Nasty metallic noise in diode clipping circuit

[Zoomby]

Hi everyone,

did someone also notice the following annoying effect?
I breadboarded a circuit which is similar to a MXR distorion+. The opamp is a TL072. I used germanium diodes (1N949) and also tried silicon ones (1N4148). The opamp gain is about 100 to 200. Th signal is lowpass filtered (about 800Hz) after the opamp.

When a single guitar note (especially high notes) decays and reaches the transition between overdriven and clean, one can hear a nasty metallic noise. It sounds a bit like a ring modulator or a fast tremolo. When the string is played very softly the effect is very dominant. If the opamp gain is low the effect seems to be gone, but the signal still gets slightly overdriven. Both diode types are affected.

If the diodes are in the opamp feedback path the effect seems to be weaker. The string decay sounds much smoother.

Is this the normal sound of MXR Distortion-type or Proco Rat-type distortions? Or is something wrong with my circuit?

[karbomusic]

Sounds like intermodulation distortion splatting in and out as the signal rides that fine line between clipping/non-clipping. You might try different opamps, the RAT if I remember used an LM308 which had a horrible slew rate comparatively. You might also add some light series resistance before the diodes to see if that helps any (couple hundred ohms or so).

That being said and in my opinion, you are in exactly the right place to be fine tuning a distortion circuit to sound good. Most all of them sound great cranked, few sound so good when riding that line between clean/dirty and more than one or two notes simultaneously... that's where all the hard work is, again in my opinion.

[blackieNYC]

You had me at Nasty Metallic Noise.
But, yeah - sometimes that's not as much fun as it sounds.  Does Ge sound better than Si?  Clips sooner, so Karb's fine line is much lower volume-wise..  Also to his point -  The MXR dist+ was a 741 op amp. Maybe the TL072's "speed" is a factor?

[Zoomby]

Thanks for responding,
intermodulation distortion was a good hint. The metallic noise was simply hum/buzz interacting with the signal. A kitchenfoil as shield connected to ground solved the problem. 

[karbomusic]

Thanks for responding,
intermodulation distortion was a good hint. The metallic noise was simply hum/buzz interacting with the signal. A kitchenfoil as shield connected to ground solved the problem. 

Duh, didn't even think of that even though I've experienced it from time to time. Good find.