RF Filtering

[grapefruit]

Just wondering what methods people use to keep RF out of a circuit. Say if you have a untiy gain buffer for a splitter box so you can't put a cap in the feedback loop of the op amp for low pass filtering.

I'm thinking an input resistor and cap to ground is not an ideal method for guitar because the source resistance (of the guitar) will change the cutoff frequency quite a bit.

Ferrite beads?
Maybe filter it after the buffer?

I know that for a buffer it's not that necessary, but for a bass fuzz when you want to buffer, split, and distort some of the signal, I'd rather knock out the RF right on the input.  I hope this made sense to someone  

[brett]

Hi.
For most pickups, a 0.001uF cap to ground at the input to your box is a good option.
For z (pickup impedance) = 10kohms, Fc (the cutoff freq) = 16kHz
For high Z pickups (?? big fat humbuckers ??) with 15kohms, reduce the cap to 680pF.
As far as I know, these caps will only ground RFI upstream of their position.
cheers
PS the formula is Fc=1/(2.pi.R.C) where Fc is in Hertz, R is in ohms and C is in Farads.

[grapefruit]

Thanks brett.
That's a big help. I thought pickups had a higher impedance than that.  For a buffer I could put a 22k series input resistor before the cap and reduce the Fc difference between different pickups.

Stew.