Different Way to Change Diode Clipping Characteristic

[Rob Strand]

I was thumbing through Jerald Graeme's cool book "Applications of Operational Amplifiers" and noticed how a resistor and silicon diodes were added to reduce the leakage of a zener clamp (fig 4.9).   It got me thinking that the knee was also made sharper since it enforces a significant current to flow through the zeners before the clipper clips.   That's going to sharpen the clipper knee and raise the clip voltage a very small amount.

So I thought we could use that to make a diode clipper clip harder.   An ltspice sim confirmed what was expected to happen.

[Download the original image so you can zoom in on the knee.]


Comments:
- Yes, you need two diodes on each side.
- You can play with the resistor value to tweak the clipping characteristic.
- You can try any diode in any position.  The diodes connecting to the output
   are ones that have a sharpened characteristic.   That means a Schottky at
   output and and Silicon at the input will be completely different to a Silicon
   at the output and Schottky at the input.
- Don't expect big changes.  It's going to be subtle like changing diodes.

[Gus]

Have you simmed this with different value caps in series with added resistor?

[Rob Strand]

Have you simmed this with different value caps in series with added resistor?

No, I just checked out the basic idea in passing (I'm working on something else). 

The added resistor inspires all sorts of variations.  Even wacky things like wiring the added resistor back to the same cap (47n on Ts9) as the gain resistor (4k7 on the TS-9).  For the idea to work you want the output set of diodes to conduct first and relatively hard so many variations are going to undermine that and just work like two diodes without the resistor.

[Gus]

I was wondering if cap coupling the added resistor would cause a leading edge "wiggle", hard then soft clipping and how the harmonics might change.

[Vivek]

Modulatable ?

[Rob Strand]

Modulatable ?

You would have to wrap another scheme around it.

It's only a subtle tweak.  The effect is in the order of changing diodes, so probably not worth modulating.

[amptramp]

An active clamp circuit allows you to set the voltage at which the diode changes from non-conducting to conducting.  This is for units that have antiparallel diodes to ground such as the DOD 250 where an active clamp with variable positive and negative thresholds can give you continuously variable diode voltages.  Add in variable resistance and you can get variable resistivity curves.  If the resistance has some capacitive or inductive element in series, you can avoid clipping at low or high frequencies.  At low frequencies, clipping makes things sound muddy and at high frequencies you get exact harmonics intermodulating with the slightly off-harmonic frequencies coming from the guitar.  Besides, why would you take high frequencies and create higher frequencies?

[iainpunk]

Besides, why would you take high frequencies and create higher frequencies?

to annoy the subjects of most of the profile pictures here?

cheers