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AM 8120 Module - Roland 100M VCO
Overview This module is a
clone of the VCO from the legendary Roland 100M modular analog synthesizer. This VCO appeared in two formats:
- Single VCO without Sync in the 110 module
- Dual VCO with Sync in the 120 module
It is this later circuit I have replicated ( as a single VCO) but with some significant changes to the VCO core. This Roland VCO design originates in
the System 700 (1976), and has been used with minor changes in the:
- SH-2 Mono-synth
- Jupiter 8 Poly-synth
- System 100 Modular
And of course ... the snazy 100M. The VCO core is based on a heated UA726 dual transistor chip driving a capacitor based integrator that produces a
sawtooth waveform.
AM8120 Module The AM circuit is an improvement over the original design with a number of modifications to improve frequency
stability and tracking. This is the most accuarte VCO I have designed so far...The dual transistor exponential generator has been retained but a modern SSM2210 is used rather than the expensive UA726. The frequency control circuit has
been modified to use a standard 1K Tempco resistor, and the octave switching has been retained using 0.1% resistors. The high frequency trim circuit is based on the latest ideas from Rene Schmitz - which is another level of accuracy.
The original Roland VCO was stable, thanks to a +/-10V reference power supply driving the core and +5V as a reference voltage for the comparator. This is an advanced design for 1976, and it has not been seen again until the
resurgence of analog design in the 1990's. I have upgraded the design using high precision voltage reference chips and op amps. This further improves the stability of the power to the core and therefore the frequency.
The
waveshaping circuit has been carefully replicated using European transistors rather than Japanese. The VCO produces the usual waveforms; sawtooth, square with pulse width modulation and triangle. There is a sync facility with
Strong and Weak settings. The circuit was designed using Eagle CAD and laided out onto two 80mm x100mm PCB. PCB-A holds the core sawtooth oscilaltor, whilst PCB-B has the waveform shaping circuitry.
The module has the following front panel controls:
Frequency Range (6 octaves available) Pitch (frequency trim) Frequency Modulation (x2) Pulse Width Pulse Width Modulation Waveform Selection (square, triangle, sawtooth waves) on a slide switch
There are six 3.5mm jack sockets mounted on the left hand side of the panel, these are:
Front Panel The front panel is a 3" wide FracRac with Alpha rotary potentiometers, with 15mm diameter plastic mixer knobs.
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