A versatile special effects and percussion synthesizer. Design by Robert Penfold

This sound generator can produce a wide range of percussive sounds, including various noise and falling tone effects. The unit can be triggered by either tapping the case, or by means of a +5 volt pulse of a few milliseconds fed to the trigger input. The output level is a few volts peak-to-peak from a fairly low source impedance, and the unit is therefore capable of driving virtually any amplifier, recorder, etc. The unit is self contained with power being provided by a 9 volt battery. The circuit is less complex than one might expect due to the use of a sound effects chip (the SN76477) which gives a wide range of sounds but requires few discrete components.

System Operation

The block diagram of Fig 1 helps to explain the way in which the unit functions. Even if you are not interested in how the circuit works, it is still worthwhile briefly studying the block diagram and this description, as a basic knowledge of the electronics used in the unit makes it much quicker and easier to set up the controls to give the desired effect.

There are three signal sources in the SN76477 chip; a voltage controlled oscillator (VCO), a super low frequency (SLF) oscillator and a noise generator. In this application the two oscillators are both used as audio frequency types, and although the SLF oscillator is designed primarily for sub-audio use, it will in fact work perfectly well at higher frequencies. Both oscillators have a frequency control, and in the case of the VCO this gives a wide operating frequency range of around 100Hz to 10kHz. The second oscillator has a more restricted but useful frequency range. The noise generator has a simple low-pass filter at its output, but this is not needed here and is given a very high cut-off frequency so that it is effectively eliminated. This gives a white noise output signal from the noise generator.

An electronic switching circuit and a mixer select the signal source or sources, and in this circuit three options are available. For simple tone or falling tone effects the VCO alone is used. For metallic "chiming" sounds the VCO plus the second oscillator are used. Normally ring modulation is used for this type of effect, but the simple mixer circuit used in the SN76477 gives what is a good approximation of the sound. For handclap and "wave"-type sounds just the noise generator is used.

The SN76477 incorporates an envelope shaping circuit, but this is only partially utilised in this circuit. It is in fact used as a straightforward voltage controlled attenuator, and the control voltage is generated by a discrete circuit. This uses a simple microphone fitted inside the case to produce a small electrical signal when the case is tapped. This signal is amplified and fed to a simple C-R circuit which produces an attack/decay type control voltage. The attack time is preset and quite short, but the decay time can be adjusted from a few milliseconds to around 5 seconds. The time constant circuit can also be driven from a 5 volt trigger pulse via a separate amplifier circuit. A buffer stage is used to provide a suitably low impedance control voltage signal to drive the SN76477's voltage controlled attenuator. This control voltage can also be fed to the control input of the VCO. This enables simple but effective falling pitch sounds to be generated, with the pitch falling with the decaying signal level.

A lowpass filter is included at the output of the unit. This is a 12dB per octave tuneable type, and it also includes a resonance control which can be adjusted to give a pronounced peak in the response just below the cutoff frequency. Some of the sounds available direct from the SN74677 are not that useful from a musical point of view, but this filter can modify such sounds to give vastly superior results. The filter is especially useful when used in conjunction with the noise generator.

Sound Generator Circuit

The circuit diagram for the sound generator and envelope shaper sections of the unit are shown in Fig 2.

The circuits of the SN76477 require a stabilised 5 volt suply, but the device has a built-in 5 volt regulator that permits operation from a 9 volt battery. The stabilised 5 volt output is available at pin 15.

Both of the oscillators require only a discrete timing resistance and capacitance for operation, and for the VCO these are provided by C3 and R2 plus VR2. The latter is the tuning control for the VCO. A similar circuitf (R1, VR1 and C2) provides the timing resistance and capacitance for the second oscillator.

The noise generator is a digital type based on a shift register. Although digital noise generators can give a rather rough sounding output, especially when subjected to filtering, the circuit utilized in the SN76477 seems to give well above average results in this respect, and also an excellent output signal. The noise generator can use either its own internal clock oscillator or an external circuit, and in this circuit the former is used. The only discrete component this requires is R3. The noise filter circuit contains R4, but as explained previously this circuit is not needed in this application, and the filter capacitor is omitted so that the filter is ineffective.

The required signal sources are selected by taking pins 25 to 27 to the appropriate logic levels. The table shown below details the logic levels which give each of the eight possible output signal combinations.

Only the three signal combinations stated earlier are needed in this case, although you can obviously experiment with the others if you wish. S1 is used to set pins 25 and 27 at the appropriate levels for the selected output type, but no switching is needed for pin 26 as this is low for all three of the required output types. No pull-down resistors are needed for pins 25 and 27 as these components are effectively built into the chip.

Pins 1 and 28 are control inputs, and these determine whether or not the signal is processed by the internal envelope shaper in some way or is simply passed through the VCA to the output. In this circuit the latter is what is needed, and the appropriate mode is selected by placing pins 1 and 28 low and high respectively. Pin 22 is also a control input, and this enables the VCO to be swept either internally by the SLF oscillator, or by an external control voltage applied to pin 16. Here pin 22 is tied low to permit external control of the VCO.

The pick-up in this application, Mic 1, is actually a ceramic resonator, but this works well and provides a reasonably strong output signal. This signal is amplified by IC3 and negative half cycles are used to charge C4. Positive half cycles are prevented from discharging C4 by the inclusion of D1. The components VR4 and R12 provide a discharge path for C4, and VR4 therefore controls the decay time of the control voltage on C4. This signal is buffered by IC2 to provide a low impedance output. The control input of the VCA is at pin 11 of IC1, and this circuit is really a current controlled amplifier. However, by adding R8 at the input the current flow becomes proportional to the input voltage, and the required voltage-controlled operation is obtained. Minimum gain is obtained with a +5 volt input, while maximum gain is produced with a 0 volt input. The control voltage is normally at +5 volts so that the unit has zero output. Operating the unit pulls the control voltage down towards earth potential and produces an output from the unit.

The unit can be activated by applying a 5 volt trigger pulse to SK1. This switches on Tr1 which then rapidly charges C4 via R9 and gives the required low control voltage to produce an output from the unit. The pulse should be a millisecond or so in duration so that C4 has time to charge, but a long pulse should not be used as this would hold the VCA open and prevent the envelope shaper from operating properly.

With S2 open the control input of the VCO is held at 0 volts by R6, and the output frequency is controlled by VR2. However, if S2 is closed, some of the control voltage from the envelope shaper circuit is fed to the VCO's input. This permits the generation of falling pitch sounds, and in this mode VR2 sets the maximum output frequency, and VR3 sets the sweep depth (in effect this sets the minimum output frequency).

Semiconductor IC1 has an output stage which can drive a pair of complementary class B output transistors. However, in this application a relatively high output impedance to drive the filter is satisfactory, and these transistors,are not needed. R5 is used to bias the output circuit of IC1.

Filter Circuit

The filter circuit is based on an LM13600N dual transconductance operational amplifier, and the circuit diagram appears in Figure 3.

This is a 2 pole Butterworth lowpass filter, and the cut-off frequency is controlled by VR7. This gives a wide frequency range which extends from the sub-audio into the ultrasonic range. The value of the filter is greatly enhanced by the inclusion of resonance control VR5. Adjust VR6 to limit the degree of feedback to a level that is not quite sufficient to drive the filter into oscillation. Adjustment of VR7 has no significant effect on the resonance level incidentally, which makes the filter easy to use.

Construction

Details of the printed circuit board are given in Figure 4. The board is straightforward to build in most respects, but the CA3140E used in the IC2 and IC3 positions is a MOS input type which can be damaged by high static voltages. Use IC sockets (8 pin DIL types) for these two components, and do not plug them into place until the unit is in other ways complete. Leave them in their protective packaging until this time, and handle them as little as possible. The IC1 and IC4 SC's are not MOS devices, but as they are not the cheapest of components either, it is probably worthwhile fitting these in sockets as well.

To complete the unit, point-to-point wiring is added. This is shown in Fig 4 and there should be no difficulty here.

Adjustment And Use

There is only one adjustment to be made before the unit is ready for use, and this is to set VR6 to prevent the filter from oscillating. This is just a matter of setting VR5 for minimum resistance (fully clockwise), and then adjusting VR6 as far as possible in a clockwise direction without the filter breaking into oscillation and producing a tone at the output of the unit. Place VR7 at a roughly midway setting when making this adjustment.

Anyone familiar with electronic instruments should have no trouble in getting the unit to produce an interesting range of sounds. Using a fairly low pitched tone, the filter to attenuate harmonics on the output, little or no sweeping, and a medium length decay, reasonable drum sounds can be generated. The noise signal, if used with the filter close to resonance, a medium filter frequency, and a long decay time, gives an interesting "sonar" type sound. A higher filter frequency and short decay gives handclap type sounds. Using the two oscillators some musical interval apart can give bell type sounds. In fact the unit has a large repertoire and these are just a selection of the vast range of sounds available.