Dist+ mod question

[powerplayj]

What is the purpose (in terms of tone) of putting a diode arrangement in parallel with the 1M distortion pot?  I have seen a few mod schematics (like Brian's below) and Alfonso Hermida's where an assymetric or symetric diose arrangement is used.   

http://aronnelson.com/gallery/albums/Brian-Wampler-Indyguitarist/dist_plus_mod_406.sized.gif

[petemoore]

  Actually that''d be a Dist+ if the second set of clipping diodes [in the feedback loop of the OA] and the tone control [10kpot 'n capacitor] were not there.
  This makes it a Double clipping Dist+ W/ LP Filter Adjustable tone control.
  I havent tried teh FBLoop diodes, but sure like having that little Tone knob to use on it.

[powerplayj]

Yeah, actually the diodes in the loop was what I was referring to (I confused the 1M resistor for the 1M pot).  So in other words, the diodes make the clipping even a bit more nastier?  I built this thing as an all out distortion pedal so that is a good thing. 

[petemoore]

Actually that''d be a Dist+ if the second set of clipping diodes [in the feedback loop of the OA] and the tone control [10kpot 'n capacitor] were not there.
  Actually...edit...
  The circuit shown has a few things in common with a DIST+, the LM741 and the bias arrangement.
  So in other words, the diodes make the clipping even a bit more nastier?
  Hard to say, never tried that...I just saw this circuit yesterday.
  1 meg gain pot in the feedback loop...
  instead of setting gain with a variable R/C to ground like the D+.
  Interesting arrangement there, I'm tempted. I like DP's and making the FBLoop diodes switchable would to restore a regular Dist+ diode to ground clipping sound.
  So as not to confuse matters, I suggest " + Dist " or " + DIST + " as good name to designate the Dual Clipping Circuit shown, for purposes of discussion.
  Nastier?...
  The Dist+'s LM741's output, after being whacked down by the to ground clipping elements, doesn't have alot of 'uumph' above unity...adding more diodes will further reduce output. Should be a good sound though, and a nice alternative to the DIST+, a bit more gain might prove useful.
 
 

[Mark Hammer]

The thing with double clipping is that the second set of clipping diodes has to be confronted with a supra-threshold signal in order for additional clipping to occur.  In pedals like the Big Muff or lesser achievements like my Chaos pedal and a whole bunch of other DIY pedals, there is an additional boost after the first bit of clipping in order to bring the signal level up again and re-clip.  Simply sticking diodes in the feedback loop and following that with diodes on the output will not really accomplish much additional clipping.....

...unless you make sure that the signal at the output of the op-amp is still well above the clipping threshold of the second set of diodes.  So, the optimal strategy is to use a much higher clipping threshold in the feedback loop than occurs after the chip.  The 2+1 diode complement that Brian popsted is in the right direction, but will not likely add a great deal more clipping since the single-diode half-cycle will not be noticeably re-clipped by the second set of diodes.  A pair of red LEDs in the feedback loop would fit that bill nicely, though.  There is enough gain in the op-amp stage that some clipping due to the LEDs ought to occur, and the LED forward voltage is high enough (about 1.5v) that the output of the op-amp will still be potentially high enough that the second set will clip as well.

Alternatively, if one is building this as a perf or vero improvisation, use a 1458 or 4558 dual op-amp and make a Dist+ equivalent of the Double Muff or Big Muff, with two op-amp/clipping stages cascaded.  Since the gain is set by a resistance to ground, you could easily stick the gain control between the cap and ground (the way the DOD 250 does it), rather than between the inverting pin and cap.  Why is this good?  Because you could stick a 250k pot as the gain control for two sections by tying the wiper to ground and using each leg of the pot as the gain-setting resistance of a different stage.  In this way you could have clipping in only one stage or in both stages, using only one pot.