A possible new clipper scheme

[Rob Strand]

For good or bad, I'm just putting this up for those who want something to play with.

There's a circuit which has been around for 50 years or so which is normally used in voltage and current regulation applications.

The circuit basically puts a resistor between the collector and 'output' terminal
https://wiki.analog.com/university/courses/electronics/text/chapter-11#the_zero_gain_amplifier
With some possible variations,
https://wiki.analog.com/university/courses/alm1k/alm-lab-7

The trick is how to use it.   

Taking  "Figure 11.15 NPN Zero Gain Amplifier" as the reference schematic.  The clip voltage appears between the collector and the emitter.

The resistor R1 is not part of the clipper it is more part of the surrounding circuit.   For example like the resistor SOS.R2 on the Son-of-Screamer circuit.
http://www.muzique.com/tech/scream.htm

The clipper would connect the emitter of Q1 to the output terminal.

We don't want to connect the base to opamp's inverting input as this will make the clipping behave like a VBE diode.   So the other option would be to connect Q1's collector to the opamp's inverting input and then include RL as part of the R1 circuit (That's SOS.R2 on the Son-of-Screamer circuit.)

The NPN circuit only clips in one direction.  So for say a symmetrical clipper (more or less) we use a PNP transistor, again with the emitter connected to the opamp output.

We can also use the circuit in an MXR Distortion+ type output clipper circuit. Here the R1 resistor is used as the 10k output resistor before the clipping diodes.

http://www.muzique.com/schem/mxrdist2.gif

One key feature is the resistor RL is used to control the hardness of the clipper.

By using some of the variant circuits, or perhaps adding silicon or Schottky diodes (whatever) in series with the base or the emitter we can further tweak the clipping behaviour.

There maybe other connections and variants which sound good/better.

I won't be playing with this circuit in a pedal myself for some time!

[Fancy Lime]

I have been toying with a similar idea but never tested it: a JFET with G and S tied together in series with the audio path and a resistor to ground after it, for example after an opamp output. Since that JFET limits the current, it forms an amplitude dependent voltage divider with the resistor. It is essentially the complement to a series resistor plus diodes to ground. The big practical problem is the large Idss variation of JFETs. So not really suited for production circuits but may be interesting for one-offs. It would also only be practical with fairly low Idss JFETs and the resistor to ground should probably be a pot. On the plus side, we can make this clip asymmetrically by inserting a resistor between the S and G. With a pot there, we can adjust the asymmetry.

Andy