[sdiy] Shielding

[harry]

Hey burnin' Man...
I'm sending this on the list also... It seems that this is something LOTS
of folk
will have a problem with. Even if your noise is different its worth a shot
!

Moho Disco wrote:

> Harry,

yeah ???

> I've done these things to get rid of the noise:

Take this slow... WHAT noise.  What kind of noise is it... white noise,
buzz,
hum, static, garbled shit ???  This is important. Looking for a little
noise is better if
you find out where the noise is... (and not bomb the whole fvcking country
to get rid of it... apolitical humor there ;^)

I'm really NOT being a wise at ss (well I am... but with good intentions ;^)

> * Tried a brand new power supply (same ilk)

OK... then the noise is noy in the power supply... OR it is and both
supplies have the
same effect...

> * Moved power supply away from cables

A good move. In my studio... AC cables and speaker leads are on the
floor... and
all audio wiring is suspended on hooks from the ceiling !

> * Used balanced/shielded cables from my mixer to my monitors

Audio monitors (speakers) I presume... not Video right ? (videos can be
real bastards with 15KHz and VERY fast rise times, as well as hell of
magnetic radiation... I had a lot of trouble with those...

Another very good move...

> * Plugged it into an outlet separate from the one my audio gear is on

A good idea. I actually (when mixing my CD) ran the power off the
refrigerator
power supply... it was more quiet than the other one with everyones light
dimmers
on it... Crazy but effective

> * Used it on a power conditioner

Good idea, but not likely to help against most noise....

> Still the problem persists.
>
> So what can I do now?

Define the noise as well as you can. Can you SEE the noise with a scope ?
does it ride the audio signal  ? Can you define the frequency of the noise
or
is it random...

> Try the "metal box around
> the line lump" thing?

I'd wait on that. You have more homework first.

I had a really bad noise problem in my apartment... it stopped the final
mix of
my CD for TWO WEEKS while I tried to kill it.  It turns out it was a radio
signal...
I suspect from someone in the building with remote speakers (carrier
current on the AC line).  It got into my mixer following the SHIELD of the
OUTPUT cables... even
if those were SHORTED tip to sleeve !!!  The solution traced to the inside
of the mixer, where the wires ran for three inches before they hit the PCB
(where the real ground was).  I put ferrite beads on the wires (inside) to
help stop radiation... and used the balanced output, as a single ended
output (to get 2X the signal) then ran that
to the tape daek with a 10:1 resistive divider... and ran the signals hot
hot HOT !!!
the large signals help improve the signal to noise. The noise was constant
but I made the signal 10X larger... then attenuated the output. So the
signal gets to the deck at unity gain, but the noise is reduced by a
factor of 10.  This got me to where the noise
became inaudible with headphones, loud !!! Good enough.

Here is the classic EMI story.  I have a transmitter (in someone else's
place... maybe
a RADIO station ?), a conductive means (the building wiring as an antenna,
braodcasting to the mixer via RF) and a receiver (the mixer itself, due to
shitty design.) BTW it was a Tascam Stage Mixer... presumably never
intended for recording !!!

I could not kill the tramnsmitter, or break the path (shielding would be
impossible) so I modified the receiver to kill the noise.

You have to do the same... but first you need to find everything you can
about the
noise.

Remember the classic scene in "Raiders of the Lost Ark" where Indiana
Jones and
his Egyptian friend do a little dance "They're dig..ging in the
wrong..place"
(why was I the ONLY one in the theatre who busted out laughing... was the
crowd
THAT slow ???).   Don't dig in the wrong place... find the noise...
eliminate it !
(or make it promise not to be too obnoxious, like my example...)

H^) harry

>
>
> Lava
>
> At 02:27 PM 11/29/2001 -0800, you wrote:
> >Hi Lava
> >
> >The rule for EMI is you have a source, a receiver, and
> >a medium for transmission.
> >
> >step one is to identify those three items. What makes
> >the noise, what recieves the noise and how does it get
> >from point 'A' to point 'B'.
> >
> >Then you see what you do about it. You might shield
> >the source. You might interrupt the medium... you
> >might shield the receiver.
> >
> >Some noise rides on the cables and power lines...
> >usually things like line filters or ferrites can help
> >there.
> >
> >Some noise goes through the air... in that case you
> >might want to use a sealed metal box with no open
> >holes of cracks. The wires would be run through
> >filtered connectors.
> >
> >Another Idea is to re-orient the transmitter or
> >receiver to get rid of the problem.
> >
> >If it is switching noise that is the problem, you
> >might
> >make a linear supply... it will be BIG and HEAVY.
> >
> >Also watch out for the backlight on the display...
> >some of the fluorescent lamps can emit RFI also.
> >
> >H^) harry
> >
> >
> >
> >
> >--- Moho Disco <moho at mohodisco.com> wrote:
> > > How does one go about shielding something?
> > > Say, a line lump laptop power supply?
> > >
> > > Thanks,
> > >
> > > Lava
> > >
> > >
> > >
> >
> >
> >__________________________________________________
> >Do You Yahoo!?
> >Yahoo! GeoCities - quick and easy web site hosting, just $8.95/month.
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