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CGS31 - Digital Noise | Cat Girl Synth

CGS31 - Digital Noise | Cat Girl Synth


This module is a very standard pseudo-random digital noise source for DIY modular synthesizers with a few enhancements.

This is a PCB only suitable for multiple formats

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Availability:In stock

This module is a very standard pseudo-random digital noise source with a few enhancements. Instead of running a fixed high frequency clock, a VCO is used instead, allowing for unusual sweeps, and for reducing the speed right down to a series of random pulses. The internal linear VCO can also be bypassed so an external source such as a 1V/Oct. VCO or LFO can be substituted. It has pink and white noise outputs, and two separate (unique) digital outputs for use in triggering other circuits.

Additionally this version has an 8 input DC mixer and a small power distribution board included. The board has been made in such a way as to allow the latter to be cut off, making the board suitable for mounting behind Euro and Frac Rack panels.

How to use this module:

Connect the CV inputs to a voltage source, or wind the speed input up to maximum. Take the required noise source from the appropriate output. The two LEDs indicate the activity of the two digital outputs. When an external CV source is used, the speed pot acts as an input attenuator. Adjust this pot for the desired effect. Alternately, feed an alternate clock pulse in to the external clock input.

A little on how it works:


The schematic of the Digital Noise.

The circuit consists of several distinct blocks. The first is the internal VCO and its associated parts. From the left, there is a voltage follower then the VCO itself, which is part of a 4046 phase locked loop.

The second circuit block is the timing generator. At the left, there is a comparator based around one half of a TL072. This is there to convert any external clock signal into something suitable for the remainder of the circuitry. With the values given, the sensitivity is set at around 2V, allowing triggering from signals with a +/- 10 volt swing, or with a 0V to +10 volt swing, both of which are common in modular synths.

The output of this comparator is fed into the clock input of the shift register, a 4006.

The exclusive OR gates form a complex feedback path and are responsible for the generation of the pseudo-random pulse train. The 1M and 0.47 mfd capacitor are there to start the sequence.

Several outputs are taken from different points of the feedback path, two being buffered use as digital outputs, the remaining being filtered to produce both pink and white noise outputs.

There are many similarities between this design and the ETI 4600 noise source, as this was developed from my heavily modified ETI unit.

The mixer is a standard, two stage DC mixer as per CGS04.

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Brand Cat Girl Synth (CGS)
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