived from groups: rec.audio.tech (More info?)

Powell wrote:

> I’m replacing the unshielded power cord on a microphone
> preamp. Besides having an insulated ground wire there is
> a raw wire lead. That raw lead is attached to the plug
> ground, of course, but in addition to the foil wire wrap and
> in turn to the stranded wire shielding. What is the purpose
> of the raw wire lead and what should be done with it? I was
> under the impression that for maximum RF/EMF rejection
> the coax shield/foil only drained into the common ground of
> the plug, not to the equipment itself.

Do you mean " I’m replacing the *shielded* power cord on a microphone
preamp " ?

What you refer to as the 'raw wire' is likely what is often termed the
'drain wire' that makes continuity with a foil or similar shield.

It should indeed be connected to ground.

RF shielding, to be truly effective, requires it to be connected at both
ends.

I've never come across anything like that. Sound kinda overkill but
whatever.


Graham
Anonymous
October 29, 2004 12:44:30 AM

Archived from groups: rec.audio.tech (More info?)

Powell wrote:

> I’m replacing the unshielded power cord on a microphone
> preamp.

I read you to effect that you replace an unshielded cord with a shielded
cord of some kind.

> Besides having an insulated ground wire there is
> a raw wire lead. That raw lead is attached to the plug
> ground, of course, but in addition to the foil wire wrap and
> in turn to the stranded wire shielding. What is the purpose
> of the raw wire lead and what should be done with it?

It would be absolutely wonderful with such a question to include make
and model of replacement cord. However - very generally speaking - the
raw wire lead appears to be intended to connect the shield to ground.

> I was under the impression that for maximum RF/EMF rejection
> the coax shield/foil only drained into the common ground of
> the plug, not to the equipment itself.

Very generally again, ie. trying to apply general signal grounding terms
to this situation, yes - that wire should probably if I understood this
right be connected to the ground terminal of the "mains end" plug.

You appear to want to prevent RF from radiating into the short piece of
mains cabling connecting the preamp to the mains supply.

You would be much better off examining cost efficient methods of
suppressing the RF that comes with the mains supply for all kinds of
reasons, defective electric motors is a likely main culprit. This should
primarily be done in the powersupply of the device in question, and it
is almost inconceivably that it isn't done.

It is a fair amount of years ago now that I made a listening test of
additional RF filtering on the mains supply to my stereo, and concluded
that additional - not very costly - filtering was cost efficient, and
overvoltage suppresion would be nice to have just for a cosy safe
feeling.

Leaving the sonic effects, if any, out of it, the deployment of those
filters - one pr. group of like devices - has been cost efficient
because I have avoided equipment damage during three known mains
over-voltage events, one a thunderstorm that killed a clock-radio and
two power outages, one that killed a drying helmet at a nearby
hairdresser and an ISDN installation in a nearby apartment and one (city
wide) that killed harddisk and powersupply of one of the computers that
I tend to end up supporting.

In short: using a moderate sum on additional mains RF filtering for that
preamp may, just may, be worthwhile and relevant. It will in fact
address the issue you are concerned about and just possibly improve
sound quality by some minute increment. It will also protect you
somewhat from mains carried RF noise caused by light dimmers in stage
lighting setups, such noise may, may mind you, cause a deterioration of
the sound if, if mind you, the power supply design of the preamp is less
than perfect, even without actually being audible as a cause of buzz.
You do need a very high quality of mic signal and of recording equipment
for this to be something to worry about, but it may, may mind you, be
worthwhile to be able to say that even such interfererence has been
ruled out.

Generally speaking: if either mains filtering or shielded powercords or
both matter clearly and decisively in terms of obtainable sonic purity,
then it might be said that the mic pre is either inadequately designed
or plain defective. I don't want to rule some kind of minute effect out,
I just want to make you understand that it is about minute effects. It
is my policy not to worry about such things during a recording, I prefer
to keep the setup as simple and "few boxed" as possible.

Worry about optimizing mic placement during recording instead .... if
you have money to spare, then get better transducers first and next a
better AD converter and/or recording device if thereby implied, you
don't say what mic pre this is about, but it may have excellent mileage
in the quality race as it is.

Please notice the disclaimatory wordings used above, it may be a very
good idea to consult a qualified electrician so as to ensure that the
wiring of the power cord is actually safe. An incorrectly wired power
cord could kill you or be a cause of fire or major equipment damage or
"all of the above". Take care!



Kind regards

Peter Larsen

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* My site is at: http://www.muyiovatki.dk *
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