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AM2200 Module - Eľ Systems VCO
Overview This module
is a replica of the main VCO used in the Eľ Systems Modular Analog Synthesizer from 1972 - 1980. Dave Rossum originally designed this VCO back in late 1972, and the core sub-module is the 1200. The sub module design was updated in the
mid-1970's as the 1201, with a few minor changes. It is this later design that this module is based upon.
Original Design The VCO produces a sawtooth from its core, which is then converted into triangle, sine and pulse
waves. The pulse width is variable from 0 - 100% and the frequency range can be switched low to turn the module into a LFO, with a low frequency of one wave every 33 seconds. The VCO is temperature compensated and it has low jitter and
excellent HF tracking (0.25%).
The VCO provides an adjustable and inverted mix of all waveforms, plus full volume outputs on every waveform at 5V peak to peak. The updated 1201 VCO added a hard sync input, as well as linear FM
control input. Some components maybe have been upgraded, and a LM394 dual matched transistor was used instead of the AD818.
The sine, triangle and square waveforms are created by the 1210 Wave Converter module. See separate builds notes for the AM2210 which replicates this module.
Original Circuit
This module was originally designed way back in 1973 probably using Op Amps and a CA3080 - like the 2000 module. The Revision 2 design came out in early 1978 using an SSM2020 dual VCA chip, LM1458 and TL082 Op Amps. The replacement of the CA3080 improved the S/N ratio by 30dB and reduced audio distortion too.
1978 saw a resurgence in Modular sales with Eľ Systems having the funds to move into a house in Santa Cruz, 417 Broadway. Dave Rossum was busy designing a range of synthesizer chips with Ron Dow at SSMT. These new synth chips
replaced the ageing transistor and Op Amp designs in much of the Modular systems during the same year.
AM220 Design The Analog Metropolis circuit is an exact replica of Dave's original design with some
enhancements to improve the frequency stability and to make it easier to setup. The underlying excellent analog design remains unaltered, but I have translated the design into modern components. The circuit is laid out one PCB. A
separate PCB handles wave conversion (AM2210). The improvements include:
A revised Audity-like HF adjustment design that uses a 20-turn cermet trimmer, rather than having to manually select the right resistor value.
Some Op Amps have been upgraded to provide improved frequency stability and to remove dependency on obsolete designs. Some Op Amps have remained as Dave specified, as improvements make no difference, for example the LM318. An ultra precision voltage reference chip replaces the original 741 Op Amp design for the critical +5V supply to the oscillator core.
The Frequency controls are driven from an ultra stable +10V and -10V
power supply (based on the same ultra precision reference chip). A re-positioning of the front panel controls enables the PCB's to be mounted to the front panel using pot brackets. Soft Sync on/off switch The circuit is temperature compensated with a 3600ppm 1K PTC resistor, and uses either an LM394 dual matched transistor array or the higher
quality SSM2210 for the exponential generator. The Tempco is mounted on top of these transistor pairs. This VCO (if untamed) will oscillate up to very high frequencies, well beyond 30 kHz. Whilst this does have musical uses as a modulation source,
it makes the FREQUENCY control range very wide. So the AM version has been tamed slightly to operate up to 20 kHz.
All of the parts are readily available, although the 1pF ceramic capacitor, dual FET 2N3958, 2N4121 and 2N3644 transistors can be harder to locate.
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