noise pickup in stompbox

[mikko.helin at nokia.com]

> I'm about to layout a PCB that suffers from noise
> pickup by the input amplifier.
> Do people think it would be worth having a fet 
> right at the input (where the input jack mounts on the board)
> to act as a preamp or just as an impedance lowerer so the trace
> to the first amp is less susceptible to noise pickup?
That would be a nice thing to do. Have you considered using the
preamp at the source end of the cable? I guess you are building
some guitar FX device, right? Usually an input impedance of about 1 Mohm
is recommended for those devices - on the other hand the pots
in guitar electronics are about 250k (strats) to 500k (LP's).
The input impedance of Fuzz Face is pretty low (<1k), though
many use the pot on guitar to increase the impedance seen by
the pickup itself (there's a mod for FF which adds
a 100k pot before the input, this is also used on Fulltone's
FF copy). Anyway it would make sense (at least for me) to lower 
the input impedance of any guitar FX gear to about 200-250k to get
rid of some noise. Better way would be to feed the signal from guitar
symmetrically (with balanced cable).

> Also, the power diodes I have are radiating right up into the RF.
> Are some rectifiers better thaqn others?
> Should I put a tiny cap straight across them? Or would thid be
> bad luck?
Try a cap in front of the input or in feedback loop of opamp.

-Mikko
> 
> looking foward to any & all answers,
> 
> paul perry Melbourne AUstralia
>