The Arp Pro Soloist is a monophonic synthesizer used by Prince on For You and Prince. Prince also used an Arp Pro/DGX, an updated model with a different filter and switching system.
It was one of the first commercially successful preset synthesizers, with a series of presets and limited programmability. It also featured a number of performance controls, including aftertouch, vibrato, portamento, "wow", "growl" and "brilliance". This made it an expressive instrument for leads and solos.
It was the most frequently used keyboard on Prince's first two albums. It can be heard on "In Love", "Soft And Wet", "Just As Long As We’re Together", "Baby", "My Love Is Forever", "When We’re Dancing Close And Slow", "With You", "Still Waiting", "I Feel For You", "It’s Gonna Be Lonely". The solos "Soft And Wet"
and "I Feel For You" were performed on a Pro Soloist.