Input Noise

[grapefruit]

From reading "The Art of Electronics" I understand that FET input Op Amps are lower noise for high source impedances, while bipolar input Op Amps are lower noise for low source impedances. I often use LM833 low noise bipolar input op amps, with a unity gain buffer on the input.

When used as a unity gain buffer, with a 1M 1/2V bias resistor (1M input impedance) would a bipolar op amp be likely to produce more noise than a FET op amp, when being driven by a guitar?

Cheers,
Stew

[Transmogrifox]

I don't have a precise answer.  However, it seems like it's in the 10k's source impedance where the JFET input op amp typically becomes the optimal solution.  Typically the minimum you'll see for output impedance on most guitar pickups is 10k.  The big issue is that the guitar output is usually a 500k pot.  Once you roll the volume back a little bit, the source impedance is in the several 10k range.  This is a bad combo: decreased signal, increased noise (thus between 25% and  75%% on the volume knob occupy the worst signal/noise ratios you can have, regardless of the preamp input).   

My guess is you're best off using an emitter follower buffer to drive the BJT op amp as it adds little noise and offers a suitably low output impedance; however, this defeats the purpose of using and op amp since the BJT follower is low distortion for only very small signals.

Taking a stab in the dark, I would say that the optimal guitar buffer would be a JFET input op amp.  I throw this out there for the sake of discussion, since I have never actually measured these things (nor done the rigorous calculations).  I do remember a professor having stated that typically the JFET op amp noise performance is better than the BJT after somewhere in the several 10k source impedance range.