Third diode on potentiometer for blending asymmetry?

[Asymmetric]

I've had an idea. Instead of a switch which adds a third diode for asymmetrical clipping, what about adding a potentiometer instead to blend the third diode so you can have varying degrees of asymmetry? I'm sure that this is possible, it's just that I wouldn't know how many ohms the pot should be. So, say that I have two BAT43's so symmetrical clipping, I could have something like a BAT48 connected to pins 2 and 3 of a potentiometer?

[slacker]

Yes you can do that, the sixth example here http://www.muzique.com/lab/warp.htm looks like what you're talking about. I'm not sure about pot values but I'd guess that 10k would be big enough.

[Mark Hammer]

Done it with two diodes and a pot.  One diode always in circuit and the other dialed backwith a 10k pot.  Same difference.  Worked well.  In fact, I sold it to one of Canada's hottest award-winning blues players (from Monkeyjunk), because he liked it so much.

EDIT: The configuration I "warped" was a 1N914 pair in a TS-808 clone.  With a higher Vf than the BAT schottky types, the degree of asymmetry would be somewhat less than what one would expect from a schottky conducting one half cycle and the other barely conducting.

[ashcat_lt]

Again, though, this only really works correctly (actual assymetric clipping for more than a brief period of time) when the clipper is not floating around at the end of a capacitor - or worse between two.  It really should be DC coupled to whatever gain stage is driving it, which means that one leg of the clipper is going to have to go to the positive rail instead of ground.

[Asymmetric]

I'll have to try it! I guess another thing that you could do is only have two diodes in the clipping section, and one of them on a potentiometer so that when you turn the pot down you're left with only one diode, which would give asymmetrical clipping.

[Mark Hammer]

The "brake-pedal" model of op-amp functioning.

Op-amps are designed to provide ridiculous amounts of gain in their "open-loop" form.  Think of them like cars designed to have the gas-pedal permanently floored.  The feedback loop allows for the user to specify the amount and kind of negative feedback from the output.  That negative feedback is like the brake pedal, such that you control the gain of the op-amp by how hard you step on the brakes..  Higher feedback resistances are analogous to lifting your foot off the brakes.

Normally, when a diode conducts, it functions like a low resistance path.  In the case of feedback diodes in a TS-type configuration, when the forward voltage is reached at the output,  the negative feedback from the output is essentially coming through the diodes, rather than the feedback resistor, dropping gain momentarily (more negative feedback = lower gain).  That sudden momentary drop in gain is what we call "clipping".

When additional resistance is placed in series with one of the two diodes, there is still some suppression of negative feedback when the forward voltage of that path is reached.  So, gain is dropped for that diode without the added resistance, but not nearly as much for that portion of the signal passing through the diode with the added resistance.

In general,  clipping effect can be varied with resistance added in series with one or both diode paths.  Vary only one of the paths and you get a more asymmetric clipping effect.  I highly doubt it is comparable to the lore about tubes, but it IS musically useful.

Note that reducing the clipping effect - even only on one 'side' - will add more dynamics to the output and greatervolume level.

[ashcat_lt]

I guess I assumed this was a diode-to-ground clipping arrangement.  I guess the OP didn't say, but slacker's link...

TS will do asymmetry I think without any changes, but a Rat won't.