Replacing Pandora's Box Switches with a Pot

[thunderaxe]

So the version of the Expandora that I want to build is essentially the same as the Aion Effects Dynamo, with the two switches replaced by a "Drive" knob instead. And I think I want to add the Bass Pot described in this thread as well (instead of the bass switch on the Dynamo). Since I was ordering a couple of PedalPCBs anyway I decided in my infinite wisdom as someone who has never built a pedal before but is an incurable tinkerer and also a cheapskate who hates paying extra for shipping, that I would just add the Pandora's Box to my order and then mod it. The particulars of replacing the switched resistors with a pot isn't my question, I pretty much just copied & pasted the resistor and pot from the Dynamo into the Pandora's Box circuit, my question is more this -- does replacing the switches with a Drive knob kind of make the Gain knob redundant? I understand that in theory they're doing different things -- as I understand it (and do please correct my if I'm wrong, and you may have to explain like I'm 5 because I am a complete noob), the switched resistors in the Opto part of the circuit decide how much the Opto can reduce the amount of gain going into the clipping stage, and when they're removed entirely for Forbidden Mode you just get an ungodly amount of gain slamming into the front end of the OpAmp causing it to freak out into insane fuzz and self-oscillation. The good stuff. Anyway, with that essentially controlling the gain, what role does the normal Gain knob play anymore? Is it just like a micro adjustment and the Drive knob is the Macro? If so, does it make sense to keep it? Can it be done away with?

 

[Feral Feline]

I'll liken it to a tripple-carb set up:

You can run all three carbs at once, each one connected directly to the gas pedal and working in concert with each other. Good all-round performance while circuit racing but a bit of a gas-guzzler setup since you don't always need that much fuel-air mix being fed into the engine on grocery runs around town.

OR

The setup can be configured to run only on the centre-carb under normal acceleration around town, but if you stomp on it (for some stoplight to stoplight street racing or for passing a semi on the highway), the other two carbs kick in and dump loads of air/fuel into the engine to produce gobs of power and your Km/L (MPG) goes down the toilet.


For the Pandora's Box
You could replace the GAIN knob with a fixed resistor. Set the GAIN to where it sounds right to you, ie the pedal behaves the way you want it to and then you remove the GAIN pot and measure its resistance at where you set it, say it measures 637k. The nearest fixed resistor to that is 620k or 680k so you'd install one of those two values in place of the pot. You could even throw the two resistors on a switch and make the first op-amp gain stage in the circuit be more like the second prior to modding it to have a pot. If you do away with the GAIN completely, either a variable resistor (pot) or a fixed resistor, and have nothing, you'll turn the first gain stage into a buffer.


The DRIVE on the Dynamo is just another GAIN control, but controlling the second op-amp in the circuit, which is also connected to the opto-coupler.


GAIN & DRIVE are each doing different jobs on different parts of the circuit.



To use a car analogy again:

GAIN is like the idle-screw on your carburettor, set too low and the engine will stumble when cold and stall. Set too high and you're racing the engine at stoplights around town wasting gobs of gas.

DRIVE is like the adjustment for the gas-pedal. Too slack and when you lift your foot completely off the gas-pedal the engine dies, too tight and the car doesn't accelerate in a smooth controlled manner but leaps away at the lightest touch of the gas-pedal.

Like a rev-limiter monitoring the engine, the opto-coupler is translating how hard you hit the strings, ie how much gas you're giving the circuit.
Forbidden Mode is like disabling the REV-LIMITER in the engine.


It's a system: With everything adjusted correctly and doing its job, you'll have a good balance of performance/fuel-consumption and you won't blow up the engine.







So, play around with the circuit, try subbing out the 1M GAIN pot for a 560k resistor, test it out. Will it still do what you want it to do?
Note how the first op-amp has GAIN B1M whereas DRIVE is a mere B1k — why do you think that is? Why not the other way around?
What if you just add a B1k pot in series with Pandora's R8 and make the resistor the same value as the Dynamo's 240Ω, then you could switch between the Dynamo pot and the Pandora's 1k1 R9. Why not make the GAIN an internal trimmer and the DRIVE, too, while you're at it. Set them and forget them and have only one knob on the pedal for LEVEL; or add a Baxandall EQ instead of the stock tone-stack...
Hey, it's your build, you can do anything you want!