Maybe you were referring to Cat Stevens 1977 album, Izitso.

I bought my first monophonic synthesizer in the late 70s. The polyphonic synth was the coolest thing I ever heard back then. Even the earlier 5 note polyphonic synths had that new sound, but very expensive.

"(Remember the Days of the) Old Schoolyard"
https://www.youtube.com/watch?v=iFwDEqLQ0bA

Marcus