DIY Turntable power supply

Hi,
After completing my dac, I turned to turntable motor power supply, by this I am talking about AC synchronous motor not the DC type. Generally, these motors are either 12V (Rega, Thorens etc) or 24V or 120V (Linn) and at 50Hz (or 60Hz) run at 300rpm(or 250rpm) and with pulley system & belt, turn the platter at 33.33rpm. But due to dimensional errors, sometimes 50.00Hz don't give you exactly 33.33rpm speed. Hence we need some fine adjustment to the frequency.

There are various commercial products doing this job but most cost over RM1K and some are non adjustable. Hence we are looking to make one DIY that has the following features:-
1) Produce pure AC (sinewave) with about 10W of power output
2) Adjustable frequency range from 40 Hz to 100Hz
3) Adjustable voltage output
4) Highly accurate and stable frequency of sinewave
5) Low distortion pure sinewave

Those days, having quartz crystal oscillator controlled is a 'big thing' but nowadays with advanced computer chips, sinewave function generator like the DDS pictured below can be had for USD45 and it outputs sinewave with less than 0.8% distortion from 0.1Hz to 1MHz in 0.01Hz resolution and stability and accuracy of about 1 part per million. The DDS (Direct Digital Synthesizer) you just 'key' in the frequency and it outputs in digital form and with its built-in DAC converts the digital signal to analogue. So this will be my starting point and I thought we just need a 'power amplifier' to complete the project.

Any comments, and anyone done this before? I have ordered the parts (DDS) and will do up one for testing soon and will post results.

Very interesting I'll say. This can link to various kinds of power amplifier to output the required voltage to the AC motor. Its essential that most AC syncronous motors are frequency driven for speed stability and accuracy. The ideal AC voltage to the motor is to maintain the motor torque especially at startup. Too low the voltage and its trouble at startup. Too high the voltage and motor vibration sets in to upset the TT sonics. Thats why a compromise for a given voltage to a particular motor in some TT. They're not all the same and vary from TT to TT. Another great implementation is to have 2 seperate power amps (much like mono block) driving 2 opposing phases, rather than using a phase shift capacitor (which have its own set of implementation issues). Phase shift capacitor is the cheapest form of inverting the other phase. Terrible actually.
I'm sure a greater idea is the AC motor can spin at full voltage for a few seconds then drop the voltage to a certain fraction (when platter is at full speed) thereon the platter is almost free spinning with the benefit of nearly void of any motor vibration. This technique is implemented in some high end TT PSU for quite sometime now, no wonder owners are usually in awe using this kind of PSU. Its not as simple as its thought, like driving the motor at constant 80V, quartz lock 50hz indefinitely. Constant voltage applied throughout isn't the end of things yet.

Hi Cmboy,
Thanks for your comments. I got the idea from here :
TT PSU

I have done this below so far, and waiting for the DDS module to arrive and will connect up to test. The small pcb in the middle is the +/- 15V dc regulated supply and the pcb on the right has the class B low power amplifier with a gain of about 5. The DDS can be programmed to output AC sinewave up to about 3V peak-to-peak. Fine tuning the voltage will achieve the required 12VAC at the amplifier output, enough to drive my turntable motor.

For 120V motor, use another transformer (say 240V primary to 24V sec) in reverse to step up the voltage (10x) before connecting to the motor.

As you mentioned about lowering the voltage to a certain point (to minimize vibration), I will use the DDS built in memory presets (it can store and recall from 10 memory presets) for this purpose, and also for different speeds.

As for dual sinewaves (with 90 deg phase angle), if you have the expertise, to write the software, it can be done easily. At the moment, I am using the good ol capacitor to do the job.

TT PSU completed, freq and waveform looks good, output voltage at 12VAC peak-to-peak is achievable with +/-15VDC supply for the power amp. My turntable motor is 12V synchronous motor so its fine. Frequency is adjustable in 0.01Hz steps and can save & load up to 10 presets. For a start I will use 50.00Hz & 67.50Hz for 331/3rpm and 45rpm respectively and measure actual turntable platter speed and fine tune & save freq.

COSTS:
1) DDS from eBay (made in China) is USD45 = RM140 (delivered)
2) local parts for power amp, op amp and transformer & box = RM100
3) Total = RM240

See pictures below, the small pcb on bottom left is the 5V digital power supply that comes with the DDS(wall wart casing thrown away), the waveform is 50.00Hz sine at output with 5V/Div vertical scale. Those interested in the schematic/details can PM me.

Project completed - runs well, sine wave checked good, platter speed checked and adjusted. The motor now runs cool - previously the motor gets quite warm after 30 mins. To achieve 33.33rpm (using digital tachometer) in my case, the frequency is 48.90Hz. Now only left the finishing touches - knobs and buttons.

Hi CLH,

Hello dear CLH,

I'm always having problems with motor turntables, here in Brazil the voltage is 220V/60Hz,
and I usually encounter motor 110V/60Hz or 240V/50Hz.
And the available Power Supply to turntable are expensive.
So I'm very interested in building a DIY TT Power Supply like this that you published.
The motor that I use today is: AIRPAX SYNCHRONOUS MOTOR 110V/60Hz 300rpm,
and I also wish you let me know if I can in the future to use MOTOR 230V/50Hz
without any need to make changes to the Power Supply? Besides changing the primary of transformer.

I thank you in advance for your kindness,
Paulo

Welcome Paulo to the forum.

Just a pointer here. You can also send a private message to CLH, just in case CLH is not online in the forum all the time.

Hi elhefe,

I am very grateful for your reception and orientation.

But unfortunately as the newbie forum, I am forbidden to send a personal message before completing seven days.
As soon as possible I will send a message directly to him.

All the best
Paulo

Hi members,

I'm very interested in building a DIY TT Power Supply That You like this published,
so I wonder if someone from the forum has this project and can deliver it to me.

Thanks Paulo