Please help me understand this volume pedal

[OldManAP]

Not sure if this belongs in this subforum, apologies if I’m in the wrong place.

I’m trying to better understand my volume pedal. It’s a Goodrich L10K active pedal. Unfortunately, I don’t have a full trace as I didn’t have time to remove the whole board to see the underside. But just looking at the components, I’m thinking it’s nothing more than a buffer. Hopefully, someone here can confirm and tell me a little about what the components are doing. It’s a tiny board built on what I believe is vero.

I see a 4K7 and 0R resistor, MC33171P opamp, a 4u7 tantalum cap, a 360K and 390K resistor, and a 220n cap. The pot is 10K linear. There is an instrument in, an amplifier out, and an unlabeled out that I believe is a tuner out (which I’ve never used).

I was unfamiliar with the MC33171P, but it is an obsolete low power single opamp, and I imagine it was chosen for long battery life as this pedal has no DC jack.

Any insight into this circuit would be appreciated. Particularly, I’m interested to know if it’s an efficient and elegant way of doing things, or if there is a better way. Thanks!

 

[spi]

yep probably an op-amp buffer. See the Kliche Mini for an example of what is probably a similar schematic.

 

[fig]

...or maybe more to the point....

Kliche Buffer - PedalPCB.com

Standalone Klon-style Buffer

www.pedalpcb.com

Nope, ignore me...no schematic yet...

 

[Robert]

 

[OldManAP]

Thanks so much for that, Robert!

So I guess my next questions are:

The pot value is more a function of loading, right? We usually see 250K or 500K in passive volume pedals, but 10-25K in active ones. I’m guessing these values work better respectively depending on whether they follow a buffer? Is that why my old Sho-Bud pedal (500K) sucks so bad if it follows my dirt pedals?

Also, the pots in volume pedals are most often driven by a string or belt, or something that ISN’T a rack and pinion. I assume this is because the rack and pinion can’t turn the pot through it’s full range. If one were to use a typical wah shell for a volume pedal, I assume you’d have to tune the gear so that heel down bottomed out the pot, and perhaps one could use some sort of simple amplifier in the pedal to bring the signal back up to unity (or higher) when the pedal is toe down?

 

[Alan W]

If you are building a new mechanism, you could use a second gear on the pot that is smaller than the pinion, so you could get full range (turn) from a smaller arc movement. If this intragues, but confuses you, let me know.

I use a volume pedal continuously as I play, and a few years ago modded my Ernie Ball mini with the Zeppelin labs kit, which converts it to a non mechanical, magnet proximity based system. The instructions included how to adjust the pedal to increase tension, because since there are no mechanical interactions with the pedal, it's nice to have at least some resistance (no pun intended) for the pedal feel.