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AM2200 Module - Eµ VCO
Overview This module is a replica of the main VCO used in the Eµ Modular from 1972 - 1980. Dave Rossum originally designed this VCO way back in late 1972, and the core sub-module
is the 1200. The sub module design was updated in the mid-70's as the 1201, with a few minor changes. In 1972 this VCO was a revelation, it was stable and precise, a leap forward from ARP and Moog.
Original Circuit 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 has a hard sync input, as well as a linear FM input. Some components maybe have been upgraded, and a LM394 dual matched transistor is used instead of the AD818. This VCO is a classic design, and
with a few improvements it can easily take its place in the 21st century.
AM Circuit
The AM 2200 module 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 and improved frequency drift. This VCO exceeds the performance of an Oakley VCO, and gets close to the precision of a MOTM VCO, the all important waveform shape has been unaltered by the improvements. The circuit is laid out one PCB, and 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.
- Op Amps upgrades to provide improved frequency stability (OP177/LT1012) and to remove dependency on obsolete designs (including LM318 replacement).
- An ultra precision voltage reference chip (0.6ppm/V) replaces the original Op Amp design, this makes a significant improvement in frequency drift.
- The Frequency controls are driven from a ultra precision voltage reference and 0.1% resistors are used in sensitive locations to further reduce frequency drift.
- A reduction in VCO range, as the upper limit is supersonic at well over 30kHz. This makes adjusting the VCO in normal use (a keyboard with a range of C0 - C5) much easier
and more precise. The waveform shape deteriorates at low frequencies, so the loss of a slow LFO is not a big deal. This is a precision VCO, not a cool LFO.
- A re-positioning of the front panel controls enables the PCB's to be mounted to the front panel using pot brackets at 40mm spacing.
- Sync selection (hard/soft)
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. We could have pushed the analog design further but that would have meant more radical changes which potentially disturb the "unique" sound
of an Eµ VCO.
Front Panel The front panel is a standard AM design, 180mm wide and 4U high, with black lettering. Controls knobs are standard 19mm diameter Eµ Modular.
Build History
The schematic was transferred into Eagle CAD in February 2005, and two 120mmx100mm boards laided out. A prototype module was operational in July 2005, and testing continued for a few months. The production design was ready in January 2006.
Specifications To follow.
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Resources
* AM FAQ's 
* AM2200 Project Notes
* AM2200 PCB layout
* AM2200 front panel