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!

 

[jhaneyzz]

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!


View attachment 79105

Epic response!!!

 

[thunderaxe]

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!


View attachment 79105

unfortunately for me i know even less about cars than i did about guitar pedals when i first asked this question. fortunately, i have learned a lot more about guitar pedals in the year and a half since. so here's what i understand about the circuit and you can let me know if i get anything wrong:

the first section is a gain boost. i thought a 1Meg pot in the feedback loop was a huge value but what really matters is the ratio between it and the input resistor, which is 100K so it's just a 10:1 ratio, giving a maximum gain of 10x or 20dB. why use such a high value then? presumably because it's right at the input, without a buffer, and so any existing impedance (guitar cable, pickup, a pedal with a poor output impedance) plugged in to the front of the pedal will be added to that of the input resistor, so setting it to a high value reduces the margin of error.

why not make it a non-inverting op amp then, which doesn't depend on input impedance to set its gain? i'm not entirely sure, but one effect of doing it the way it is, is that because there is no fixed resistor in the feedback loop to set the minimum gain, once the pot gets below 100K resistance, which would be the bottom 10% of the sweep, the ratio with the input resistor is less than 1:1 and so it actually begins attenuating the signal rather than amplifying it, all the way down to silence at the bottom of the pot. i don't know if that's intentional or has an real useful advantages, but it's the only reason i can think of to not use a non-inverting configuration. whatever the gain from this stage is, it's multiplied by the second stage.

after the first gain stage, the signal is split and one branch is used to control an optocoupler. the other branch goes into the second gain stage, which is clearly based off of a rat, except that its gain is mostly controlled by the optocoupler. so basically, the louder the input signal (the harder you hit your strings), the more resistance the optocoupler creates in the feedback loop, and thus the more gain is applied. because the controlling signal is taken from after the first gain stage, the first gain control kind of does double duty, because it increases the gain of both the controlled and controlling signals. so you kind of get an exponential result from a linear pot. in parallel to the optocoupler are the switchable resistors which change the overall resistance. the lower the value of these resistors, the more the gain is reduced overall and the less effect the optocoupler has on the feedback loop resistance. in "forbidden" mode there are no parallel resistors so the gain is entirely controlled by the optocoupler. because the gain level is set by the ratio between the feedback loop resistance and that of the ground branch coming off the feedback loop, and the resistance in the ground branch is quite low, even a moderate amount of resistance in the feedback loop can create a pretty big gain. all of the non-forbidden resistance are quite low, so letting the optocoupler work by itself can produce some pretty big resistance and thus absurd amounts of gain when it's set this way.

i will have to play around with the circuit the way it's set up in the aion dynamo to find out how differently the two controls work, but it does seem like they work together in tandem and you could maybe get pretty good results by working both at the same with a dual gang pot. that would mean getting the values of both pots down to the same value, and obviously there's a huge difference between the 1K "drive" pot and 1M gain, but i do feel like the drive could use a greater range, so maybe 10K, and by converting the first section to non-inverting we could control it with a 10K pot too. worth trying anyway.

 

[Feral Feline]

The Padawan has become the Master.
The questions I asked in post #2 weren't from a position of knowledge, but of not having the answers — I had no clue, which is why I asked 'em!

I've read your post above and hopefully understand it; I think you understand the circuit better than I ever will — I have much to learn, still.
I'll need to re-read about inverting vs non-inverting op-amps, for starters.


Are you saying in the last para that you'd mod a dual-gang pot to be 1M on one gang, and 10k on the remaining gang?
That'd be cool. Aion has good instructions on pulling apart and re-assembling a pot in the CHROMA doc, N1 or Legacy.

In the Chroma doc, Kevin shows how to pull apart a couple of pots to make a custom A10/C10 dual-ganger.
You could take a dual-gang 1M and a 10k single apart, swap wafers and have a 1M/10k dual-gang and 1M single-gang when done (or vice versa, take apart a dual-gang 10k and 1M single).

Have you got a schematic for your (very cool) idea?

 

[thunderaxe]

with the values the way they are now, it would require a dual-gang pot. i was talking about what it would take to make them the same value so you don't need a special pot. i feel like the drive section could use more gain, so i would maybe up that to 10K. and then in order to get the first gain section to also be a 10K pot you could convert it to a non-inverting configuration so that its gain isn't affected by input impedance.

 

[thunderaxe]

the marshall bluesbreaker/guv'nor circuits have a very clever way of controlling the gain of two cascading gain sections via a single-gang pot but it requires the second stage to be inverting with its gain controlled by a variable input resistor, and i think it would be way too much to convert this circuit to that.