Input Biasing

[sepulchre]

Hello all,

I've got this little Opamp preamp circuit and I was wondering if I could substitute some values for the Input Biasing resistors. (I'm not really sure if that's the proper term for them, but hey . . .)

Here's the circuit:



I'd like to change R1 & R2 to 1 meg. Would that effect anything?

Also, the output is going into a guitar amp or possibly a mixer. Is the 100k pot at the output (R7) going to load anything down, or dampen treble?

Thanks for looking. I'd appreciate any help you may offer.

Ken

[sepulchre]

Oops! I forgot to include R6 in the schematic. Here is the proper circuit:



Sorry about that.

Ken

[AndrewT]

you must ensure that, despite how catastrophic the failure, you cannot get mains voltage sent down the J1 input to the guitar strings.

[kaos]

Changing R1 & R2 to 1M shouldn’t be a problem with most op-amps, though it will shift the DC output point a little bit with bipolar op-amps (shouldn’t be much of an issue since it’s capacitor coupled). If it were me I’d change the output pot R7 to 10K for lower noise and less of an effect from cable capacitance on the treble.

[sepulchre]

Thanks Kaos,
I'm using a TL071 at the moment. How much of a difference would the 1 meg resistors make? Would 470k be that much different? I do have 470k parts but they're 1/2 watt and I'm trying to keep the size down. I guess I could order some 1/8 watt ones. I live in a rather rural area where the tiny Radio Shack in our "mall" (yeah, right -- 5 whole stores) has no parts to speak of.
For the same reason, I'm going to give the 100k output pot a try. If it's noisy or suffers from cable capacitance I'll order a 10k one. I take it you see no reason that such a low value would effect the input of an amp or mixer.

And to AndrewT, thank you as well. I generally take all the safety precautions I can. Good tip.

Thanks again,
Ken

[Mr Evil]

Find the datasheet for the TL071 and look at the input bias current. Multiply the worst-case value by the input resistance (note that R1 and R2 appear in parallel to the input, so the total input resistance is half the value of one resistor) to get the input offset voltage. Multiply that by the DC gain of the op-amp (it's just 1 in this case because of C3) and that's the worst possible DC offset at the output.

[kaos]

The TL071 is a FET input amp with miniscule bias currents (leakage really). The 1M resistors won’t matter with it (unless it gets really hot, 100+C, even then it won’t matter in this application).

[sepulchre]

Thanks a million! Got it built and it works Great!

Ken