Thursday, October 11, 2018

Discovered by accident: Low parts count waveshaper/modulator/distortion circuits!

At the bench trying to learn more about inductors--something I've not worked with a lot with before--so bought a few Neutrik NTE1s from Amazon.

It's a 1:1 transformer (making the math easier I figure) and has decent audio specs.  And it's cheap--about $14USD each, for decent audio performance; good for experimenting.

What can we do with this little gadget?

Benchtime! my goal one day is to come up with something cool like the Moog 914 (yeh right, I have a long way to go).  breadboarding up some transformer/cap widgets, got bored, some inductor/transformer/RC/op amp widgets, got bored again, so breadboarded some transformer/zener widgets,  when all of a sudden I heard what sounded like a balanced modulator coming from the my bench's audio monitor.


I looked over my work and realized (after a bit) an op amp I was using as an output buffer was oscillating (unintentionally) and I had wired things backwards, confusing the breadboard inputs with its outputs!!

So instead of 1 in and 2 outs, I had created 2 ins and one out.

I stopped everything and drew out what I had done since i liked the way this accident sounded. it's this:

Almost forgot, output resistor is 1K!

How does it sound?  Well, "It depends".  For low frequencies (say less than 2K) maybe like a cheap guitar distortion pedal.  For higher frequencies, especially when you sweep different sine or tri waves from say 8K down to < 500hz through the 2 inputs, more like a classic synth balanced modulator.  From there I can get all different sounds--fat sounds, thin sounds, odd sounds.  Waveforms beating with each other, with beat frequency changing as I lowered the input frequencies.  Odd.  Not everything sounds good, but some things do.

What waveform goes into in 1 and in 2 makes a huge difference, and what amplitude/what dc offset matters as well.  Sometimes if I didn't like the sound at output I'd switch what was plugged into inputs 1 and 2 with one another, and I'd have a completely different thing going on.

The son of zener above creates about a -3db drop in amplitude, input to output, so I added the x2 gain op amp above....up to 9 parts!  Damn! but if you're serious about making this passive only (no batteries! No power supply!), you probably could do without it.

Figuring I might have stumbled on something? I got out another breadboard and came up with this 8 part variation:

Since I like this one's sound better this is "zener"  other is "son of zener"

From design #2 I got weird beat frequency stuff, balanced modulator stuff, all sorts of things.  For this one the 4x op amp gain stage is necessary since the transformer (? something?) introduces a pretty big amplitude hit.

Anyway, I strip boarded it, and used some simple op amp PCBs I had created for other projects.

Here are the obligatory bench photos--the zener prototypes literally took minutes to build.  If anyone wants to screw around, or knows of other designs like this (I don't, other than zener based guitar stomp boxes--which I guess these are? But zener and son of zener seem to require the transformers to get the odd BM sounds? I couldn't reproduce the exact same timbres with op amps) let me know.

Still to do: make some demo wav files, and panelize this.  UPDATE: Done!  Go here!  Until then, don't breathe the fumes!


  1. Hi - thank you for sharing the ZI Waveshaper. Interesting stuff and well done! Can you tell me what the two circled A's are for in the first schematic?

  2. Just realised is connect A to A. Sorry being a bit dense!


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