This Pabst motor was originally built for 50Hz and
Miichell sold a power supply for it that converted 60Hz
to 50 Hz and the received wisdom was that the motor
is quieter at 50 Hz. The supply sold for $800 which
seems whacky even though it looks nicely designed
with two oscillators and two op-amps. Wish I could
get my hands on a schematic or a cheap damaged
model.
Michael
Miichell sold a power supply for it that converted 60Hz
to 50 Hz and the received wisdom was that the motor
is quieter at 50 Hz. The supply sold for $800 which
seems whacky even though it looks nicely designed
with two oscillators and two op-amps. Wish I could
get my hands on a schematic or a cheap damaged
model.
Michael
EC8010 said:
This is interesting, how did you get the impresion that the 60Hz are better? This is for all your tt? Could you please elaborate a bit on this?
After all this time working on the ps, my conclusions are that the voltage divisor MUST be avoided, and that the correct voltage is critical for low vibration.
Agreed, 50Hz reduces the rotational speed of the motor compared to 60Hz, but the mechanical noise of a decent sleeve bearing is minimal.
Electrically, a motor is similar to a perfect transformer in parallel with an inductor. The inductor's reactance is proportional to frequency, so the current it passes is inversely proportional to frequency. If that inductor passes too much current its core will saturate and buzz. We want our applied power to go into rotating the shaft, not saturating the parallel inductor. It's my belief that this is why 60Hz allows a lower noise floor than 50Hz. This idea seems to work for old-fashioned 4 pole motors (Garrard 301) and the more modern 24 pole synchronous motors. Remember that turntable wow is not an additive effect - it doesn't just add 0.55Hz to the audio, it's a modulating/multiplying effect that adds noise sidebands either side of each and every audio frequency.
Electrically, a motor is similar to a perfect transformer in parallel with an inductor. The inductor's reactance is proportional to frequency, so the current it passes is inversely proportional to frequency. If that inductor passes too much current its core will saturate and buzz. We want our applied power to go into rotating the shaft, not saturating the parallel inductor. It's my belief that this is why 60Hz allows a lower noise floor than 50Hz. This idea seems to work for old-fashioned 4 pole motors (Garrard 301) and the more modern 24 pole synchronous motors. Remember that turntable wow is not an additive effect - it doesn't just add 0.55Hz to the audio, it's a modulating/multiplying effect that adds noise sidebands either side of each and every audio frequency.
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