AM3030 Module - SCI Prophet 5 Revision 1 VCO

Overview  This Voltage Controlled Oscillator (VCO) is based around the classic SSM2030 chip that was used in the original Prophet 5 (Rev 1 and 2). The SSM2030 was the first non-proprietary VCO chip ever produced, Dave Rossum created it in 1977. The chip was the basis for a number of custom
E-mu Systems synthesizers, as well as the original Prophet 5 and 10. This is a good VCO design which has high frequency trimming, and the relatively similar performance of each chip within a batch enabled polyphonic designs. However its performance has been exceeded by more recent discrete designs, although an ultra stable power supply helps alot.

Original Circuit The original design in the Prophet 5 is very simple, with basic pulse, triangle and sawtooth waveforms being created from the sawtooth core. This is exactly out of the datasheet with an external tempco resistor (1K 3600ppm) and some simple CV and modulation summing Op Amps.

AM3030 Design The Analog Metropolis circuit is based on the original but with soft and hard sync options. Originally I designed the VCO so it could be switched into LFO mode with a large capacitor, but the cross talk in the wiring makes this an unstable feature and is therefore not included in the final AM design.

The base PCB produces Triangle, Pulse and Sawtooth waveforms. I have added push button control to select the waveforms and sync on/off, using the same momentary buttons as on the Prophet 5. This makes for a more interesting control surface, complete with red LED indication of the selected waveforms. A high quality audio Op Amp on the back of the switch assembly mixes the selected waveforms into both a normal and inverted audio output. The switch PCB's are generic and can be used to control any analog synth module.

The front panels controls are:

• Frequency
• Fine frequency
• Frequency Modulation 1
• Frequency Modulation 2
• Pulse Width
• Pulse Width Modulation
• Soft or Hard Sync
• Push buttons for waveform selection

There are six 3.5mm jack sockets mounted on the base of the panel, these are:

• Pulse Width In
• FM1 and FM2 Inputs
• Sync Input
• Outputs (+ and -)

Panel The front panel is a standard AM design, 90mm wide and 4U high, with black lettering. Controls knobs are standard ARP2500-like designs.

Build History The schematic was transferred into Eagle CAD in July 2005, and prototype boards were sucessfully tested in November 2005. Production boards have been available since January 2006.

Prophet 5 Rev2