The Pro-Soloist is a thirty preset monophonic analog synth from the early 1970's, originally designed to sit on-top of, and accompany a large Organ. It had 30 preset sounds to choose from, including Flute, Bassoon, Brass, Fuzz Guitar etc. Other features included pitch, portamento, vibrato, growl and wow effects. The keyboard has 37 keys with aftertouch sensitivity for volume, brilliance, vibrato, wow, and even bend effects. A few basic filter (brilliance), envelope and LFO controls allow some tweaking of the preset sounds, however there is no memory to store your settings.
The Pro-Soloist is a compact and basic monosynth best suited to leads. Because of its aftertouch feature, it's simply one of the most playable and expressive solo instruments Arp ever made. It was later followed up by the Pro/DGX in 1977. The Pro-Soloist has been used by Tangerine Dream, Tony Banks (Genesis), Coldcut, Gary Numan, Herbie Hancock, Patrick Moraz, Billy Preston, John Entwistle, Billy Preston, and Vangelis.
The Pro-Soloist was based on the Soloist from 1970/71. The Soloist - also designed to sit atop an electronic organ - had its preset sound selection switches located under the keyboard in the same tradition as the Roland SH-1000/2000 and Elka Soloist synths.
Demos & Media
Images from Tone Tweakers and Audio Playground Synthesizer Museum
Thanks to Lee Matthews for providing info.