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A combination polyphonic / monophonic synth introduced by ARP Instruments in 1978. It was made from components and functional blocks from other ARP products, and is widely considered a hack job; it was introduced shortly after the market failure of the Avatar became apparent, and it was intended to keep the company alive until the Chroma, which had languished in the lab during the Avatar's development, could be brought to market.

The Quadra had four functional sections, which could be shared or split across the 61-note keyboard in various combinations. There was a monophonic bass synthesizer section, based on the Solus, that could be mapped to the lower two octaves; this was a single-VCO preset system with two presets. The "lead" section was basically the guts of an Odyssey, with some simplified controls and fewer routing options. The "string" and "polyphonic" sections were based on the Omni string synthesizer, and offered a few string and organ sounds with some adjustments for filter cutoff and waveform mixing. The keyboard had aftertouch, but there was not a proper pitch wheel (a slider to the left of the keyboard allowed for some tuning variation) or mod wheel. Possibly the most outstanding sonic feature was the built-in phase shifter; when used with the string section, it produced a characteristic varying string timbre that was a hallmark of late-1970s popular music, particularly in the disco genre.

Each of the sections had some membrane switches for selecting certain functions to be turned on or off, and sliders for adjusting parameter values that were adjustable; LEDs in the membrane indicated which functions were on and which sliders were active. A primitive form of patch memory memorized only the switch settings; when a patch was selected, the performer had to note which sliders were active and then adjust them manually to complete the desired patch. The configuration of the functional sections relative to the keyboard was not memorized and had to be set manually for each patch.

Several shortcuts and quality compromises were taken to get the Quadra out the door and keep the cost down, which gave it a reputation for unreliability. The power supply was undersized and ran hot. The case construction methods, which had been adequate on smaller synths such as the Odyssey, was not stiff enough for the larger and heavier Quadra, and case flex damaged circuit boards and gave the keyboard a mushy feel. The ends of the keys protruded beyond the bottom front edge of the case, resulting in keys frequently being broken off in touring use. The paint flaked, and the membrane panel was not repairable if a switch failed.

The Quadra's biggest misfortune, however, had nothing to do with itself. Within a few months of the Quadra's introduction in 1978, Sequential Circuits introduced the Prophet-5, a proper polyphonic synth with full patch memory and a much more capable voice architecture. The result was that only a few hundred Quadras were sold. Of the ones that remain now, few are in working condition.

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