PAiA 2720 modular synthesizer

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The Radio-Electronics May 1973 cover, featuring a PAiA 2720 with button keyboard.

The PAiA 2720 modular synthesizer, 'a playable but limited instrument', was designed in 1972. The construction was published in Radio-Electronics magazine, May to Oct 1973. While a Moog system at the time cost from $5,000 to $10,000, the kit of all parts for the 2720 from PAiA cost $139.[1][2]

PAiA 4700 and 2700 modules are mutually compatible.[3]


The 2720/A included:

  • Build instructions
  • User's manual
  • Parts for 6 audio patch cords
  • Parts for 12 control voltage patch cords

Patch cables

The patch cables for the 2720 consisted of 3.5 mm jacks for audio pin jacks for the control voltages/triggers. Since the modules already share a common ground via the power supply the patch cables between modules only needs a single conductor. A common ground will need establishing when connecting tyo external equipment.[4]


Voltage controlled amplifier

The 2720-1 voltage controlled amplifier is an op-amp driven by a differential pair.[5][6]

Voltage controlled oscillator

The 2720-2A Voltage controlled oscillator outputs with a frequency range of 20Hz to 5kHz ramp, triangle and variable duration pulse waveforms. It has on board voltage regulation.[2][6]

Function (envelope) generator

The 2720-4 function generator accepts a trigger pulse from the controller and generates a 0 to 5v output waveform with independently adjustable attack and decay times. Attack is variable from .002 to 1 second and decay is variable from .005 to 1 second. Sustain level is maintained as long as there is an input trigger. The amplitude output is fixed or variable up to 5V.[5][6]

Voltage controlled filters

The 2720-3B band pass filter center frequency range is from 350 to 1200 Hz, with resonance adjustable by a potentometer. The 2720-3L low pass filter has a roll off rate of 12 dB/octave adjustable by control voltages.[5][6]

Control oscillator/noise source

The 2720-5 control oscillator is a combined control oscillator (LFO) and noise source. The range of the LFO is 1 to 25 Hz in three ranges and both fixed 5V peak to peak and variable amplitude outputs are available. The noise source is a reverse base-emitter junction.[6][7][8]

Power supply

The 2720-7 PSU provides at up to 100 mA (sufficient for up to twenty modules), + 18 Volts, + and - 9 Volts and a rail variable between + and - 5 Volts. The module also an isolated potentiometer and a patch panel allowing more than one module to be driven from the same control source or patching between audio and control lines.[2][6]


The intial 2720-6 economical keyboard consisted of shirt buttons on music wire springs. The 2720-8 keyboard/case is a three octave Pratt-Read keyboard. Pressing any key generates a pulse trigger and a gate signal while the key is held down. There is a pitch control to raise or lower the keyboards range by an octave. A sample and hold circuit holds the control voltage for a minimum of thirty seconds after keys are released. The case measures 31 inches x 12 inches by 11 inches (78.7 cm x 30.5 cm x 27.9 cm), sufficent for fifteen single width modules.[6][9][10]


The 2720/R expanded the system with:

Envelope follower/trigger

The 2720-11 envelope follower/trigger converts any low level input signal (e.g. from a musical instrument or microphones) to a pulse waveform whenever the signal exceeds a preset level.[6]


The 2720-12 inverter/buffer can be used as an audio inverter, buffer, inverting summer or level shifter.[6]

Sine converter/pulse width modulator

The 2720-14 sine converter/PWM converts a triangle wave to a sine wave and a voltage controlled pulse wave with voltage controlled 5% to 50% variable duty cycle, both at the same frequency as the triangle input.[6]

Glide retro-fit

The 2720-9 glide retro-fit replaces the control panel on the 2720-8 keyboard/Case to provide a variable rate glissando.[6]


  1. ^ Vintage Synthesizers by Mark Vail, Miller Freeman, 1993, ISBN 0-87930-603-3
  2. ^ a b c Radio-Electronics, May 1973, pp. 38-41
  3. ^ Editorial, Polyphony 1/76
  4. ^ a b 2720 and 4700 series patch cords, MIDI Control via 9308 FatMan or the 9700 MIDI2CV8, and possibly the 9756 L2E.
  5. ^ a b c Radio-Electronics, Sep 1973, pp. 53-55
  6. ^ a b c d e f g h i j k PAiA catalogue 1978, pp. 8-9
  7. ^ Radio-Electronics, Oct 1973, p. 88
  8. ^ Electronic Music Circuit Guidebook by Brice Ward, Tab Books, 1975, ISBN 0-8306-5743-6, p. 168
  9. ^ Radio-Electronics, Jun 1973, pp. 56-57,
  10. ^ Radio-Electronics, Jul 1973, pp. 46-47

Further reading

  • Electronic Music Circuit Guidebook by Brice Ward, Tab Books, 1975, ISBN 0-8306-5743-6 - includes PAiA 2700 circuits
  • The A-Z of -Analogue Synthesisers Part Two: N-Z by Peter Forrest, Susurreal Publishing, 2003, ISBN 0-9524377-3-2, pp. 47-48

External links