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
- 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
- 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
- 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
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
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
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