VDRs/Varistors for distortion?

[Andreas]

I just read something about Voltage Dependent Resistors, also called Varistors. These vary their resistance depending on the voltage across them, so I though, hey this could make a distortion device. Has anyone tried this or any idea if it might work, apart from the obvious problem that those are usually designed to work at higher voltages than our typical 9V.

My idea was basically to make a voltage divider from a resistor and a varistor, taking the output from the varistor. As the signal goes up, the resistance of the varistor goes down, so the increase of voltage is lowered and this might look like a soft form of clipping. Who knows, perhaps this does work, perhaps it doesn't at all...

[Paul Perry (Frostwave)]

To get over the voltage problem, you could have a couple of audio transformers to jack up the voltage & back, perhaps.

[Peter Snowberg]

Today those buggers get called MOVs and you find them in almost every "power strip" that claims to supress surges.

Magnatone used them to generate phase delay to produce the only tube amps with real vibrato, but I've never seen them used for distortion before.

At Digikey, they have 2.5VAC/3.5VDC varistors for $0.56 in singles:

http://www.digikey.com/scripts/us/dksus.dll?Detail?Ref=29055&Row=184512

The only problem is that those come in 0805 surface mount packages. The 1206 package costs more than twice as much, and they don't have anything with that low of a voltage that has leads. If you don't mind higher voltages, the options open up.

Happy hacking! I can't wait to hear what you find.

[drew]

http://www.musicsynthesizer.com/Waveshapers/movs.html

This stuff seems to be optimized for higher signal levels than what goes thru guitar pedals.... but who's to say you can't use a fourth of an opamp to boost the level? Or a J201 booster, or similar. (The Snarling Dogs "Very-Tone" has a 2SK30-based booster which has about six parts, and it gets pretty loud & clean...)

[drew]

Oh, FWIW, I suppose in that last schematic there's a transformer, good call Paul.

[puretube]

Today those buggers get called MOVs and you find them in almost every "power strip" that claims to supress surges.

Magnatone used them to generate phase delay to produce the only tube amps with real vibrato, but I've never seen them used for distortion before...

Happy hacking! I can't wait to hear what you find.

impatient?

[R.G.]

As in all things, there is a devil hiding in the details of varistors.

Varistors have different amounts of resistance change per volt. Modern varistors have been carefully designed to have almost no conduction at all until some threshold is reached, then to snap violently into conduction. They are this way to clamp over voltages, much like a zener diode tripping over. The distortion with one of these is not at all soft - it's abrupt and sharp.

Older voltage variable resistors did have a softer characteristic. The ones in the old Magnatone amps that did the vibrato were the soft kind. You need voltages in the 50-100V range to get appreciable changes in resistance on these.

I don't know if the soft-curve MOVs are being manufactured any more. I've looked for them and can't find them.

Like germanium transistors, just because it's a MOV doesn't mean it's a good MOV.

[Paul Perry (Frostwave)]

Low-voltage 'soft' varistors did exist back in the Old Days, because I remember reading about them being used in making non-linear function generators for the old analog computers. But I don't even see them in surplus lists.
You can make something that feels like a varistor - though a lumpy one - from enough diodes and resistors, but that's what we do already..
Here's an idea for someone with an enormous amount of time and sharp ears: maybe -just maybe - there is some IC somewhere that just happens to give the right voltage/current curve between two of its pins..... you never know....!!!!

[Nasse]

Yeah I remember seeing lightbulbs used in somewhat similar way, but dunno

[DDD]

Yes, tiny lightbulbs are widely used to stabilize output voltage of sinewave oscillators. In that case they behave as inertial non-linear resistors.
They also can be used for low-distortion compressing of signals.
JFET channel (drain-source conductance) is also a non-linear resistor, but who knows what to do with the gate terminal in that case? Connecting gate to the source or to the drain makes JFET to be the "current source" - well-known JFET application. Leaving the gate terminal "free" makes the JFET drain-source conductance to be extremely sensitive to electric charges, air humidity etc etc.
Low-guality ceramic capacitors are a some kind of non-linear capacitors as well as high-Ohm resistors, but they are very unstable from this point of view.
Old carbon resistors are non-linear ones, too ("carbon resistors make the tubed amp sound better"). But their non-lineariy is negligible and can be achieved only in the high-voltage circuits.
"Exotic" diodes such as PIN-diodes and UHF diodes can be also tried as non-linear resistors, but these things are very complicated.

[puretube]

A while ago, these were for sale...