Variable clipping diode circuit

[FSFX]

This is a schematic for a distortion pedal where you can continuously vary the clipping level of the diodes over quite a large range.
I am not sure whether this particular method has been used in any other pedals. So apologies if I am doing just what others have done before me.

[johngreene]

This is a schematic for a distortion pedal where you can continuously vary the clipping level of the diodes over quite a large range.
I am not sure whether this particular method has been used in any other pedals. So apologies if I am doing just what others have done before me.

I did a design similar back in 1999 only I used transistors. I later replaced the center 10k resistor with a pot. I think quite a few years ago Gus Smally posted a circuit about a 'programmable diode' that can also be used in the feedback loop of an opamp. Same principle using a transistor with a voltage divider on the base to change the threshold.

[johngreene]

This is a schematic for a distortion pedal where you can continuously vary the clipping level of the diodes over quite a large range.
I am not sure whether this particular method has been used in any other pedals. So apologies if I am doing just what others have done before me.

Here is something I was experimenting back in 2009 that is similar to yours. The 'R25' mentioned in the note on the schematic is in reference to my previous schematic posted. This circuit replaced R25 with a variable threshold and the additional transistors allowed the clipping threshold to go all the way to zero.

I did some work on designing a pedal that gradually raised the clipping threshold as the note decayed thinking it would have a dramatic effect on the distortion as you really dug into the strings. Problem is that as you lower the threshold you also lower the volume. So it needed a reverse control of some kind that would dynamically adjust the volume to compensate for the threshold level. By then it just got too complicated and I shelved the idea.

[johngreene]

This is a schematic for a distortion pedal where you can continuously vary the clipping level of the diodes over quite a large range.
I am not sure whether this particular method has been used in any other pedals. So apologies if I am doing just what others have done before me.

This circuit can be used in the feedback connection of an opamp and you can set the threshold to be whatever you want. This one clips at +/- 2V.

[m4268588]

more stupid.
DC fluctuates when cap charged.

https://electronics.stackexchange.com/questions/60927/what-does-this-circuit-do-op-amp-with-transistors-in-feedback
Rubber diodes can be NFB loops.

[amptramp]

There is a diode clamp circuit with the diode inside the feedback loop of an op amp, so the diode breakover voltage is set by the level going into the non-inverting input and the diode forward voltage is reduced by the gain of the op amp:



This gives one polarity.  Turning the diode around would give another.  A dual clamp would look like this:

[FSFX]

There are many reasons why I did it the way that I did in the circuit I posted.

1.  The use of a differential output pair of op amps ensures a low impedance for diode reference voltage.
2.  The reference voltage tracks to ensure symmetrical clipping as it is adjusted.
3.  Changing the gain of the inverting part of the op amp arrangement allows you to vary the symmetry of the clipping.
4.  The control voltage input can be modulated with an LFO to create dynamic changes to the clipping such as a tremolo effect.
5.  The control voltage can be varied from an external source or an expression pedal or even using a digital pot and microcontroller.
6.  It retains the knee characteristic of the diodes whilst allowing the clipping level to be adjusted.

No other design I have seen offers such.

[johngreene]

There are many reasons why I did it the way that I did in the circuit I posted.

1.  The use of a differential output pair of op amps ensures a low impedance for diode reference voltage.
2.  The reference voltage tracks to ensure symmetrical clipping as it is adjusted.
3.  Changing the gain of the inverting part of the op amp arrangement allows you to vary the symmetry of the clipping.
4.  The control voltage input can be modulated with an LFO to create dynamic changes to the clipping such as a tremolo effect.
5.  The control voltage can be varied from an external source or an expression pedal. 

No other design I have seen offers such.

No other 'published' design. 

[FSFX]

Doesn't that holds true for everything. That is why innovators employ researchers when filing patent applications 

[johngreene]

Doesn't that holds true for everything. That is why innovators employ researchers when filing patent applications 

A taste of some of the work I did along those lines.

[FSFX]

A taste of some of the work I did along those lines.

Nothing is really new or innovative anymore in electronics. That holds true for virtually everything in the pedal world where most designs have their origins in circuit designs and principles dating back to the 1940 and 1950s. They just get modified and reborn using modern components.

[johngreene]

A taste of some of the work I did along those lines.

Nothing is really new or innovative anymore in electronics. That holds true for virtually everything in the pedal world where most designs have their origins in circuit designs and principles dating back to the 1940 and 1950s. They just get modified and reborn using modern components.

Some of us have been at it a really long time too. 

I remember back in 1999? or so, when I posted the idea of using a MOSFET as a diode clipper because it clips along 3rd order curve like a tube, not 2nd order like a typical silicon diode and the next thing I know there are a bunch of pedal designs coming out using MOSFET-diodes as clippers. Point being, should you come up with something new and innovative it will become common place so quickly that the 'newness' vaporizes almost instantly.

[FSFX]

Point being, should you come up with something new and innovative it will become common place so quickly that the 'newness' vaporizes almost instantly.

I have never claimed that anything I post is new or innovative.

All the circuits I design are based on things I was taught many years ago during my technical college electronics education and my formal training. Also the many seminars and manufacturer courses I have attended taught me how their professionals designed their circuits. That is why I said what I did in my initial post about others doing similar things and why, growing up in the early days of transistors, I know just how much innovation took place at that time which many here are just rediscovering.