"mick" wrote in message

On Tue, 06 Jul 2010 16:52:16 +0100, Rob wrote:

Why do belt drive turntables, usually British, have
noisy motors? Is it really beyond the wit of
designers/manufacturers to use a quiet motor?

I've just bought a Roksan Xerxes, curiosity buy, and the
motor vibrates to the point that it can be felt through
the plinth. Apparently (having been through forums etc)
this is quite normal. It's a testimony to the design
that very little of this finds its way to the platter or
arm, but why bother designing in such compromise, only
to have to design it out?

I thought the Xerxes had a DC motor and a fancy power
supply?

All AC motors will vibrate to some extent. It's because
they have to have a finite number of poles, causing the
armature to "step" between them. The inertia of the
armature reduces this somewhat, but it's always there -
it doesn't matter when or where the turntable was made.
The more poles the better, but there is a limit to how
many can be usefully manufactured. Thorens used to use 16
poles IIRC, and Linn used 24 poles.


All motors are in some sense AC motors. If you apply pure DC to the armature
coils of a motor, it will be locked down solid.

So called DC motors have commutators, which are in essence electromechanical
DC-to-AC converters.

So-called brushless motors simply cut to the chase and replace the
electromechanical commuator with a multiphase solid state inverter.

Therefore, all DC motors are effectively AC motors and they will similarly
all vibrate to some extent.

The spectrum of noise from motors has a few dominant sources. One is at the
motor's rotational speed, and another is at the motor's rotational speed
multiplied by the number of poles.

The primary means of isolating the turntable platter from these vibrations
is a number of mechanical low pass filters. One is formed by putting the
motor on compliant mounts and another is formed by the drive belt and the
flywheel effect of the turntable platter.