Variable bass drive control

[Transistor-Transistor]

Mark Hammer gave me the idea to add variable bass drive to my pedal I'm working on. I didn't really like the Joe Gagan one that he recommended (unless I didn't do it right)
Two caps are in parallel. I want one cap to be out when the pot is turned all the way one way and the other cap to be out when it's turned the other way. Is this even possible with a standard pot or do I have to use a dual?




Theres no pot in this schematic, I was using a switch to achieve this earlier, but the caps are C2 and C3

Thanks, Atticus M

[Kevin Mitchell]

I suggest that you pull up one of the Joe Gagan's Easy Face schematics to see exactly what Mark meant.

Edit due to your edit while I was typing: no pot in drawing
You could try to raise the value of the pot and lower the value of C2.

[Transistor-Transistor]

On my board it looks like how it should on the schematic

[Kevin Mitchell]

I've edited my above reply - maybe worth trying.

[Transistor-Transistor]

Oh hey that worked
Thanks a ton!

[Mark Hammer]

Thank Joe, who demonstrated the worth of this mod, over 20 years ago.  Here's one of his circuits.  You'll note that this circuit is essentially a PNP Fuzz Face.  The .005uf input cap to Q1's base sets the "default" bass content.  The 2u2/100k path adjusts how much lower frequency content is added to that default content.  The lower the pot resistance, the more bass drive.

[Transistor-Transistor]

Okay so after a day of playing around theres another problem about this thats bothering me. The pot is linear and I think by the way this is configured it may need to be reverse log to get it to sound like there's no big jumps in freq/distortion. It doesn't really sound like a bunch of changes are being made left of noon and right of noon there's a ton of really good bits that are hard to get to. (Typing this out makes this sound like a value issue but all the values I've tried have been like this too, just cutting out other good territory left of noon) I've done research and I'm still really fuzzy about how the whole "log-simulation-with-a-linear-pot" works, and if thats even a viable option in this scenario.

Sorry if this is a bit much but I thank you all sincerely for your time and for helping me out with this.

Have a great rest of your day!
Atticus M.

[Mark Hammer]

Sounds like you're onto it now.  Keep in mind that there's nothing about it that compels use of a pot.  You could also use an on-off-on SPDT toggle and several series resistors to achieve three different levels of bass drive. 

For instance, use a 33k and 68k in series with the added cap.  The cntre lug of the toggle goes to the junction of the cap and first of those resistors.  One outside lug goes to the junction of the two resistors, and the other outside lug goes to the "far" end of the 2nd resistor.  That yields a direct paralleling of the two caps (no series resistance) for most bass boost, one setting for modest bass boost, and another for no boost.

Or, like you suggest, just use a C-taper pot (or A-taper, wired the other way).

[Transistor-Transistor]

Well whats compelling me to use a pot at this point is that it affects the gain in a way I find nicer than the actual gain pot tied to the emitter (its less muddy at low settings)
C and A pots are expensive. is there a way to simulate a reverse log with a linear pot in this config? Or should I just spring for different pots?

[Mark Hammer]

If you have a regular log pot, you can simply wire it up "in reverse".  If you only have a higher-value pot, just stick a resistor in parallel with the contacts to be used on the pot, o bring it down to a reasonable value.

[Transistor-Transistor]

Well the log pot in reverse worked except that the knob direction is reversed and that'll drive me nuts. I'm going bonkers trying to find a suitable parallel resistor value. I might as well just buy the C pots

[GibsonGM]

We've all been there.  Sometimes it's not so bad, but something you use a lot, ya - the C taper pot is probably a good buy!

[amz-fx]

Thank Joe, who demonstrated the worth of this mod, over 20 years ago.  Here's one of his circuits.  You'll note that this circuit is essentially a PNP Fuzz Face.

I actually sent that tone circuit to Joe along with a bunch of other ideas when he was getting into pedal making. I certainly did not invent it, and the general idea had been used for decades prior!

Best regards, Jack

[Mark Hammer]

In which case, thank YOU, Jack.  I was going by the 20 year-old schematics I had, and was unaware of the back-channel stuff.  Apologies.