Author Topic: picking up radio signal with mixer  (Read 1305 times)

travis

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picking up radio signal with mixer
« on: November 19, 2011, 06:38:39 PM »
I have a peavey PV6 mixer that picks up radio signals (and some other noise) for some reason.  I can just power it on and plug headphones in and there it is! :o  It changes a little when I move the mixer or the power supply around.  My guess is that it is something with the power cable /psu but I am not sure.  Any ideas on where to start?
"some dive into the sea, some toil upon the stone"


sodderboy

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Re: picking up radio signal with mixer
« Reply #1 on: November 20, 2011, 09:04:03 AM »
Does it have RF in different outlets and electrical circuits?  Is there RF on only the mix buss and/or external inputs?  All channel faders down?  Only the headphone amp?  RF is almost always improper shielding with wiring or chassis.  If you have a DMM check for continuity, like less that 1/4 ohms between the console metal and PSU chassis and the AC ground at the wall.  They should all connect.
And is everything stock?  Cause if something has been modded you look at the mods first.
Mike

abbey road d enfer

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Re: picking up radio signal with mixer
« Reply #2 on: November 20, 2011, 11:15:26 AM »
Most MI mixers do not have significant RFI protection; JR could shed more light on this.
You need to try something: put 2x 0.01uF caps on one input, one between pin 2 and 1, the other between 3 and 1. You dont have to do it on the mixer, you can do it on the plug. If it solves the problem, you know what you have to do...
I had to do that a number of times in the past.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

JohnRoberts

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Re: picking up radio signal with mixer
« Reply #3 on: November 20, 2011, 11:59:55 AM »
The PV6 was designed after I left the (peavey) reservation so I have no first hand knowledge, while live SR mixers are often required to work in close proximity to wireless microphones and the like so not immune to RF issues. 

First step is some basic troubleshooting like Mike suggested.

Over several decades of working with mixers large and small I've seen exactly one (small) mixer where an internal bus sum amp rectified RF, while the vast majority of RF issues are associated with mic preamps due to the high gain and therefore high slew rate required to amplify raw RF signals without slew limiting.

Sometimes the RF sensitivity is evidence of some broken circuit that is oscillating and detecting radio stations, or perhaps the mixer is in an unusually high RF field. Could be something as simple as an open solder connection preventing some passive filter from doing it's job.

If the RF is in an input stage, you should be able to troubleshoot that with mute/fader. Sometimes the RF can come into a final output and be rectified by an active output stage.

You need to try to isolate where the rectification is occurring.

JR
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travis

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Re: picking up radio signal with mixer
« Reply #4 on: November 21, 2011, 02:45:26 PM »
Wow thank you for the great responses!   

It picks up RF in outlets all over my apartment.  It is a very old building and it might be a good idea to test it elsewhere (although this is where I most need it). 

This mixer has 2 stereo channels and 2 mono channels with no mods.  With all channels down it is silent.  With gain and volume cranked on either mono channels there is some white noise.  With gain and volume cranked on either stereo channel there is noise and RF.  This is all with nothing plugged into the inputs or outputs besides headphones (tried with several pairs) getting the same results from all outputs.

I do have a meter but I am not sure how to access the psu chassis as it is encased in plastic with no screws.  I assume it must pop open somehow?  when I check the mixer case to ground it is .5 ohms.
 
(I will try the .01 caps but I have some holiday things going on over the next few days) 

Thanks again, I would love to be able to sort this out!     
"some dive into the sea, some toil upon the stone"

travis

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Re: picking up radio signal with mixer
« Reply #5 on: November 23, 2011, 12:59:20 PM »
hey abbey road d enfer,
I have a two .01 uf caps in a project I am wrapping up.  I was thinking of removing them temporarily to try the test that you suggested.  Could it damage them?

Thanks again for all of the input.  I also checked very closely for solder problems and found nothing. 
"some dive into the sea, some toil upon the stone"

JohnRoberts

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Re: picking up radio signal with mixer
« Reply #6 on: November 23, 2011, 01:06:23 PM »
That is kind of unexpected. The mic inputs should be more sensitive to RF.

How does it sound with actual inputs plugged into the stereo inputs?

JR
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travis

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Re: picking up radio signal with mixer
« Reply #7 on: November 23, 2011, 01:45:39 PM »
Hi Jon,
RF stops when there is a stereo input plugged in!  This was bothering me so much because when I record vocals it is so distracting.  I suppose that it is not a huge problem becase I can just turn stereo inputs to minimum volume when not in use.  It is still a little fuzzy and I would rather not have to think about it, but I am happy to be able to work around it!  Any other Ideas?  I would like to try the cap trick still but otherwise I think I can live with it.         :)
"some dive into the sea, some toil upon the stone"

abbey road d enfer

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Re: picking up radio signal with mixer
« Reply #8 on: November 23, 2011, 02:10:45 PM »
I don't know the specs for the stereo inputs, but I suspect they are about 10k ohms impedance.
0.01uF may be too much and suck treble. You just have to experiment with any value; there's no harm in connecting/disconnecting them.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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