A preset synthesizer introduced by ARP Instruments in 1972. It was a monophonic unit with 30 presets, an aftertouch sensitive keyboard of three-octave span (C to C). The Pro Soloist had an internal architecture which use a form of digitally controlled oscillator and patch memory, both unusual and forward-looking for their time. The synth was a sales success for ARP, but it also possibly represents a missed opportunity.
The Pro Soloist replaced an earlier model called the Soloist, which had been one of ARP's first products after the 2500. The Soloist, like other small synths of the day, had been marketed as an adjunct to an organ, intended for both home and professional use. Its packaging was rather awkward; it had organ-like tabs which protruded from the front of the case for preset selection, which got broken off when used by touring musicians. It was also said that its conventional analog circuitry was not very reliable. Accordingly, ARP set out to fix those problems with improved packaging (the preset controls were moved to a tilted rear panel), and vastly improved and innovative circuitry for its era.
The Pro Soloist's voice architecture consisted of the aforementioned DCO, a set of fixed filters, a VCF, a VCA, and two envelope generators. The parameters for all of these functional components were stored as digital values in ROM, which were converted by crude D/A converters to drive the circuitry when a preset was selected. This was done using discrete digital logic. The DCO consisted of a high-frequency oscillatory which drove a digital counter, which in turn produced a train of pulse waves. This was output directly, and also used in a combination of higher-frequency pulses to produce an approximation of a sawtooth wave (see Walsh functions).
The DCO waveforms were mixed and then sent through the fixed filters, in some combination according to the preset selected. These were tuned so as to help produce realistic tones for the presets that were labeled with names of real instruments, e.g., "cello", "bassoon", etc. The output of the fixed filters was then mixed and routed to the VCF and then the VCA, which were of conventional design. The VCA was under the control of an ADSR envelope generator, and the VCF could be driven by an AR envelope generator depending on the preset chosen.
(Early production versions of the Pro Soloist used a version of the ARP 4035 VCF, which infringed on a Moog Music patent. After lawsuit threats from Moog, ARP replaced this with a VCF of its own design in later production, as it did for other models of the era.)
The aftertouch was routeable to several destinations, using panel controls separate from the presets. Since the Pro Soloist lacked a pitch wheel or mod wheel, the aftertouch performed the usual pitch bend and vibrato functions, as well as being routeable to VCF cutoff frequency. Additional controls provided some control over the output of the fixed filters.
Despite all this, the Pro Soloist would probably be a footnote in synth history had it not been for the efforts of Tony Banks, who used one extensively on Genesis recordings from 1973 to 1976. (His "In The Cage" solo, well known to progressive rock fans, was played on the Pro Soloist.) The advanced circuit design contained many of the building blocks of a voice-allocating polyphonic synth, and it is somewhat surprising that ARP never attempted to extend the design into that territory, which (with a proper microprocessor replacing the discrete logic, which would have been practical by about 1975) would have given them one of the first practical polyphonic synths. Instead, they did not explore this territory until much later with the Chroma.
The Pro Soloist was replaced by the Pro/DGX in 1977. This was very similar, but replaced the preset select switches with pushbuttons. The pushbuttons had LEDs to indicate which preset was active. The case was also made stronger. The Pro/DGX remained in production until ARP folded in 1981.