|
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
analog modular synthesizer System 700 (1976), and has been used with minor changes in the:
- SH-2 Mono synth
- Jupiter 8 Polysynth
- System 100 Modular
And of course ... the snazzy 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 accurate VCO I have designed so far...
The dual transistor exponential generator has been retained but a modern SSM2210 (but now obsolete in 2009) 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 original wave shaping 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 laid out onto two 80mm x100mm PCB. PCB-A holds the core sawtooth oscillator, 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
The sync is internally wired to the 2nd VCO of the pair in my Roland 100M clone. There are four 3.5 mm 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. The VCO Range rotary switch is mounted on a sub panel to ensure it is at the same knob height as the rest of the module. The production aluminum panel will have the Range switch mounted higher.
|
