PAiA 2720 modular synthesizer

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 PAiA 2720 kit of all parts cost $139.

PAiA 4700 and 2700 modules are mutually compatible.

2720/A
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.

Mouser

 * 171-PA3191-1-E, 3.5 mm phone connector
 * 530-105-0302-1, Red tip plug
 * 602-1635-100-03, Red EPR hook up wire

Voltage controlled amplifier
The 2720-1 voltage controlled amplifier is an op-amp driven by a differential pair.

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.

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.

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.

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.

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.

Keyboard/case
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.

2720/R
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.

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

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.

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.

YouTube

 * 1974 Paia Synthesizers demo record
 * Paia 2700 system walk through
 * Sounds of the Paia 2720 by Tinfoil Cat, Dec 2019