RF Interference in an audio circuit

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captainate

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Hi all,

I just solved a problem through this forum yesterday, so I figured I'd give another issue a shot.

I have a wah-wah pedal that has been heavily modified. I am experiencing RF interference when the wah is active, and being an audio circuit, it is extremely unacceptable. It sounds like a grounding issue to me, especially since the circuit is only grounded through the mounted 1/4" phone jacks that are in contact with the beefy metal casing that surrounds the pedal. Thanks for your help, if more information is needed I can try to provide it. I am working on ascertaining the schematic.

~Nate
 
Does the input cable to the circuit have poor or missing shielding?
Disconnect the input cable. Then is the interference gone?
 
The input and output cables, although unbalanced, are of the highest quality and do not present such problems with other equipment. I thought about twisting all the signal wires in pairs but I can't imagine that would be enough to combat such pervasive interference. Thanks,

~Nate
 
What is the interference?
AM or FM radio stations?
TV stations?
CB radios?
Taxi cab, police or aircraft communications?
Ghosts?
 
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What is the interference?

Ghosts?

Haha no I think that's from a movie though, a bad horror movie I can't remember the name of.

In terms of troubleshooting, the "modifications" replaced every component in the pedal. It's essentially a completely new and different pedal. I hear a radio station, I believe New Country 93.3 FM, although since I don't listen to the radio (especially country) I can't be 100% sure. Thanks,

~Nate
 
There is not... at first I thought it was because the circuit was no longer active without a cable inserted in the input jack. It appears, though, that the filter effect is still active when the input cable is unplugged.

This means the interference is from the cable (or less likely from the guitar), correct? This doesn't make sense, though, because at the same volume there is no interference when the wah is in bypass (true dpdt bypass). I'm lost......

~Nate
 
Sounds like the pedal is picking up the radio station, due to faulty shielding or grounding, and some nonlinear circuit, probably a base-emitter junction in a transistor, is rectifying the RF. This amounts to an RF detector, which yields the audio modulation envelope. When you switch to bypass, that circuit is apparently bypassed.
A schematic would help.
 
Yeah, that would be funny if I weren't so fed up with the problem. If it turns out to be a design flaw, I think I'll try and sell the fix to him and give the proceeds to the person who solves it!!!

~Nate
 
Yeah, that would be funny if I weren't so fed up with the problem. If it turns out to be a design flaw, I think I'll try and sell the fix to him and give the proceeds to the person who solves it!!!

~Nate

Without seeing the circuit it is hard to make recommendations. High frequency bypassing should be distributed throughout you power lines and near all power connection pins on IC's. If you have a high gain audio stage, take care of long leads. Try localizing the interference by placing a 470pf cap from ground to various inputs and outputs on the circuit.

Since the problem is not present with cable disconnected try adding RF chokes at the Input connector. Just a guess.
 
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Attached is the scan of a page from a National Semiconductor audio handbook from the 1980's. The problem was more common then, with interference from AM radio stations, but the rules apply to other RF interference sources. Might give you some ideas.

Note the last bullet point -'pray' - and that from a text book! So a solution can be challenging!
 

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Cool, your app-note supports my suggestion to add an RF choke on the input.
 
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