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Comp Resistors

June 5th, 2011

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Removing or filtering RF from music effects pedals?

I am trying to remove some very invasive RF from my guitar effects pedals. I realize that cable shielding does help as well as the transistors I have used to construct the circuit are probably picking some of the frequency up too. Should I use carbon comp resistors? I have shielded the metal box but that did nothing. I have heard that putting a 100 pf capacitor across the input and output jack can help but I have not yet tried this. Any suggestions? I am mostly picking up local radio stations and a few at a time.

RC filter? Any insight would be great!
I did use this formula to calculate the desired capacitance:

C=1/(2pi)*(f)*Xsubc

=1/6.28*100*(365,000/10)
=approx 400 pf
ECKO:

http://members.fortunecity.com/uzzfay/iswf/iswfsch.html

here is the circuit!

The interfering radio signal can be picked up by any of the connections to the box. The most likely is the input lead though. The shielded box, cable shields and the common rail of the amplifier should all be connected at one point. The ground connection also connects to the power supply common (power point ground). Cable shields are grounded at one end only, though there are occasional reasons to try otherwise.
The resistors, carbon or metal film, no different from this point of view.

Here are some ideas to try, but no guarantees.

Use ceramic disc capacitors for these suggestions.

At the input of the amplifier, a series 1K resistor from the input lead and a shunt 1000pF capacitor to common, at the cable shield.

From the power supply leads a series 22 ohm resistor and a shunt 0.1uF (100nF) capacitor to common.

Output lead, series 100 ohms resistor, shunt 100pF to ground, another series 22 ohm to the cable.

On the controls, just make sure the shields are properly connected.

You can also try shield breaks - these are those ferrite clamps like you see on computer VGA cables, USB cables and so on. It can be a ferrite toroid with one or two turns of the cable through it.

That formula, the radio frequencies are say 1MHz, the Xc may be 1592 ohms, and that is 100pF.

If you have the circuit I can be more helpful.

So quixdraw built his first computer in 1970, yeh way to go!

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