On 07/07/2010 20:51, Arny Krueger wrote:
wrote in message

On 07/07/2010 12:47, Arny Krueger wrote:
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.

There's endless chatter on 'net about relative merits.

And this differs from other audio subjects how? ;-)

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.

And have a motor that's as quiet as possible.

Slow is good from a noise perspective.

Or mechanically isolate the motor completely. I would have
thought.

That's that I meant by mechanical low pass filter...


Ah, got you, thanks.