The ARP Pro Soloist was one of the first commercially successful preset synthesizers. Introduced in 1972, it replaced the ARP Soloist (1970–1971), one of the first portable synthesizers. The Pro Soloist, expanding the number of preset patches to 30, and incorporating digital electronics for preset memory and keyboard control, was much more reliable. A novel tone generator eliminated tuning problems suffered by the Soloist. It found its way into the hands of famous musicians as Tony Banks of Genesis, Josef Zawinul, Billy Preston, Vangelis and Tangerine Dream.
The Pro Soloist is monophonic and has 37 keys for three octaves with aftertouch. It uses a digital ROM chip to program the signal paths. The expression knobs are analog controlled. Sliders control volume, velocity sensitivity, brilliance (VCF cutoff) and portamento. An octave switch allows the 3-octave keyboard to be transposed one octave up or down to extend the range of the instrument to five playable octaves. A rotary potentiometer controls the vibrato or tremolo rate as well as the repeat function, which causes the LFO to retrigger the envelopes of the selected voices.
The instrument has a single oscillator that simultaneously generates available pulse and sawtooth waveforms. The signal from the waves is passed through a mixer, followed by a high-pass filter with four selectable settings, a low-pass filter and an amplifier, each controlled by an attack-release and/or ADSR envelope generator. The original 24 dB/octave low-pass filter was very similar to the Moog transistor ladder filter and had to be replaced. (Source: Wikipedia)
