Am I seeing things?? (diodes won't clip)

[Skidood]

Hi all,

I built a DOD Overdrive 250 the other day. The schematic from GGG's site was used. In essence, it is nothing more than an op-amp set up to deliver a sizable gain, and the output signal is fed across the tops of some clipping diodes that are tied to ground.  The weird thing is, the op-amp starts to clip before the diodes do.  With a DC supply of 9 volts, the output signal would start to clip at around 3 volts peak-to-peak.  The diodes had nothing to do with it.  I used a signal generator to feed the circuit an input signal, and increased the input voltage until the signal coming out of the op-amp started to clip, then backed it off a bit. Using a breadboard and an oscilloscope, I could see that when I insert/connect the diode(s) onto the AC output signal, with the other end to ground, they would not clip at all !!
I tried good ole 1N914's as well as a variety of LEDs. 

Then I cranked up the DC power supply to 32 volts (absolute max for the op-amp)  then cranked up the input signal till I was measuring 11 volts peak to peak. on the output.....and STILL the diodes would not clip the signal.

What the heck???

[JasonG]

Thats crazy. I guess you have done this already but, have you checked your connections? Breadboards can go bad maby something is not really making contact.

[Skidood]

OK  I figured it out..you have to put a fairly high resistance between the op-amp output and the diodes.  It will create a current limiting effect, when the diodes conduct. Only then do you see the clipping.  Duh...
The DD Overdrive 250 used a 10k resistor..I guess I should have tried a 20K or more.

[petemoore]

  welcome news, I'm not the only one who has experienced this.
  I have a DIST+, and have diddled with the diodes, when they 'do work, noticably clipping occurs only when they're in'.
  It has to be inexplicable, a ghost, or some other .. more tangible explanation.