Cartridge-- M/M or M/C

The brushes of DC motors are the main culprit of EM noise. The motor rotates from the physical contact from the brushes-collectors and due to physically unstable contact the transient EM noise occurs while the motor is working.
To visualize or audiolize that effect larger you can bring a drill(corded or cordless) closer to the cartridge and turn it on. You may even see your speaker drivers moove picking up transient pulses. Certainly such are much smaller from the supplied motor but they're present and definitely affect the performance of the system.
Cartridge pickups, microphones and speakers are working on the same effect of electromagnetics. when the current flows through the conductor it induces a magnetic field and bacwads: the moving magnet inside the electric field creates emf i.e. electro-motive force. One way or the other all magnetic systems are vulnerable to EM interfearance.

In AC motors there are no collector brushes and so is EM noise is minimized. The quartz-digital motors we can consider as a special case of AC motor where the freequency of pulse is dictated by quartz.

I never compared or analyzed issues of stability of EM interfearance, but more often myself had issues especially in cheap analogue systems where the system picked up too much of the motor noise that had been a primary reason of poor cartridge performance.

Marakanetz, to pursue this conversation...I'm no electrical engineer, so don't string me up if I get this wrong. I am currenlty using a Grado Reference on an idler-wheel 'table, and it's as quiet as the grave, which is significant as the Grados are very sensitive to e-m fields. Now, looking at a prime example of an idler-wheel motor - a Lenco L75 (weight something like 3 pounds) - I see that it does not physically contact the brushes but spins freely like a well-designed platter bearing, which it in fact resembles, with the caveat that it has two bearings, one at each end, of polished stainless steel. So your comparison with the spark-throwing drill motor is incorrect in this case. From previous research into these motors (due to my first and shocking exposure to the idler-wheel principle) I had identified the motor as an "induction" motor. Maybe I was wrong, I never had a chance to find out as I was traveling at the time. Now I am only trying to remember what I read years ago, but the idler-wheel motors are "non-cogging" and so actually give out less physical vibration than AC motors (assuming my induction motor is not an AC motor itself!), assuming they are balanced, as all idler-wheel motors are. And, for instance, Origin Live is marketing an after-market DC motor which has received universally good reviews as providing a significant upgrade over AC motors and the expensive power supply regulation they require. Maybe you could elucidate these arcane matters for me, as I truly am interested. Probably I should get off my ass and take some sort of course in electronics. Much appreciated.

Without looking inside of your Lenco motor which is I guess an advanced tech stuff, I can't say wether it's idler-wheel, AC or digital one.
If you're saying that the motor doesn't physically contact the brushes, than I assume the motor is driven by magnetic field that could either be due to the AC or quartz clocking pulse.
I can also assume that there are no brushes and ball bearings instead. In this case the noise will be much smaller but the fact of surfed contact is present still...

The drill example is only an example of an open problem arround all DC motors. Certainly there are ways to tame such noise and make it's influence area much smaller.

AC motors while do have vibrations have much less EM noise than DC ones. Kill one bitch and another will rise.

EM noise can interact with cartridge's magnet and change it's specified properties almost as well as Vibrations and speed instability.

Thanks for the info, Marakanetz. The Lenco is an idler-wheel drive with an extremely heavy four-pole motor, which I calculate spins at about 1600-1800 RPMs. It has polished stainless steel bearings with pointed ends (but the motor lies horizontally), in bronze bushings. There's no quartz regulation, but this is not necessary, as the heavy and hand-balanced motor creates its own flywheel effect. The heavy and hand-balanced platter,with roughly 50% of its mass concentrated on the periphery, further smooths it out, because of the direct contact via the idler wheel. Garrards have similar motors, but different from the Lenco implementation. Thanks to you, now I begin to understand their inner workings.