drive+level knob

[ashcat_lt]

Every overdrive/distortion pedal available has the same two knobs.  They may be labeled differently, but they do the same thing.  One is a gain/drive/distortion, the other is level/volume/output.  We all know why: when you turn up the "gain" in order to get more overdrive out of the circuit, this also increases the output volume, so you need to turn down the "volume" so that it's not such a huge jump when you step on the pedal.  Of course there are exceptions.  If you actually want all that extra output volume (to hit your amps input harder, for example) you obviously would just crank both knobs.

For the project I'm fantasizing about, however, I would like to be able to vary the amount of overdrive without affecting the output volume much.  I've had a few different ideas of how to do this. 

The simplest way would seem to be simply replacing the two knobs with a single ganged pot and wiring one of them "backwards."  This creates all kinds of problems though.  Obviously, if the pot is anything other than linear you'll have strange results.  Also, you may not want all of the sweep of the pot for both "knobs". 

Another way I considered was to use a ganged pot where one part controls the drive, and the other attenuates a control voltage and sets the output level of a VCA somewhere later in the circuit.

One issue with the whole thing is that overdrive not only changes the peak voltage output, but also the relative RMS voltage.  In other words, given the same "volume" an overdriven signal will have a higher percieved volume than a clean signal.  This led me to the idea of possibly somehow "fading" between a clean compression and an overdriven sound.

I understand that the implimentation of any given idea would depend greatly on the individual circuit we're talking about, but I'm looking for a general discussion of the concept.  I'd be happy to fill in details of my project if anyone is interested.

Thanks

[petemoore]

  Two controls is alot easier than working an avaliable value dual ganged into exactly the right tapers to achieve a constant level w/ increase of gain.
  Of course it can [has been?] done...the 'best worst' way would be to figure out the tapers you want using two pots to get settings for charting the special tapers and values of a dual pot design, best because it would work the way you wanted [if everything was charted right and the DG pot did exactly what you expected it to] and worst because you can't just 'get' a special taper pot.
  Another avenue would be to work with one or more circuits to decide which ones seem to lend themselves to this modification.
  ...from there the use of taper or potvalue resistors across the lugs of the pot could be used to further 'align' the pot's values to hold volume level constant and increase gain. Adding resistance values outside the pots [stop resistors] may also be of some closer targetting.
  ...Other than that you're stuck with the actual tapers of whatever you can get in dual ganged...perhaps circuit mods and circuit searches may turn up a 'match' to the available values and tapers, and the ratios of all the 'inbetweens' settings.
  I suspect getting the charts to mesh...volume line staying horizontal while gain line rises offers a challenge, also gain knobs sometimes influence EQ more than others...quite a few things to simultaneously think about for a volume constant gain set knob.
  It 'could' be done, but working with available dual ganged tapers and values presents a steep challenge at this point, having only researched it enough to let someone else take over...

[stumper1]

It's a great idea, but....

What if you WANT to adjust them differently.  Gain/Drive/Dist can vary from guitar to guitar, not to mention amp to amp.  What if you want a volume boost/cut when the effect is engaged?

Of course your use for your project may allow this to work.  I suspect the needs of the end user as well as ease/dificulty of execution of this type of control are why we see the 2 pot set-up more often.

Either way, I do think it's a cool idea!!   Good Luck!

[Processaurus]

Good thinking, I guess I got so used to that that I forgot how much it annoyed me when I was starting with distortion pedals.  The way clipping works as level limiting, though, makes it a challenge.  An obstacle I see is the difference in the output levels of different guitars and playing styles affecting the desired output level.  Quieter clean guitar means you turn the volume knob down lower on the distortion to match levels, but with hotter pickups or harder playing, you have to have more output from the distortion.  Also once you reach a point were the clipping level is getting reached, the output stops getting louder despite more gain being added, it just gets more saturated.  So at that point even though the gain is getting increased, that percieved output volume won't get much louder.

A rotary switch with two poles might work better, for simultaniously changing the gain and the output level.  I've seen that on some studio preamps (though those were set up for clean gain).  Then the taper (even though its now stepped rather than a smooth transition you'd get from a dual pot) can be as weird as you need.

On a side note, chorus/vibrato pedals have the same problem, when you turn up the speed, you have to turn the depth down to get the same amount of pitch bending.  That would actually be an easier one to solve than the gain-level relationship.

[MartyMart]

http://www.diystompboxes.com/smfforum/index.php?topic=45216.msg330386#msg330386

Tried that one in above post , idea's nice but doesn't really pan out well :-(

MM.

[petemoore]

  Work.. as in experiment/calculate/test/BTT Drawing board...
  Limiting 'where' you come from and go 'to'...IE pick a couple Gain Levels you want and try to bend the circuit to supply just two settings you want ...and the inbetweens...
  Mostly I think we use volume set at above 1/2, so attempting to set up a gain w/volume constant that goes below 1/2 volume would be added work that doesn't necessarily lend itself to being used.
  Picking out what you 'need' and getting the gain to go from just say medium to high without much volume change would probably simplify matters considerably.
  I also suppose certain circuits lend themselves to being modded in this fashion more easily than others...
  I would start with something like a BMP because the volume And Gain are...both wired like volume controls...starting with something like a 100k or 500k DGpot, then adding resistors between the lugs and outside the lugs to try to bend them to achieve the gain increases without volume changes.
  A trimpot in the mix, on the board, or even socketting resistors, having a switch for a resistor or two...so that choices like more gain with a little more volume, or gain up with same volume could be had, and to compensate for different pickup output levels and diode clamping.
  Pick a Distorter and go to town !!!...tell 'em BYOSB sent ya !!