Introduced in 1979 the 61 key ARP Quadra is an analog musical synthesizer that was produced by ARP Instruments, Inc. from 1978 to 1981 in a failed attempt to catch up to the "lead" taken by Sequential Circuits Prophet 5 with its fully programmable memory. Moog made a similar attempt with the Polymoog. The machine combined pre-existing products: the Omni, Odyssey, a Solina-esque string unit, a phase shifter and a divide-down organ with ADSR, and a 4075 24 db low pass filter into one box. It has four sections. Bass is on the bottom two octaves, has two unison bass circuits (electric and string), with AR and a single pole low pass filter and a related AD envelope for cutoff. A string section is similar to the ARP Omni. Poly Synth, and a two voice Lead Synth similar to the Odyssey and a five way mixer with four unit outputs, a stereo pair, a line mono and an XLR out.

There are 16 memory locations, but these do not provide storage of slider settings. The 8048 microcomputer and 128k of CMOS SRAM memory recalls 16 stored sets of on/off decisions about which waveforms and modulations are turned on in each of the four sections, and lead intervals. Using LEDs the player is visually directed to the sliders critical to the sound selected but their settings are not updated. This necessary tweaking and fine tuning is the weakness of the Quadra.

Like the Moog Polymoog keyboard, the Quadra was not a truly polyphonic. It used divided-down circuits for the Omni-style string section, including ADSR, LFO, filter and VCAs for the Polyphonic Synth section. Both Polyphonic Synth and string sections share the hollow wave button which changes waveforms for both sections.

It has a simple and useful arpeggiator for the lead section and memorizes intervals between the two lead VCOs. Vibrato, LFO and a fast square wave trill can be added to the arpeggiation. Very unusual is the two independent portamento rates for each VCO, whether locked together or in interval mode. This is useful in either multiple or single trigger mode.