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Old 30th October 2002, 03:49 PM   #1
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Default Designing a motor speed controller/PS - querying motors

Hello, this is my first post on this forum.

I'm designing a new speed controller / power supply to feed AC sync motors on turntables. Primary objective is to use it on the project perspective TT (my own) but I want to build it so it can be used on other TTs.
So I'm trying to find out which TTs have a low-voltage AC sync motor say up to 50 volts. I know of a couple from project and rega P9 also uses one. Do you know of any ?

By the way, suggestions on such a device are very wellcome.

Any help will be much appreciated and will keep you posted on work progress.

Best Regards,

Guilherme.
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Old 31st October 2002, 02:12 PM   #2
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Default AC REG.

Hello,

Maybe one of our new mebers can chip in here?

see this link and there must be some other in the Analogue DIY TT section as well.

New from BC Canada.

Hope this will help you along,
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Old 31st October 2002, 03:28 PM   #3
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Thanks Frank.

The link you sent me shows that the TD160 will not work on my project since it needs 110v. What I was trying to do was to do a sort of a poll. People would reply and say: I have a TT that works with a 24volt or 20volt motor or whatever. The result of that poll would enable me to better decide what the max voltage of the device would be. I dont want to design for those motors that run on mains supply (110/220v) but rather those that are low-voltage.

Anyway, my approach to speed control is very different from Steven's. I'm using DSP: transfer a sinewave to RAM and use a counter to run through the RAM which outputs to a DAC. Counter is clocked by a PLL locked to a quartz oscillator. The PLL loop has a divide by 1024 which sets the "sample" frequency. This design allows for a very precise and small step increment of the frequency output.
Then, since I have a microcontroler in the design, I'll add some goodies like the possibility to store up to say 8 different speed settings (for those old 78s), soft start/stop to avoid those pops when the motor starts, mileage count to know how many hours your cartridge has, phase and torque control to smoth motor operation and I have some ideas on slightly "bending" the sinewave to achive achieve a quieter operation, but I'll have to confirm that once I have a prototype running.

It sounds more complicated than it is and parts cost (first estimate) should be somewhere in $250.

Best to You,

Guilherme.
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Old 31st October 2002, 04:03 PM   #4
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Default TT MOTORS

Hello Guilherme,

Quote:
The link you sent me shows that the TD160 will not work on my project since it needs 110v
TD160 I had always ran straight off 220VAC/50Hz??
And apparently his problem was adapting the European sourced (my educated guess) TD160 to US voltage and frequency,he probably just used a step down xformer and regenerated the 50 Hz AC cycle with a Wien oscilator?(I'm simplifying)

Anyway it doesn't really matter.


I think I follow you but what I don't quite understand is why you don't use DC motors iso AC in that case?
They are a lot easier to bring under control I think.

Do you see any advantage there?

Our resident moderator Dice45 knows a lot more than I do about this and I think if you twist his arm a bit and offer lots of bribe money he'll give you a good number of pointers.(just kidding)

I think what you're trying to do is similar to what Dice's buddy Manfred Huber has accomplished with DC motors.
If you go down that road I think it is also a good idea to install some modicum of control/check the actual speed of the platter and introduce a comparator +diff amp to correct any differences as well.
Not an easy task!

In my designs (years ago) I always relied an low torque AC motors and high platter mass since IMO the hard part is controlling overshoot,hence bringing the speed down without audible deterioration to playback.
Then again I'm pretty much old school and can be pigheaded.

You can do a search in the DIY TT thread we've running as well if you haven't done so already.

Let's make a DIYAUDIO TT


See you,
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Old 31st October 2002, 04:55 PM   #5
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Frank,

I'm not building a TT. I just bought one (the Pro-Ject Perspective) which has an AC motor and thats a very good reason for going AC .

If you care for my opinion (motors are not my main field of expertise), I think that if you can control the "kick and jerk" problem of AC motors, then they have the potential to produce more stable speed without drift since the rotor is just following the frequency the motor is beeing fed.

I agree with you that low-torque AC motors are the way to go and thats one of the reasons I got the Perspective (though it does not have a very high mass). I think that if you can control torque so that you raise it to win initial platter inertia and then lower it to a minimum just to keep it going, then you may get some surprising results (I hope).

In DC motors (and correct me if I'm wrong) voltage controls speed and torque. So if you use them in a system that does not have some sort of feedback to correct speed, you'll have problems like temp drift and speed change by just loading the platter more or less (record weights).
OTOH, if you use feedback servo, you'll have the problem of over/undershot as the servo tryes to zero-in on the correct speed. There was an article published somewhere where a 5Khz oscillation was detected on such a system (I think it was on stereophile, not sure).

I'll put a diagram on this forum when I have one. Right now its just a bunch of drawings on paper and IC specs scattered all over the place.

Best to You,

Guilherme.
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Old 31st October 2002, 06:11 PM   #6
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Default PRO-JECT

Hello Guilherme,


Quote:
If you care for my opinion (motors are not my main field of expertise), I think that if you can control the "kick and jerk" problem of AC motors, then they have the potential to produce more stable speed without drift since the rotor is just following the frequency the motor is beeing fed.
Of course I do care for yr. opinion and hope you value mine reciprocally.

I looked up some info on this TT and noticed it has some sort of suspension on the platter and armbase assembly.The motor however seems to be fixed to the plinth.
If you then use an AC motor and a mediumweight platter the "kick and jerk" effect of the AC motor can very well become measurable and audible IMO.(due to low platter inertia)
Regulated it will most problably improve on the motors' behaviour but I don't think it will negate the problem entirely.

A better (meaning more complete) sollution to this would be to block the suspension (and locate the TT on a wallmounted shelf out of the direct soundfield of the speakers) and if the bearing allows for it I would opt for a platter of at least twice the weight (maybe it is available as an upgrade?) and a belt that has enough elasticity to even out the ripple caused by the 50Hz pull on it.
By doing so one will very likely gain in lower noise floor and therefore gain in dynamic contrast.

Don't get me wrong here:I don't want you to think you have a bad TT on your hands,I never even heard it.
This is just my view of what you could try out.

Best regards,
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Old 31st October 2002, 06:54 PM   #7
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Default 3 SPEEDS 3 MOTORS??

Guilherme,

Sorry I didn't notice this at first but it seems from the spec sheet that they actually provide 2 motors (33rpm + 45rpm) + an optional one for 78 rpm!!

In that case an electronic speed control would make a lot of sense.

Our local TT fan Dice45 has a far more economical way of resolving that:he suggests 3 different pulley diamaters that would be alternatively fixed with a collet chuck to the motors' spindle.
(a solution put forward due to limitation of the sideway torque allowed on the Maxon motors)

Rgds,
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Old 31st October 2002, 11:09 PM   #8
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Default Re: TT MOTORS

Quote:
Originally posted by fdegrove




TD160 I had always ran straight off 220VAC/50Hz??
And apparently his problem was adapting the European sourced (my educated guess) TD160 to US voltage and frequency,he probably just used a step down xformer and regenerated the 50 Hz AC cycle with a Wien oscilator?(I'm simplifying)

Anyway it doesn't really matter.


See you,
Frank,

My TD160 was originallly run straight off 220VAC/50Hz. There is a resistor in series with the synchronous motor. If you by-pass this series resistor, the syn motor will run with 110VAC/50Hz.

The power supply is very simple. It uses a step-down transformer and 3-terminal regulator to get a +/- 15 volt for the Wien-bridge oscillator and a low power PNP/NPN output at about 5.5 VAC at 50Hz. This 5.5 VAC is then step-up by another transformer to 110 VAC to feed the TD160 syn motor.

This electronic 50 Hz supply is very sinusodal and free from digital and line interference. It improves the sonical effect of the
audio system.

Alternatively, the 5.5 VAC could be stepped-up another useful voltage if you like (say 24 VAC).

One could also change the speed electronically by generating another frequency (say 50 Hz for 33 1/3 rpm and 67.5 Hz for 45 rpm) with different set RC constant.


Quartz controlled AC supply for the turn-table syn motor will give a very accurate speed control. However, the AC waveform is usually reconstructed digitally and may pose some digital interference to the audio system.

DC sevro motors for turn-table rely mainly on feedback for speed control. This could degrade the sonic quality because they tend to oscillate around the mean zero error.

The Seven
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Old 31st October 2002, 11:19 PM   #9
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Frank,
please re-read my posts!
I am using a maxon with one pulley diameter. Different pulleys are used for either string or tape belt use and it also is thinkable to provide rough and smooth pulley surface, depending if the user wants to have much slip or much grip.

The maxon motor covers speeds between 16 and 121 rpm. It is an asynchonous DC motor and its speed is linearly determined by the supply voltage.

I find it ridiculous to fiddle with different pulley diameters to change speed.
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Old 31st October 2002, 11:30 PM   #10
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Default OOOPS

Bernhard,

Quote:
Different pulleys are used for either string or tape belt use and it also is thinkable to provide rough and smooth pulley surface, depending if the user wants to have much slip or much grip.
You're absolutely correct I confused this.Sorry my mistake.

What I meant to say is that I don't quite understand why a manufacturer of TT's using an AC motor is not offering your system of exchangeable pulleys to handle requirements in speed changes:

One for 33rpm,45 and 78 .
Ok optional perhaps but a lot easier to mount than swapping motor units.
Not to mention the expense involved.

Regards,
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