Maxon Brushed motors for Hi-Fi turntables are now available from Maxon Motors.
For most people, particularly novices, a brushed motor is a good choice. The control electronics are comparatively simple and as they can be linear, low noise and the cost is lower. The wiring is also simpler, just two wires to the motor. The drive system is inherently smooth and quiet as long as we are aware of some basic points.
The basic requirements:
- Sufficient power, ie speed x torque
- Low noise, electrical and mechanical
- Minimal torque ripple
- Reasonable cost
For a retrofit situation, there is often have little control over the mechanics, the motor speed is much as determined by the original design. In the case of TT’s employing synchronous type motors this may be quite low, much less than 1000 rpm.
This requirement is usually at odds with the brushed motors available, which tend to be high revving when run at their design voltage, typically 4000 to 6000rpm.
Fortunately the solution is at hand. By using a large diameter motor (which provides good torque, and more power than is ever likely to be required) we can under run it from a lower voltage than its nominal design voltage. This will reduce the speed whilst torque remains purely proportional to the supply current. From a datasheet perspective this means selecting motors with a low rpm/volt speed constant, line 13 in the maxon data sheet tables.
Taking the A-max 26 110191 as an example, a nominal 48V motor, this has a speed constant of 127 rpm/V. So, in a typical turntable application (~600rpm), the supply voltage will be only approximately 4-5V. This is perfectly fine as long as the motor has sufficient torque. From the datasheet the motor makes 15mNm torque at max current.
As the turntable belt drive multiplies the torque by 20X approximately this is quite substantial. It is possible to go into more details as regards start up times if we know the moments of inertia, frictions etc etc, but unless we are designing for broadcasting applications this is probably not required here. In most real world situations belt slip will limit the torque in any case.
So it is necessary to look at some large diameter brushed motors, large in this context is ~>25mm diameter, with low volt/rpm..
What else is important? We want the lowest noise, mechanical and electrical. This means minimal brush noise and low torque ripple. So the choice is now towards precious metal brushes (PMBs), for minimal drag, noise and best efficiency, a long life capacitor (CLL) module to prevent brush sparking and as many poles as possible for smooth torque delivery.
A-max 26 series brushed motors
A-max 26 7W 13 pole 110191 an old favourite and still a good choice
RE-max 29 series brush motors
RE-max 29 9W 13 pole 226779 (bit more power has extended shaft)
RE-max 29 15W 13 pole 226761 (more power for heavier platters, faster start etc)
RE 25 series brushed motors
RE 25 10W 11 pole 118748 (with extended shaft, more power smaller size, tacho option)
Can have DC Tacho 118909 factory fitted for closed loop control
The RE 25 comes with the option of a factory fitted DC Tacho 118909, and so if required a true ‘closed loop’ control can be closed around the motor and the speed controller. It is also the most powerful motor for its diameter, which may help in some conversions, although it is also the longest.