Modding an OD pedal (gain and volume knob relationship)

LionAquaLooper

Take a simple true bypass 3--knob OD pedal (gain, volume, tone) as an example.  How much work would need to be done to mod it so when at gain 0, there is little to no output volume loss.  Right now this cheap OD pedal I'm messing around with, at gain 0, needs volume to be at max to just reach unity with my amp.  Some of my other ODs don't suffer from a big output volume loss when I set gain to 0. 

Is there a simple mod to have it so output volume is roughly the same regardless where gain is set?  Would be ideal if I can set volume to unity with my amp, and I'm free to adjust gain without having to tweak the volume knob constantly.  Or am I asking for the impossible?

Dave_Mc

I could be wrong, and hopefully someone who knows more than I do will chip in, but the gain basically is another volume as far as I'm aware. When the volume goes over a certain amount, the circuit will clip. And then the actual "volume" is usually at the end of the circuit to turn the overall output volume down, but still retaining the clipping/overdrive. So that would complicate things... you'd need to have some way of more or less turning one up while the other one turns down- and they still might not match exactly in terms of volume.

thermionic

You wouldn’t be able to do this by modding, you’d have to design the circuit from scratch. If you started with a dual-gang pot where one has a reverse taper it might be an idea.

ICBM

There are two ways of doing it - one is as thermionic said, to have a dual-gang control which adjusts gain and volume in opposite directions, but even that won’t be perfect because the relationship between the two changes depending on how heavily clipped the signal is - and the other is to make the sound so compressed that the output level doesn’t change significantly as the gain and amount of clipping goes up, which will probably mean it has to be overdriven even with the gain on 0 (which I suspect is not what you want).

Or you could try putting a compressor after the overdrive, rather than (as more usual) before it.

Danny1969

In theory you can achieve this mechanically on some pedals if the gain and volume knobs are close. Just fit rubber surrounds on both knobs so they touch.  Then when you turn the gain knob to the left it will rotate the volume knob clockwise and visa versa

JayGee

I suspect the simplest way of doing this would be to replace the gain and volume pots with digital potentiometers and add a dinky little microcontroller with a simple mapping between gain and volume settings which could be tweaked to give constant output. 

The crazy thing is that while on the face of it this is complete technological overkill the chips and/or modules you’d need to do this actually wouldn’t cost very much at all. The only remotely tricky bit would be preventing noise from the digital bits getting into the analog bits, and there are plenty of reasonably priced bits of audio kit around (pretty much anything with a remote volume control) which manage that, so it’s not insoluble…

LionAquaLooper

Thanks for the tips everyone.  Just a quick question though for anyone who might know - why is it that on some OD pedals the loss in volume at gain 0 isn't as drastic compared to others?  I've noticed this particularly on Boss ODs like my SD1 and OD3 and even my EHX Crayon.  Even at gain 0 the output volume isn't far from unity with amp.  But on other ODs I've tried gain 0 = output volume 0. Is it to do with:

True Bypass vs Buffered?
Input/Output impedance? 
Resistor/Capacitor values within? 

I suppose this is what makes some ODs better clean boosts than others but I'm just wondering what it is about them that does this.  

thermionic

This is simply because overdrive pedals contain two or more gain stages. I mean gain in the strict physics/engineering sense, i.e., the ratio of voltage out to voltage in - the term “gain” is commonly misused when describing “amount of clipping/distortion”. The circuit desgner decides how much gain range is available by say, a level control and a drive control. You can design the stage controlled by the level control to narrower or wider range.

ICBM

LionAquaLooper said:

Just a quick question though for anyone who might know - why is it that on some OD pedals the loss in volume at gain 0 isn't as drastic compared to others?  I've noticed this particularly on Boss ODs like my SD1 and OD3 and even my EHX Crayon.  Even at gain 0 the output volume isn't far from unity with amp.  But on other ODs I've tried gain 0 = output volume 0. Is it to do with:

True Bypass vs Buffered?
Input/Output impedance? 
Resistor/Capacitor values within?

None of those. In some circuits, the control is a true gain control for a stage, usually an op-amp with the gain controlled by a negative feedback loop. Setting the control to 0 (full negative feedback) produces unity gain. On others the control is actually a signal level potential divider between gain stages - if this is referenced directly to ground (many are), then setting the control to 0 gives no signal at all.

It’s perfectly possible to have a resistor in series to ground with the second type, so turning the control to 0 gives a pre-set signal level above zero.