Rumble and Motor Vibration Are Greater in Direct Drive Designs
If you have been in this long enough, you know that the '1200 has come along way in terms of overall critisism. Why, at one time it was an accepted rule that the '1200 was not worthy of quality reproduction on any level, plinth, motor and arm. Well, times have really changed and as more folks learn what the 'table is all about, and more serious people use it and approve of it, the things left to criticize get smaller.
The technical evolution of motor drives in phonographs is as follows:
Each evolutionary step provided lower rumble and greater speed control. What may not be clear is that all drive systems except 6 and 7 have thier motors running at full power all the time. The brushless direct drive systems, because they rotate at the platter speed, need to be controlled by adjusting the power factor. This is the main reason brushless direct drive systems have such low motor noise. When running at speed, brushless direct drive systems use a very low amount of internal power.
The actual rumble frequency created by a drive system determines its possible audibility. That fundamental frequency is determined by dividing the motor RPM by 60 to get a value in cycles per second.
For the average belt drive that would be 300rpm/60 or 5 Hz. There would be harmonics at 10 and 15 as well. You will note that this is in the stylus resonance area (8-15 Hz) where it can be amplified and reproduced as woofer pumping.. Motors operating on the AC line voltage will also self vibrate at 60Hz as well. The low frequency rumble enters through the belt, the 60Hz self vibrating noise enters through the motor mount system into the plinth.
[ref. A 4-pole motor would rotate at 1800 rpm, a 12-pole motor at 600 rpm, a 24 pole at 300]
Direct drive motors genereate a rumble frequency of 33.33rpm/ 60 or 0.5 hz. with harmonics at 1 and 2 hz. The 3 phase drive system will self vibrate at 3 x (33.33/60) or 1.66 Hz. Even with second and third harmonics, this is still outside the stylus resonance area. When you combine that ultra low frequency with the very low power factor involved, it should be clear that the direct drive motor is very quiet indeed.
What is truly quiet
A turntable drive system can be considered truly quiet if it generates an unweighted rumble of -50dB. That means that any and all motor noise is 50dB or 316 times smaller than a cartridge producing a 1khz tone at 5 cm/s. Typically that is where a MM cart will produce 3-5 mv., the rumble output would be 15uV.
I stress unweighted, for that takes all drive system noise into consideration. The weighted measurement will always be signigicantly higher. Weighting is supposed to discard frequencies that are considered inaudible. That may not always be the case. So always insists on both measurements. For the Technics 1200 the weighted is -78dB and the unweighted is -56 dB. An unweighted measurement will also reveal hum from an internal transformer if present, and hi frequency whine from certain types of DC motors, as well as bearing noises.
In summary the main thing to grasp is that, in a direct drive system the motor design is significantly different because it runs at platter speed. These are controlled power factor motors. Rumble can come from many sources. It can reveal itself as a deep hum and as woofer pumping exacerbated by the arm-cartridge resonance. We have also seen that it can reveal itself as a high frequency whine. The unweighted rumble measurement will tell you whether this is a problem or not.
A simple test
A simple test to detect motor noise in a belt drive system is to remove the belt. Cue the needle onto a record. Turn the table on, and increase the volume and listen for hum. In a direct drive sysem, this test can be done to detect internal transformer hum. Some test records have quiet tracks that let you test further for the lower frequency components of drive system rumble.