PS301 - Diode Wavefolder

by Antti Pussinen and Wolfgang Spahn

Ps301 Diode Wavefolder is a dynamic waveform processor module, that changes the color and the overtones of a sound, while keeping the same base note. The design is originally Jürgen Haible's take on “Serge Wave Multiplier, middle section”. Yves Usson of YuSynth added an exponential VCA to the design, making voltage control possible, and we modified the circuit further to fit our design requirements.

The module takes in a 10Vpp sine, triangle or saw wave and folds this waveform back into itself up to 7 times at the crossover point of the wave.The effect sounds a kind of mix of frequency modulation and mid resonance filtering.

The Controls Include “CV attenuation”, “Fold Range” (which can also be contolled with 0-5v control voltage) and “Fold Amount” which changes the amount of folds. Inputs and outputs conform to eurorack standards of 10Vpp, and it runs on eurorack +-12v power supply, and even with +15v power supply from Frack Rack or MOTM synthesizer systems.




Theory of operation

A PS-301 Wavefolder adds overtones to a waveform, a bit like a distortion unit or a resonant filter. But instead of clipping or ringing, when the waveform peak goes over a certain limit, it folds back and creates a new odd harmonic component. Jürgen Haible had a really good explanation of the process on his site, and since its impossible to know how long the legacy site is kept up, i copy it here:

A string of diodes is driven with the amplified and level shifted input signal. After each diode there is a “tap” with a resistor that will contribute positive or negative to the output of a two-opamp subtracting amplifier. There are two virtual GND nodes for these summing and subtracting operation. By feeding these virtual GND nodes alternately from the taps of the diode string, you can have the output voltage increased when one diode drop voltage is reached, and have the output voltage decreased again when the next diode in row starts conducting. The number of valleys and peaks is only determined by the length of the diode string, and not by the number of opamps. It turned out that choosing equal resistor values results in an almost regular pattern of the voltage transfer function.”



Audio & Video


Schematic


Board


Faceplate


Parts

Resistor
470R 1
680 1
2.7k 1
5.6k 2
6.8k 1
10k 2
15k 11
22k 2
27k 1
33k 2
56k 2
68k 1
100k 7

Semiconductors

1n4148 20
TL074 1
TL072 1
2N3904 3

Socket

14p ic socket 1
8p ic socket 1
16p IDC male 1

Potentiometers

100k poti 2
50k poti 1
1k multiturn trim 1
50k multiturn trim 1

Other

pinstrip 90 deg 4 pins
Thonkiconn 3.5mm jack socket 3


Scope Pictures

Input Waveworm: SAW