Controlling the amplitude of the output signal from a pedal?

[GibsonGM]

That COULD be bleeding off bias voltage...and perhaps the cap is wasted now...so if you have another just replace it.  Another of those things one may ASSUME someone knows, but they don't.     

Since the DC is biasing the opamp, it's going to be at the "+" potential; a quick look at things like transistors show the same condition...many schematics show the "+" on the caps if polarized, or have the straight line on the + side, showing us which is at a higher potential.

[YurkshireLad]

Are you kidding me? I swapped the polarity of C1 and the op amp is biased at 4.5V now. 

The distortion doesn't sound right; it kind of fizzles out, but there's a lot of noise in the signal and I'm also picking up radio from somewhere. Still it's progress, right!? 

If C1 was ceramic, instead of electrolytic, would that block DC in both directions?

[GibsonGM]

Yes but - Ceramic caps suck tho   If it was a POLY type, like .1u, it would be better...better quality caps, and the lower value will mean less bass input to the stage - more crisp-sounding.  So less distortion (often caused by too much bass content).   

It'll pick up RF and junk if it's not in an enclosure.....you're actually using a lot of gain there, so that is normal.  There are tricks for if it does that once it's assembled properly (like a .1u cap to ground from input etc)...don't worry about that now tho...

If you're TRYING to get distortion...using the opamp to do it probably isn't going to be a nice sound...you'd want to use clipping diodes with it (look up "MXR Distortion Plus Schematic" to see that).    I figured you just want a clean boost for right now - so maybe the feedback resistor needs to be a lower value (like 10k instead of 20k?).     What we want to do for the 1st stage is raise the tiny guitar signal above the noise floor and get a nice, clean, solidly amplified signal to work with.     

Also - what amp are you using? Is it a tube amp?  Feeding highish level signals into a solid state amp can sound like crap, where with a tube amp it would sound very nice!

[YurkshireLad]

I have a Peavey Blazer 158.

[GibsonGM]

They SAY it behaves just like a tube amp - not sure about that, but we'll go with it.   I'll just leave it at - if the amp sounds good when you add more and more drive to the input with your boost circuit, then great! If not, the cause may well not be the boost, but the transistors in the preamp.

Some solid state amps will sound good over a small increase in input, and then turn to BLAHHHHH after that.  The amount of bass content you feed the amp can be a MAJOR player in how the drive will sound.   If takes way more power from the amp transformer to amplify bass than higher frequencies, which can over-tax it.  Why I suggest a smaller cap (and that means doesn't have to be electrolytic).  Just something to be aware of, there are limits to how much boost you can get away with. Even with tube amps; eventually, they'll just turn to a solid wall of non-dynamic fuzz.


Anything non-polarized will block DC in both directions...ceramic, a poly type (favored for stomp circuits), mylar, or non-polarized electrolytic (abbrev. "NP" in their documentation). 

[YurkshireLad]

I have no doubt it's an average amp, because it was pretty cheap when I bought it.

Do you mean use a 0.1uF for C1? I just rebuilt the circuit so I'll try it as is and see if 0.1uF makes a difference.

[GibsonGM]

If we're still going by the schematic that has the 10u input cap...yes, .1 would improve it I think. Esp. in terms of noise.

Also try, as was mentioned, lowering the feedback resistor from 20k to like 10k.  You don't need massive gain out of one stage, LOL.   If your guitar input signal is say, 1V on average...asking it for a gain of 21 WILL make the chip distort, because the 9V supply can't GIVE 20V out.  It clips.
At 10k, still kind of a lot of gain but possibly you won't notice distortion, or it'll be less nasty.

Can try a 6.8K in there too, see if that dials it back a bit....or a 10k or 20k (etc) pot wired as variable resistor, adjust to taste...