Hum when setting frequency of P500 to 81 to play 45 RPM LPs

I use my P500 as a speed controller through regenerated port. No issues whatsoever @ 33.3 (60 hz). When I adjust to 81, system takes on high pitched hum. Sound is only present when stylus is in the groove. Also, sound is eliminated when I lower frequency into the 70s. Any idea what gives?