The Crank

[soggybag]

I built The Crank, it's working but I'm not sure I have the gain pot wired correctly. There is a cryptic note in the schematic that says "reverse wired".

Is meant to to say the two sections of the dual pot are wired opposite to each other? For example when the gain on one section is going up the other is going down? Or should both sections of the pot be reversed?

[ElectricDruid]

If it's a dual-pot, then perhaps that's it - the two sections are wired opposite to each other. That can be a handy arrangement in various circuits.

[Mark Hammer]

They're wired opposite because one stage (the first stage) is using the ground leg to adjust gain, and the other (2nd stage) is using the feedback loop.

Non-inverting op-amps need to be thought of like a car that has no gas pedal, only a brake pedal.  The car "wants" to go full out, and the only way to regulate its speed is by using the brake pedal.

What is the equivalent of a brake pedal in a non-inverting op-amp?  Negative feedback from the output will reduce the gain from the maximum the chip is capable of.  If more of the negative feedback is preserved then the gain is reduced.  If more of the negative feedback is bled off, then that's like taking your foot off the brakes.

The feedback and ground-leg resistance operate like a voltage-divider (volume control) to determine how much of the negative feedback is conserved or bled off.  If feedback resistance is high and ground leg resistance is low, that is equivalent to "turning down" the negative feedback (taking your foot off the brake).  If ground-leg resistance is higher, and feedback resistance is lower, then more of the negative feedback is conserved, reducing the gain, by stepping on the brake harder.

The thing is that reducing the ground-leg resistance ALSO results in reduction of low end, and increasing feedback resistance reduces gain at the higher end of the spectrum.  Owners of the MXR Distortion+ (which uses ONLY the ground leg to set gain) will know that cranking the gain also gets a thinner sound.  Owners of a Tube Screamer (which only uses feedback resistance to set gain) will know that turning gain UP will also remove some high end.

I decided to use only a bit of change in ground leg and feedback resistance over the two stages, and rely on their multiplicative gain (i.e., this gain times that gain) so that bandwidth would not change much and only tame the top end when overall gain was set high enough to result in audible clipping.

Long story short, pot section A should reduce resistance as gain is increased, and pot section B should increase resistance as gain is turned up.

[soggybag]

Thanks for the replies. For clarity, you're saying both pots are wired reverse? Or is that one is reserve of the other?

[Mark Hammer]

One is the reverse of the other.  So if you are looking at the pot, and the lugs are pointing down, the first stage would use the wiper and lug to the right, and the 2nd stage would use the wiper and lug to the left.  That way, when you rotate the pot clockwise, the ground leg resistance gets smaller, and the feedback resistance gets bigger.  At the 7:00 position, stage one's gain is 3x, and stage 2's gain is 4.2x, yielding a combined gain of just over 12x.  At the 5:00 position, stage one's gain is 7.4x, and stage two's gain is 5.7x, yielding a combined gain of just over 42x.  That should get you some clipping when you dig in, but not so much that you'd consider it an "overdrive" circuit.  A clean gain of 12x is certainly a big boost, volume-wise, and enough to push an amp hard.

[ElectricDruid]

+1 what Mark said.

The two sections are wired with one the reverse of the other.