The Delta Seven

The Delta Seven Monophonic Synnhesizer
The Delta Seven is basically your standard analog monophonic synthesiser. It was built in the early eighties, and started its life as a subset of the Practical Electronics Minisonic. Over time, various modules got replaced with more up-to-date units, and the only remaining parts of the original design are the high-frequency keyboard gate generator and a bit of the keyboard controller circuit board. The machine has lost a bit of its originality in the process, but has gained a lot in reliability and is still nice to have around. For quite a while, it was the only synthesiser I had.

The Delta Seven architecture is quite standard, and contains the following modules:

Two Voltage Controlled Oscillators.
One -24dB/octave Voltage Controlled Low Pass Filter.
One VCA.
Two ADSR Envelope Generators.
One wide-range LFO with saw up/down, triangle, square and two random waveshapes.
One keyboard controller for a three-octave keyboard, incorporating a portamento circuit.

The machine is completely analog, and based on the Curtis CEM 3340 oscillator chips and the Curtis CEM 3320 filter chip. Sound-wise it is therefore not too far from a Sequential Circuits Pro-One, although the Delta Seven has some interesting modulation capabilities not found on other machines:

Variable synchronization between oscillators, where the sync level can be continuously varied from zero to conventional hard sync. This means you can get some odd "half-synced" sounds, as well as syncing a lower-frequency oscillator to a higher-frequency one.
Oscillator frequency modulation - FM from oscillator 2 to oscillator 1.
Filter modulation by oscillator 2.
A special semi-random waveshape on the LFO which creates a repetitive stepped waveshape by sampling an internal sawtooth oscillator within the LFO.

Interfacing is extensive, and apart from the standard audio output and CV/gate inputs and outputs, there are audio and modulation inputs, as well as outputs from the filter envelope generator and the LFO for use with other synthesisers. (And not only synthesisers: using the Delta Seven and an oscilloscope, I have successfully managed to set the high-frequency bias of several tape recorders, by sweeping the filter (set to self-oscillation) with a sawtooth wave from the LFO, which at the same time feeds the oscilloscope X input.)

The secret to putting together an analog synthesiser is not so much in being good at massive circuit design, but rather taking the best bits of other machines around and combining them. For instance, the envelope generators originally appeared as an article in the January 1979 issue of Practical Electronics. Using this as a base, and incorporating a VCA based on the 1973 Minisonic design, I put together the filter envelope generator and loudness envelope generator in the Delta Seven.

For an example of what it sounds like, check out my music page.

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