AM radio interference in guitar buffer...

[therecordingart]

So I built a JFET buffer on perfboard...something I've done at least 10 times. I put one inside a guitar, but it pickups up AM radio when I turn down the 500k volume pots. Full on I don't really hear any radio noise, but roll them back and I get radio.

http://www.muzique.com/lab/buffers.htm

Scratching my head over something seemingly simple.

[psychedelicfish]

Is the inside of your guitar shielded? What have you put the other buffers in?

[chptunes]

..there are a couple of mods that *should* fix it..

1. place a small capacitor from the FETs Gate to Ground.. something like 100pf or 470pf..

-OR-

2. replace C1 with a 33k resistor..

[Pojo]

How do you have it wired in the guitar? Pre volume pot, post volume pot?

[R.G.]

JFETs in general have very, very high frequency abilities. Many are used for preamps in the hundreds of MHz. If you get any RF to the gate of a JFET, it will likely get amplified.

The way to fix this is to either (1) keep the RF away from the gate or (2) make the gate insensitive to it. Or both.

Item number one includes grounded metal boxes, shielding wires, filtering power supplies with good high frequency capacitors (i.e. monolithic ceramic).

Item two is a bit more involved. Simply putting a cap from gate to ground may not help, because it is possible for this to resonate with the input wires and actually tune in some radio stations better. This is especially bad if your capacitor fixes the radio station that happens to be there locally, but tunes in some other station that is a problem when you're about to step up onto stage. Ooops.

To prevent this, you have to add some attenuation to spoil resonances. This can take the form of a series resistor on the input, followed by a capacitor to ground. 100 ohms to 1k is pretty effective when followed by a 22 to 470pF cap.

Notice that even a small cap to ground can cause resonance and tone issues with the 2H to 4H inductive nature of a pickup and the distributed capacitance of an input cord. The resistor helps with this issue too.