The board described provides a very low cost method for producing the more common sounds associated with automatic rhythm generators. The nine instruments fit on a single, six-inch square PCB and the total component cost is less than £20. Facilities are provided for accenting, and the unit can be driven from a rhythm generator IC or a control system such as the Electric Drummer, or it may be connected to a microcomputer.

The Resonant Sounds

The instruments producing a sound which may be represented by a decaying sinewave are:

Low & High Tom-Toms
Low & High Bongos
Bass Drum
Wood Block

The decaying sinewave can be produced by applying a pulse to a resonant tuned circuit, i.e. a capacitor and an inductor connected in parallel. However, to produce the sorts of frequencies we require (down to 50Hz for the bass drum) the necessary inductors would be bulky and expensive.

For this reason, the circuits employ a very simple gyrator. Whilst the theory of operation of gyrators is too lengthy to explain here, one common application is to make a small capacitor look like a large inductor. For example, in the "block" circuit C30 is the small capacitor, and IC10, R59 and R60 complete the gyrator. This apparent inductor is in parallel with C29 to form the resonant circuit. C28, R58 and D12 ensure that the circuit is only triggered on negative edges of the input signal.

To accomplish the accenting, a CMOS OR gate is used. A low level on the input to the gate will allow trigger pulses to pass through it, causing a higher voltage "spike" on the gyrator input and therefore a greater "jolt" to the resonant circuit.

The following hints should help the constructor to develop different resonant sounds: the ratio between R56 and R57 controls the level of accenting. C29 controls the frequency, R59 and R60 control frequency and damping. It is possible, when using very low values of R60 and high values at R59, to get the circuit to self-oscillate. As this situation is approached, the circuit becomes less stable and more sensitive to noise. This is the case with the wood block sound, and for this reason the use of a low noise op-amp such as the LH0042C is recommended for IC10.

When developing sounds on the PCB, insert Veropins into the holes in that section of the board as continuous desoldering can cause the tracks to lift off.

The Noise Sounds

Three sounds are provided which are basically noise: Cymbal, Snare and Hi-hat. The SN76477 complex sound generator provides a cheap and reliable digital noise generator. By changing the voltages on its control pins the IC can be made to produce frequency modulated output, and by modifying the board wiring slightly sounds similar to the Syntom can be obtained.

In the cymbal circuit the time constant C2 x R6 determines the length of the cymbal decay. If C2 is too large, the current required through TR1 is too great and a slow attack results.

The hi-hat is essentially the same as the cymbal, except that a bilateral switch (IC4) is used to discharge C10 rapidly when its gate input goes high. This simulates the operation of the hi-hat foot pedal.

If it is not required to have the hi-hat open facility, the IC4 may be omitted and pin positions 1 and 2 shorted together. Similarly, if the accent feature is not required the three 4071 chips can be omitted. In that case it is recommended that the input resistor in the resonant generators be changed for a value equal to the parallel combination of the input resistor and the accent resistor to obtain maximum dynamic range.

Construction

A double sided PCB will be available for this circuit — see parts list. This has been laid out so that nearly all the through connections may be made by soldering component leads on both sides of the board. Only a few through pins are needed, and these are shown in Figure 2.

The output voltage is variable, and the signal is suitable for feeding into an amplifier, PA system, etc. But beware — the bass drum output is very powerful, so start at low volume.

The trigger inputs are negative edge triggered. If you are driving the system from 15V CMOS or other +15V triggers, use +15V supplies. Note that the hi-hat open/close input should only go between 0V and +5V.

If you connect to TTL levels (e.g. from a microcomputer port) you should drive the board from +5V supplies. Remove TR7, R73, C33 and D13 and wire the collector and emitter of TR7 together. But be careful not to connect +15V again without reversing the procedure.