As the motor testing has shown, some motors benefit from the quality of the drive. Here is the development of a state of the art ‘Pure Analog Drive’ motor supply.
The goal was to develop a pure analog, 3 phase, ultra-low distortion motor drive (primarily for the XXXX motor). Yes there is a digital element to monitor the status but this can be switched off (micro-controller held in reset, display and uC clock off) so in this mode the supply is pure analog – there are zero digital elements switching which makes sense for a turntable which is an analog device…. The drive is as stable as the best digital ‘synthesised’ units but is pure analog…..
The performance is darn good; 2nd, 3rd etc. harmonic distortion of the generator is -116dB. This can also be described as less than 1.5 parts per million distortion or 0.00015% or 66uV. The output distortion at full voltage (44V RMS, 124V pk-pk), including the 3 phase amplifiers is -110dB (3ppm/0.0003%) and the phase accuracy is better than 0.1 degrees for the 120 and 240 degree signals. Under real motor load it achieves -107dB (4ppm/0.0004%) which took some work utilising the effective constant current of 3 phases and the drive amplifiers power supply rejection ratio (PSRR) which has a measurable influence at these ultra low distortion levels. Also any 50Hz mains component is -112dB. The frequency/speed tracks within 0.01rpm with no impact on the high Q and thus distortion of the R-C oscillator. The diagnostics have been very carefully implemented so as to i) have no influence on the pure analog signal generation and ii) be accurate. (For context the Voyd power supplies have 2nd harmonic distortion of -45dB, 5600 parts per million or 0.56% – I wish I had had this equipment back then, 40 years ago…)
The main generator board is 6 layers with dedicated planes for +15, -15, +5, 0V and two signal routing layers. The critical R-C elements are on a plug in (gold contacts) board. The unit can be configured to generate any frequencies from 25 – 200Hz. In order to achieve this level of performance a significant amount of development work went into this no holds barred design.
This is a key development, as the results attest, and allows turntable development without any concerns there is a better power supply… Oh, the ‘MF-1’ stands for Motor First 😉 – It appears the majority of turntable design is based on mechanical first approaches, whereas I firmly believe turntables should be developed from a motor first principle, hence the MF.
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Spectrum – limited to -80dB by HP3561A Signal Analyser 33.33Hz – yes 33.33Hz (& 45.00Hz) running the XXXX motor at 1000rpm (1 Joule of energy!). 1350rpm for 45.
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Spectrum of Total PSU using an active notch filter to increase the dynamic range of the signal analyser. Note the signal level (after a divider to pad down the 44V rms to a safe level) is +12dB. The small content at 50Hz is likely due to the case of the active notch filter being aluminium and not providing any magnetic screening.
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Here are the accelerometer plots….. The peak at 200Hz is the 3 phase cycles at twice the drive frequency (AC drives on both the positive and negative peaks) – essentially the drive frequency. The peaks that used to exist at lower frequencies are much reduced and can now be seen dropping to a null during the precise adjustment of phase and voltage. Also running true 3 phase drive allows lower voltages to be used and still achieve significant torque.
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As we know – vibration in these plots is primarily due to speed variation along with a small contribution from the electro-mechanical energisation of the coils/winding which is 200Hz. Touching to the frame shows how small this energy level is. A new speed counter is needed as the noise floor of the current 16bit one is maxed out…
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And the noise floor….
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