CGS drum simulator

CGS18 the CGS drum simulator module is the embodiment of the classic twin-T circuit in a form that is suitable for connection to modular synthesizers. It contains two separate drum sounds, each individually triggered, as well as something unique - adjustable harmonic content.

This updated version contains some minor changes. TL072 are used throughout. Hysteresis has been added to the trigger inputs. All connections are now in a single row, with strain-relief holes provided for the wires.

While untested, the module should work on +/-12 volts.

How to use this module
Connect the inputs to a LFO, a gate sequencer or some other rhythmic pulse source. Connect the outputs to a mixer, or feed them into some relevant part of the synthesizer. Adjust the harmonics from the basic sine tone to something rich in harmonics.

A little on how it works
Each of the two drums consists of three distinct blocks, the trigger, the ringing oscillator and the buffer/distortion amp.

The trigger takes any positive going signal that goes over approximately 3 volts and converts it into a narrow positive going pulse. This pulse is fed to the reference ground of the ringing oscillator, de-stabilizing it, thus causing it to start oscillating.

Depending on the resonance setting, the ringing oscillator output will decay until the circuit again reaches stability with no audio output.

The output of the ringing oscillator is fed past a clipper consisting of a single LED, the current of which is limited by the series resistance of the 50k pot. The result is buffered by the inverting unity gain amplifier.

The 22k/2k2 resistor combination at pin 3 of IC2A and IC3A controls the amount of DC that makes it through to the output of the drum simulators, simulating the impact sound. Reducing the 2k2 resistors to 1k or even down to 680 ohms, will reduce the impact sound. It would be possible to replace the 2k2 resistors with a 1k linear pot (wired as a variable resistor) in series with a 680 ohm resistor so the level of the impact sound could be varied from the front panel.

Construction
Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges.

When you are happy with the printed circuit board, construction can proceed as normal, starting with the resistors first, followed by the IC sockets if used, then moving onto the taller components.

Take particular care with the orientation of the polarized components, the electrolytics, LEDs and ICs.

When inserting the ICs in their sockets, take care not to accidentally bend any of the pins under the chip. Also, make sure the notch on the chip is aligned with the notch marked on the PCB overlay.

There are two 100n 1206 SMT decoupling capacitors on the rear of the board.

To use the wire anchor holes: 1) Trim off the end of a suitable gauge of hookup wire 2) Pass it through the larger (pad-less) hole from below and pull a couple of inches through. 3) Strip the insulation from the end of the wire, twist and tin it. 4) Bend the wire over and pass the tinned part through the associated pad hole. 5) Flip the board and solder the tinned wire to the pad. 6) Pull the excess wire back through the first hole so only a short length remains between the hole and the second pad. Make sure this is the LAST step, or the insulation will peel back from the wire as it is soldered.

Setup
When you power it up for the first time, you will need to adjust the resonance trimpots until the oscillators just stop oscillating. Do this with no trigger inputs.

Parts list
This is a guide only. Parts needed will vary with individual constructor's needs.

Suppliers

 * CGS18 Drum Simulator, revision 1.1, Elby Designs