PPG 301 Voltage Controlled Oscillator


Introduction The PPG 301 VCO dates back to 1976 and the beginnings of PPG when they were a modular synthesizer company based in Hamburg. The first set of modules was named the 100 series are are visually similar to the Moog modular but with different circuits and panel lettering in German. In the 100 series the 101 VCO module was actually three VCO’s and a driver, very similar to the Moog 921A.

For the 300 series VCO Wolfgang designed a standalone single oscillator module, the 301. This proved popular with Klaus Schulze and Tangerine Dream, as there were really no alternative VCO modules other than Moog in 1976.

Description There are at least two versions of the 301 VCO. An early version with straight PCB traces and two CA3086 transistor array chips. One chip is used for the VCO core and the other for wave shaping, there are no Op Amps at all. The later version has curved PCB traces and the addition of a TL044 Quad Op Amp. This 16-pin chip is an early low offset design which is used to buffer the VCO control voltages. This would have been a significant improvement over the first version in terms of frequency stability.

The 301 provides four traditional wave shapes; saw, square, triangle and sine. There is a 12-way rotary switch for selecting octaves (+/-5) and a potentiometer for fine tuning across +/-10 semitones. There are four frequency CV inputs, two of which go via control potentiometers. The square wave has pulse width control, both front panel and via two inputs, one of which goes via a front panel potentiometer. There is provision for hard sync, with an input and output jack socket but no switch.

TheĀ  PPG301 design uses a heated CA3086 as the means of temperature stability rather than a tempco. The are a few resistor and capacitor kludges to the top and bottom of the PCB, which is expected with PPG!

Trimmers There are six open frame single turn trimmers on the PCB and one mounted as a kludge underneath, they are:

  • Octave CV (marked oc or octave)
  • CV3 input (marked 3)
  • CV4 input (marked 4)
  • Pulse width adjustment (marked pu)
  • Sine wave adjustment (marked si)
  • Temperature adjustment (marked t)
  • Unknown adjustment (kludge)

Recreation There are no schematics for the 300 series modules, so the AM301 is based on tracing out the 301 PCB layout, knowledge of VCO circuits, although this design is unique and not a copy of the CA3086 based ARP VCO. Moog developed a heated CA3086 design for the Prodigy and Roland used heated transistor pairs (ua726) in their designs.

Outcomes & Availability This a long job of tracing out the tracks and trying to understand the circuit, progress so far;

  • The Octave switch uses a 100R resistor ladder
  • The TL044 is used to buffer the frequency CV inputs and octave ladder

 

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