Boost pedal receives an AM radio station

[audioguy]

I built a LPB1 form GGG and from time to time I get an AM radio station-
Any possible solutions?

Thanks!
Audio guy

[RDV]

From the DIY FAQ:

I am picking up radio stations, how can I stop this?

From Eric Hensel:A 47pf cap to,ground, after the input cap will probably do it --you can adjust this up or down --higher values will start to cut treble. use the lowest value that works.
From Zachary Vex:

Mount the pc board as close as you can physically to the metal box. this will serve as a ground plane to reduce the heterodyning that can pick up rf. don't let the input and output wires cross... make sure the layout doesn't allow input and output connections to get too close to each other. if you can, solder in ground connections around (near) the sensitive input circuitry. if rf is being picked up by rectification, these fixes won't help, but if it is being picked up due to beat frequencies generated by heterodyning (internal rf oscillation in the circuit beating against rf from radio stations) you can get rid of it through the use of careful layout and grounding to reduce oscillation.
You might try using miniature shielded cable inside of the enclosure for the input to the board too.

HTH

RDV

[audioguy]

Thanks RDV-
I think I'll try the 47pf cap.

[Lonestarjohnny]

get a Patent on it Brother, and you need to tweak on it til it get's all the local station's ! Just Joken, I had an old Gibson Maestro amp that picked up every cop car in town, got to know the 10 code very well,  :lol:
if it's got High Gain it need's Shielding and Shielded wire on all tone control,input's/output's.
JD

[R.G.]

A 47pf cap to,ground, after the input cap will probably do it

It will in many cases. But if you're near a strong RF source, it will not help. The idea is that the 47pF attenuates the RF by shunting it to signal ground, making it common mode with respect to the input. Strong RF signals will still get through.

Much better is to use a 1K or a ferrite bead in series with the input wire from the input jack and a 47pf **ceramic disk** cap to signal ground after the resistor or bead. The resistor/bead inserts some series impedance that the RF can't simply overpower in conjunction with the cap, and you get a lot of RF attenuation.

Mount the pc board as close as you can physically to the metal box. this will serve as a ground plane to reduce the heterodyning that can pick up rf. don't let the input and output wires cross... make sure the layout doesn't allow input and output connections to get too close to each other. if you can, solder in ground connections around (near) the sensitive input circuitry. if rf is being picked up by rectification, these fixes won't help, but if it is being picked up due to beat frequencies generated by heterodyning (internal rf oscillation in the circuit beating against rf from radio stations) you can get rid of it through the use of careful layout and grounding to reduce oscillation.

It makes a lot more sense to not make effects that oscillate internally that can heterodyne. No oscillation, no heterodyne pickup. The theremin and its ilk are the only effects that I can think of just offhand that oscillation is really useful or needed for.  This is correct that poor layout is responsible for internal RF oscillation, though. Every new-design effect really ought to be looked at on a scope to see if it's screaming in the VHF range. That is the only way to be sure.

Well, OK, maybe some of the RF-sniffer bug detectors would work. Hmmm... maybe I need to gen one of those up.

You might try using miniature shielded cable inside of the enclosure for the input to the board too.

Not a bad idea - the cable capacitance helps shunt RF to ground in the same manner as the 47pf ceramic cap, and the grounded shield braid helps prevent internal oscillations. A resistor or bead will improve RF rejection of a shielded input cable as well.