Inside the Collection

Beginning Electronic Music- part 4 Voltage-Controlled Oscillator / Ring Modulator

Of the objects that the Powerhouse Museum collected from the Tristram Cary estate there were several that obviously pre-dated the EMS gear and which, by my guess, were built in the early to mid 1960s.

One of these is an early version of a Voltage Controlled Oscillator (VCO). In its original state it was hand mounted on a black metal panel (approx 40 x 45cm) which has notches cut into it to allow it to be mounted in a standard 19-inch rack. There are numerous controls on the panel. Across the top it has two rows of 12 pre-settable potentiometers (24 in all) that are wired to a 30-position rotary switch, which can sequentially select from each of the presets. The potentiometers lack knobs and are set with a screwdriver so that once set the user would tend not to change that setting. Below the potentiometer pre-sets are two potentiometers with large hand-sized knobs that would give some measure of fine control and whose wipers are also applied to the 30 position switch. These control knobs are mounted on a rectangular panel with a white surface on which there are traces of control position markers written in wax pencil. The rotary selector switch also has labels for a further four external control voltage inputs.

Voltage-Controlled Oscillator / Ring Modulator
The front of the voltage controlled oscillator, PHM collection [2009/83/1
Obviously with the pre-set potentiometers and using the rotary switch the composer could step through a sequence of voltage levels each of which is sent to the oscillator control voltage input. Thus we have a device for which the output frequency of the oscillator could be left set, or operated as a kind of manually timed analogue sequencer, or swept through a frequency range, as the composer desired.

To the right hand of this central panel there is a voltage level meter. Below the white panel are an external control voltage input, a two position switch to select between internal and external control voltages, and the controls for the oscillator. The oscillator has three output jacks for square, triangle and sawtooth wave outputs plus a fourth summed output. Its design may be as a relaxation oscillator that generates a sawtooth waveform which is then put through wave-shapers to produce the triangle and square wave outputs. Each shaped output is decoupled from the supply voltage, sent to an amplitude setting potential divider and thence to the output jacks marked DC outputs. These outputs could then function as control voltages. The oscillator outputs are also summed together to produce an AC output signal.

Back of the oscillator
The back of the oscillator. You can see the printed circuit boards of the oscillator and the prototype boards of what may have been the ring modulator.

The oscillator section consists of a pair of printed circuit cards. On one is the oscillator and on the other is a hi-fi pre-amp style tone control (treble and bass cut and boost). Both cards were manufactured by Martin Electronics of Middlesex, (London, UK) and were presumably sold as kits ready to be wired in to an amplifier or test instrument project. These were presumably purchased ‘off the shelf’ and built onto the panel with the voltage setting controls, by Cary. The tone filters appear to be primarily passive and are set between the summed AC output of the wave-shapers and the output jack, giving a further level of wave-form shaping. The output must have been of rather high impedance. If anyone can tell me more about the Martin audio kit products I would be very interested.

Block Diagram of the VCO/Ring Modulator from Tristram Cary's Adelaide Studio
Block Diagram of the VCO/RingModulator from Tristram Cary’s Adelaide Studio.

Cary has labelled some of the controls on the panel with dymotape to indicate their purpose and has run arrows in strips of white tape indicating the connections of the circuits and the signal flow between them. Many of the controls are marked with coloured paper dots to enable the user to distinguish which controls appear at which outputs or contribute to those output signals.

This device can be seen in photographs of Cary’s Fressingfield studio (see the picture in part 2), and in a photograph in his paper on Electronic Music published in Audio Annual 1971 [1] it is labelled as being a Double Ring Modulator. The object the Powerhouse Museum received is missing the large knob that switches through five positions labelled 1 – 5. The connections on the back of the switch have been cut and the whole unit appears to have been modified at some point. On the back of the panel are a pair of circuit boards which have a hand-built circuit of presently unknown purpose although these may have been the ring modulator circuit. Sadly I haven’t had the opportunity yet to reverse engineer these custom-built boards so that I can work out what the circuit actually does, but the paired transistors suggest some sort of modulation device.

This ring modulator cum voltage controlled oscillator cum sequencer and sweep generator is a really interesting example of the kind of carefully considered purpose-built devices that were used in many of the studios where electronic music developed. It is a one-off. I don’t know what pieces of music is was used in (more work to do there) and though it was probably built in the mid-1960s it was still in use when Cary added his VCS3s to the studio, and he brought it to Australia when he moved here in 1974.

Stephen Jones

[1] Tristram Cary, ‘Electronic music – background to a developing art’, Audio Annual 1971 , (pub. Hi-Fi News), pp. 42-49.

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