Cartridge-- M/M or M/C

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.