Opamp preamp

[lowell]

If I clone a circuit that I dig, but use a 9v supply instead of an 18v supply, do I need to compensate all the opamps' closed-loop gain? I ask because my circuit is distorting quite a bit. This seems correct to me, but I've found that even though I think I understand opamp circuits, I typically find that I have a lot more to learn.

[loudthud]

The opamp gain stage(s) before the first volume control should have lower gain to compensate for the reduced Vcc voltage. This will preserve headroom at the input.

[R.G.]

LT is correct, but to amplify on that a bit, opamp compensation has nothing whatsoever to do with supply voltage and the resulting limitations on output signal level. Compensation is concerned with how close to oscillating it is. Power supply voltage changes will increase or decrease the output signal which can be produced before distortion is heard.

I believe you're running out of power supply voltage for the output you're asking it to do. As LT says, lower the gain, or increase the power supply voltage.

[J M Fahey]

Or maybe you are not even biasing it properly for a 9V supply.
Please post schematic.

[lowell]

thanks guys. JM they are biased correctly with the 4.5v bias voltage. I had been through this before on here, when originally the pedal didn't work at all. :-) So now it works, just is distorting early. Sounds like my assumption was correct, the feedback/gain of each stage is probably to large. Just wanted to make sure I was on the right track. I can't post the schematic as my external drive is acting up. If I get it goin' I will.

[J M Fahey]

Ok, just checking.
Consider this: *most* Op Amps, can *NOT* reach rails by about 2 volts (check datasheets), so with 18V +B you have max. 18-4=14V peak to peak, or 5V RMS.
So if your first stage has 10X gain, it can stand no more than 500mV RMS ... what a "good" humbucker puts out on a chord.

Now if you use 9V B+, you have only 9-4=5 VPP , around 1.8V RMS. **BAD**
Your preamp will distort even with a Strat
Unless you lower gain a lot, probably to 4X or thereabouts.

[lowell]

Very informative detailed response. Thanks!

[J M Fahey]

Same as loudthud had already said in post #2.
Only added a little extra detail so you know *how much* to pad gain.
Just as a curiosity: very well designed Sansamp and other products from the same design team, use "rail to rail" Op Amps.
Some cloners don't find that important and use conventional TL072 and such ... and find that the clone does not work as the original.

Not suggesting you use them, just an example.

[loudthud]

The TL072 does not like big signals on the input with low voltage rails. If the input swings too low, the output slams the positive rail. It's called input common mode range on the spec sheet. If you don't want to use one of those new fangled rail to rail parts, try the old 4558 or only use inverting configurations.