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Sprouting Fuzz part II – Clamping a Signal

A clamp in the feedback circuit.

A clamp in the feedback circuit.

In the last post we added distortion by simply over-driving an amplifier to the point where it clipped, or squared-off the top of the waveform. It basically said…”that’s all I can do… I can’t do no more”. In this post and video I show two simple op-amp circuits where a diode is installed in the circuit to limit the signal above or below a certain voltage and also cuts off the signal. However, things can become very interesting in these circuits and I will give you a direction to experiment should you decide to “nerd out” as I call it.

There are two ways you can experiment.  One way is to use Qucs and try a lot of things quickly.  The second way would be to buy a few fairly inexpensive components and a bread-board and try it.  (See my post: Equipment and Supplies to by for Electronics Experimentation)  If I had the time, I would probably do both.   The first circuit I show in the video is an output clamp.  This is achieved by simply installing some diodes from the output pin of the op-amp to ground.   Once the output voltage is greater than the forward bias voltage of the diode the diode will effectively “short out” the excess voltage.   There can be many ways to experiment with that circuit.  For example:

  • What would happen with a different diode?  Possibly an LED or a Zener Diode or a germanium instead of the silicon diodes I used.
  • What would be the effect of resistors in series with the diodes?

I guarantee you can get some very interesting wave shapes and sounds.

Output from Diodes in FB Circuit

Output from Diodes in FB Circuit

The 2nd circuit is already very interesting.  The diodes in parallel with the feedback resistor do not simply cut off the top of the output waveform but allow the output to remain very rounded.  The reason for this the forward bias voltage is not just a simple point where the diode switches on.  That point actually follows an exponential equation and the voltage increase is not linear as the current increases.  The current flowing through the feedback circuit is controlled by the output voltage and the gain resistor.  Things get kind of complicated!!! And complicated things create a complicated waveform.   Since this waveform does not have “sharp” edges it probably contains few high harmonics and is probably much less buzzy sounding than the other ways of creating distortion.   Notice the word probably was used a lot.  I have some more to discover about this myself.

Again there are lots of experiments that could be done with this circuit.   Many are very similar to the ones described above for the output clamp.   Especially interesting would be to try different types of diodes and various resistors in series with the diodes.   Series-parallel resistor networks with the diodes could be very interesting.

If you happen to be a musician, you probably do not want the voltages we are generating at the output and probably would want to add a voltage divider at the output so the voltage to the next stage, the actual guitar amplifier would be limited to probably around 0.5 V at maximum.  (I have read various numbers for the output of a guitar, so again this would require some experimentation.)   If you are a musician, the actual wave shape is not what you are interested in.  The quality of the sound produced is your goal.

The video associated with this post is embedded below:

As always I hope you find this post enjoyable and entertaining and well worth your time.

Gary

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