series input resistor and feedback resistor interaction on a fuzz face

[mordechai]

I remember reading a brief post where someone said that placing a 10K resistor in series with the input will allow the feedback resistor to have more of an effect on the gain of the circuit.  But there was nothing said beyond this, so I don't know exactly how/why this works, or what the actual effect would be.  More gain?  Curious to know your thoughts.

[R.G.]

The two-transistor gain stage of the Fuzz Face is closer to a current-in/voltage-out than the voltage gain mose people think of when they say "gain". It has a very high voltage gain, but the input impedance is so low that it sucks down the signal source current a lot.

It's actually much more like an inverting-input opamp circuit than a voltage gain stage. So putting a resistor in series with the input lowers the gain below what it would have been with no (that is, zero ohms) extra input resistor.

Adding an additional resistor lowers the gain, never raises it.

[rousejeremy]

You could put a 50k pot at the input like the Fulltone 69 and it would act like a "pre gain" knob. But of course it would only decrease the overall gain when turned down.
You could get the more gain effect by adding a bias knob like the sunface. When the collector of Q2 is a bit higher than the recommended 4.5 volts it gets a more high gain sound.

[wavley]

You could put a 50k pot at the input like the Fulltone 69 and it would act like a "pre gain" knob. But of course it would only decrease the overall gain when turned down.
You could get the more gain effect by adding a bias knob like the sunface. When the collector of Q2 is a bit higher than the recommended 4.5 volts it gets a more high gain sound.

Personally, I really love the pre-gain pot, I think it's one of the most useful things I put on mine the others being a bias pot and the amz pickup sim, it's a lot to cram in a little box but it makes for a very versatile pedal.

[mordechai]

Well, I originally put a 10K resistor in series with the input in an earlier project to get rid of radio freuqency interference, and it worked very well, but that was also a VERY high gain circuit so I gues the additional resistor didn't have as much of a noticable effect.  On a fuzz face that doesn't have quite as much gain on tap I can see how putting the resistor in there would have a greater impact...