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THE NEW MOTOR
As before the motor is built into the platter, rather than being a separate unit onto which the platter is placed. This was – and still is – a distinguishing feature of the Technics turntable; many others – especially budget designs – were built on a separate brushless, low speed d.c. motor (BLDC). The drawback of this approach is that the large platter diameter to motor diameter ratio serves to amplify the impact of dynamic stylus drag upon the motor, a phenomenon readily seen in the feedback signal, by the way; it’s not hypothesis.
The best way to combat this phenomenon, and that of platter colouration caused by internal resonances, is to use a heavy platter with high rotational inertia, and well damped construction. Where the original SL-120 Mk2 platter was a relatively light aluminium die-casting to achieve fast start up needed for DJ work, with a 1970s motor of limited torque, the new one is radically different beneath its similar exterior.
The platter of the SL-1200GAE is now a twin rotor design carrying powerful magnets both above and beneath the plinth-mounted, pancake shaped drive coils. Commutation – a technical term for switching current into the coils so the magnetic field they produce rotates – is entirely electronic. This is the distinguishing feature of a BLDC and why they appeared in the 1970s, when transistors able to manage such a task became available.
Nowadays BLDCs are used widely because of their advantages, and the art of motor control has come a long way since the SL-1210 first appeared. Technics knew it was time to improve their motor – and a common way to do this is use more powerful cobalt-samarium or neodymium magnets, replacing earlier and weaker ferrite magnets. However, the original motor was so good that little has changed in terms of measured speed stability and the so-called “cogging” effect Technics mention, our measurements show.
Motor torque has been greatly increased, allowing use of a heavier ‘hi-fi’ platter. The new SL-1200GAE platter uses a less resonant bi-metallic construction of die-cast aluminium with a brass top plate and underside damping. The old motor would slow if a finger was placed against the platter’s edge, the new one will not: it is obviously a far more powerful motor.
The Timestep modified Technics with SME 9in arm mounting plate and switch mode power supply removed.
The SL-1200GAE has a cast aluminium chassis.
Which brings me to cogging. We encountered this after fitting a Mike New bearing to our Timestep Evo SL-1210 Mk2. It increased drag a little by being larger in diameter and lubricated by heavier oil, showing the old motor didn’t have ‘enough in reserve’ to cope with the change. The cogging that appeared was at 6.6Hz (see our Feb 15 issue) but since it measured a very low 0.05% it was hardly a mechanical disaster and unlikely to have any major influence on sound quality.
Ironically, our analysis of the new motor shows an identical component at 6.6Hz so it too is a 12 pole motor spinning at 33rpm (12 x 0.55Hz = 6.6Hz) and this is what our analysis makes clear. So cogging has not been eliminated; it exists at a very low level, much like before. But platter weight and quality of construction has improved – the important point.
What I’ll finally note with regard to the platter is that it is all-metal and not a large, heavy acrylic disc of the sort common to belt drives – and this likely influences its sound more than the minimal amounts of cogging our high resolution spectrum analysis is able to detect.
Cogging that is intrinsic to a 12 pole motor can be seen at 12 x 0.55Hz = 6.6Hz. But it is at a very low level of 0.022%.