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'(OT) Radio Noise in my mixer'
1999\01\18@190518 by Bruce Turrentine

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I have a Radio Shack  A/V mixer for editing videos. I am having a terrible
time trying to eliminate radio stations that somehow get picked up through
unit and cables, even though they are supposedly shielded. The microphone cord
and the position of my body relative to the mixer seem to have the most effect
on whether the interference is stronger or weaker. Does anybody have any ideas
on how to eliminate this noise? Maybe a cap at the inputs?
Thanks in advance,
               Bruce Turrentine

1999\01\18@192727 by ryan pogge

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face
or a line noise filter ?


>I have a Radio Shack  A/V mixer for editing videos. I am having a terrible
>time trying to eliminate radio stations that somehow get picked up through
>unit and cables, even though they are supposedly shielded. The microphone
cord
>and the position of my body relative to the mixer seem to have the most
effect
>on whether the interference is stronger or weaker. Does anybody have any
ideas
>on how to eliminate this noise? Maybe a cap at the inputs?
>Thanks in advance,
>                Bruce Turrentine

1999\01\19@062954 by Nigel Orr

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At 19:03 18/01/99 EST, you wrote:
>I have a Radio Shack  A/V mixer for editing videos. I am having a terrible
>time trying to eliminate radio stations that somehow get picked up through
>unit and cables, even though they are supposedly shielded. The microphone
cord
>and the position of my body relative to the mixer seem to have the most
effect
>on whether the interference is stronger or weaker. Does anybody have any
ideas
>on how to eliminate this noise? Maybe a cap at the inputs?

Are the inputs balanced? (is the connector a mono jack or a 3-pin XLR
connection)
If the latter, it might be a faulty cable, or a dry joint somewhere.
If the former, you could try any of the following without risk:

Clip-on ferrites on the cable
A cap across the input
Shorter cables
Lower impedance sources (ie a better microphone)
Check the grounding- does the mixer have an unconnected 'mixer ground'
terminal?

There are probably others, but that should get you started!

Nigel
--
Nigel Orr                  Research Associate   O   ______
       Underwater Acoustics Group,              o / o    \_/(
Dept of Electrical and Electronic Engineering     (_   <   _ (
    University of Newcastle Upon Tyne             \______/ \(

1999\01\19@082953 by mjurras

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Try 0.01uF caps on the A/V input and output pins to gnd. I would start
with the audio output pins. Sometimes the signals are picked up buy
the output amp and rectified thus demodulating the audio. I assume
that these are AM stations that are being recieved.

Let me know how you make out.

- -Mark

Date:
       Mon, 18 Jan 1999 19:03:40 EST
  From:
       Bruce Turrentine <spam_OUTTurrbieTakeThisOuTspamAOL.COM>
Subject:
       (OT) Radio Noise in my mixer


I have a Radio Shack  A/V mixer for editing videos. I am having a
terrible
time trying to eliminate radio stations that somehow get picked up
through
unit and cables, even though they are supposedly shielded. The
microphone cord
and the position of my body relative to the mixer seem to have the
most effect
on whether the interference is stronger or weaker. Does anybody have
any ideas
on how to eliminate this noise? Maybe a cap at the inputs?
Thanks in advance,
               Bruce Turrentine



_________________________________________________________
DO YOU YAHOO!?
Get your free @yahoo.com address at http://mail.yahoo.com

1999\01\19@095936 by Greg Cormier

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Hmm, I'm no smartie, but wouldn't a capacitor eliminate your voice as well,
as it's an AC type thing? So you'd just get a DC current, or silence. Maybe
one of the experts can comment on it...

-Greg

At 05:33 AM 01/19/99 , you wrote:
>I have a Radio Shack  A/V mixer for editing videos. I am having a terrible
>time trying to eliminate radio stations that somehow get picked up through
>unit and cables, even though they are supposedly shielded. The microphone
cord
>and the position of my body relative to the mixer seem to have the most
effect
>on whether the interference is stronger or weaker. Does anybody have any
ideas
>on how to eliminate this noise? Maybe a cap at the inputs?
>Thanks in advance,
>                Bruce Turrentine
>

-----------------------
Greg Cormier
Kathmandu, Nepal
Local time : GMT + 5h30m
.....gcormierKILLspamspam@spam@wlink.com.np
ICQ # : 565465

1999\01\19@112303 by Nigel Orr

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At 15:10 19/01/99 +0530, you wrote:
>Hmm, I'm no smartie, but wouldn't a capacitor eliminate your voice as well,
>as it's an AC type thing? So you'd just get a DC current, or silence. Maybe

If the capacitor is a suitable value, it should attenuate AM radio
(probably above 100kHz) a lot more than audio (under 20kHz).  The reactance
due to the capacitance is 1/(2*pi*f*C).

Nigel

1999\01\19@115255 by dave vanhorn

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>If the capacitor is a suitable value, it should attenuate AM radio
>(probably above 100kHz) a lot more than audio (under 20kHz).  The reactance
>due to the capacitance is 1/(2*pi*f*C).


.01uF should be ok, even on a high impedance line, but ferrites around the
line are even better, and no modification of the audio at all.

Basically, start with the amp connected to the speakers, and nothing else.
Get that to where you can max the gain on all inputs, and hear nothing.
Then, add inputs, one by one, fixing each as needed.

Look at your grounding as well, everything should bring ground back to ONE
point, where it connects with earth ground or electrical building ground.
If you have multiple connections to "ground" at different outlets, then you
have a ground loop, and RF current can circulate in it.  If you MUST
maintain multiple ground connections, then ferrite them all, except the one
at the amp. That way, they aren't RF grounds.  Note, this does NOT help you
with 60 Hz hum induced by ground loops. There, you have to choose between
NEC grounding, and signal grounding..

1999\01\19@115843 by Dennis Merrill

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It depends on the value you choose.  In the frequency domain, a cap looks like
1/(2*PI*f*C) which means for higher frequencies, the impedance is higher.
So, choosing a smaller value cap may provide a reasonably small impedance
at lower audio frequencies and a high enough attenuation at radio frequencies.

Plug .01uF into the equation. Use audio frequencies for f and then try
radio frequencies for f.  If this is a resonable value, use that, otherwise
try other values of C.

                                       - Dennis

At 03:10 PM 1/19/99 +0530, you wrote:
>Hmm, I'm no smartie, but wouldn't a capacitor eliminate your voice as well,
>as it's an AC type thing? So you'd just get a DC current, or silence. Maybe
>one of the experts can comment on it...
>

1999\01\19@145101 by Gerhard Fiedler

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At 16:21 01/19/99 +0000, Nigel Orr wrote:
>At 15:10 19/01/99 +0530, you wrote:
>>Hmm, I'm no smartie, but wouldn't a capacitor eliminate your voice as well,
>>as it's an AC type thing? So you'd just get a DC current, or silence. Maybe
>
>If the capacitor is a suitable value, it should attenuate AM radio
>(probably above 100kHz) a lot more than audio (under 20kHz).  The reactance
>due to the capacitance is 1/(2*pi*f*C).

true, but in my experience there's something =wrong= when you get radio on
your audio equipment, and the caps would rather work on the symptoms than
fixing what's wrong. properly shielded, proper ground connections, the
right impedances -- and no radio. (of course what's wrong might be in the
circuit of the mixer, or any other connected equipment.)

try finding out where exactly the radio comes from, maybe by disconnecting
everything and then connecting back one piece at a time, from the
speaker/amp downwards. and since you always connect a number of parts at
the same time (an input, a cable and an output of a new piece of
equipment), when you found the part that's causing trouble, switch it
around a bit: use a different cable, put the newly connected equipment on a
different input, put another equipment on the same cable and the same
input. this then should give you a clue where exactly the problem comes
from. then you take it from there. (how exactly depends on what you find out.)

ge

1999\01\19@225030 by Thomas McGahee

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Is the case plastic? A metal case properly grounded acts like a
Faraday Cage and prevents interference from getting in. A
plastic case just lets the RF right on through.

They do make a metallic spray that you can spray all over the inside of
a plastic case, but I do not have a source to give you. I have had
good luck in the past removing all components and spraying the
inside of the case with adhesive, then applying aluminum foil
everywhere. Make sure you have a ground connection to the foil.
By that I mean make sure that the foil ultimately gets connected
to the safety ground of an AC outlet. Without the solid ground
the aluminum foil will only act like an antenna and actually
increase the RF problem.

If you want to quickly determine if this method will help
in your particular case, do the following: Set up your
mixer and then wrap the whole thing in a plastic bag
such as a garbage bag. Then wrap aluminum foil around
the entire assembly (except the cables, of course),
and connect the aluminum foil to a good solid external
ground, such as the safety ground on an AC receptacle.
If the problem disappears, then a Faraday Cage is
indeed the answer to your problems.

Hope this helps.
Fr. Tom McGahee

----------
{Quote hidden}

1999\01\20@075506 by paulb

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Dennis Merrill wrote:

> It depends on the value you choose.  In the frequency domain, a cap
> looks like 1/(2*PI*f*C) which means for higher frequencies, the
> impedance is higher.  So, choosing a smaller value cap may provide a
> reasonably small impedance at lower audio frequencies and a high
> enough attenuation at radio frequencies.

> Plug .01uF into the equation.

 I'd start with a tenth of that.  1nF.

 Note one reason why a capacitor or inductor is effective.  You may be
working with audio levels of a few millivolts or tens of millivolts.
In order for the RF to be detected, it has to achieve a level which
causes rectification (distortion); generally a few hundred millivolts.

 If you get the level below this, even if it is still comparable to the
audio level, the breakthrough will stop.
--
 Cheers,
       Paul B.

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