DC vs AC motors - a newbie needs help

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Also some AC motors have recently gone out of production and
as DC is marketing flavour of the month, switches to DC motors
with inevitable claims of improved performance have been made.

:)/sreten.

So as far as you are concerned, sreten, the agreed (by everybody who has listened to it) improvement of the Linn Radikal DC motor over the previous Lingo AC motor is pure BS? :)

Regards,

Andy
 
I was searching for articles around Radikal, and I see that it calibrates the motor power during powerup. I guess this means that at powerup it sets the DC level (I guess not PWM in this application, but pure, clear DC, am I right?) and leaves it until it is calibrated again.
Than what happens with the breaking effect of the pickup ? Just do not care about it ? I do know if that matters at all, or just marketing things from people who sell syncro motors. I have never had a DC motored TT. Yet, I thought the DC motor is good for heavy platter TT, because the changes in the breaking effect of the pickup smoothed by the high rotating mass. But, the LP12 does not have high mass platter, so I do not even have theory now :)

The feedback is on the motor directly or on the platter in case of Linn Radikal?

Thanks,
JG
 
Although there are some turntables with really heavy platters, the Linn (about 4Kg IIRC?) has most of the mass in the rim, so the moment of rotational inertia is quite high. Many modern turntables use plastics in a solid disc, which for the same mass would have less inertia than the Linn. Think flywheels.
 
Basic AC motor drives have a couple of problems.

First they rely on the mains for freq stability but its not stable. In the UK there are legal limits to this variation but its there all the same. If you use a mains energised strobe to
adjust the speed of a turntable this must be taken into account.

Second the motor requires more voltage to start up than when its running. As the
motors vibration is directly related to applied voltage it means that the motor will
be vibrating more than it should when running.

A synthetically generated AC supply can fix both these problems. I helped design
a commercial version and in my opinion its well worth doing.

Recently the increased cost and availability problems of the AC motor used (Originally Impex) made a servo DC version cost competitive even with the electronics included so I have just completed the design of a DC supply to replace the AC one.

However in all honesty I cant say that I hear much difference, at least on our turntable between the AC one and the DC one though I think the DC one may just be a little bit
better. In my opinion I think its more down to specific implementation than to type.
Ive seen and measured some pretty poor examples of both !
 
So as far as you are concerned, sreten, the agreed (by everybody
who has listened to it) improvement of the Linn Radikal DC motor
over the previous Lingo AC motor is pure BS? :)

Regards,

Andy

Hi,

Not saying that, but am saying be wary of marketing BS generally ;) .

DC motors now seem derigeur for any "high end" turntable just on marketing.

I am getting cynical in my old age, when i was young belt drive DC motored
japanese decks were remorselessly slagged off, some of them correctly due
to over-enthusiastic speed feedback loops, but they weren't all wrong.

35+ years interest in hifi qualifies me as an old duffer, I like still like
technical evidence to accompany glowing subjective appraisals.

When will DC motors for idler drives become the next big thing ?
Not likely .... if it ain't bust don't fix it ....

Personally I cannot see much technical difference in the drive issues.

rgds, sreten.
 
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Hello,
I have tried AC motors today again. In my hands, it feels far better with the accurate 2 phase drive than with the phase shift cap. I will try soon in the TT.
In the other hand, I give another try for the 3 phase motors. A pulley will be made now for one of the HDD motor to try. I make a simple drive circuit now which does not require half of my desk :)
Regards,
JG
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An externally hosted image should be here but it was not working when we last tested it.
 
So, I've got these two fans I bought quite a while ago when I was thinking of building a class A amplifier. They are PC fans by Sunon and they turn very smoothly and quietly. Perhaps their motors would work as TT motors?

They are rated for 220V AC and 0.14 amps. What would happen if I ran them at 120V?

Unfortunately, they seem to be VERY well built. I'm not sure I'll be able to disassemble them without damaging them.

Any other ideas for inexpensive, yet high quality TT motors?

I had a few old PCs laying around that could have sourced some fans or HDD motors, but, alas, my wife got rid of them a few weeks back.
 
I wrote a mail to McLennan. Do you think they sell 1 motor ?

Paul, I think you should not worry about the HDD motors. I think they are not good for slow RPM. I have tried several and Y and in delta also, with 3 sine wave. You need to spin up gradually and they get smooth above about 1000RPM, which is too much.

I think the best cheap solution is a TT syncro motor, like the premotec.

If you really want something else, FDD motors maybe, or, I think VHS capstan motor, maybe.

Regards,
JG
 
Thanks, JG. You've saved me the time messing with HDD motors.

Yeah, it might be that a premotec is the best way to go, but they're around $100 here, best I can see, and that pretty much kills the project. The goal is to build something cheaper than the Project Debut III.

I'll look into the VHS capstan motor as a possibility. That's a good idea. I suppose a cassette deck might be another possibility.
 
I've spent at least twice as much on the generation of the 3 phase signal. (but it was fun and good experience). So, the HDD motors are expensive for me :) The good thing is that I use 2/3 of the same circuit on driving the premotec sync motor.

I'm started to make a DC motor control circuit also.

Do you guys know examples of DC motor control for TT ?
I know (and have) Mark Kelly's pcb which compensate sensed current increase with driving voltage increase, but I want something different.

Do you know if the commercial DC motor drivers use clean DC or PWM ? RPM control during playback or just at startup ?

I plan to make one with a modern microcontroller (not for performance in circuit, but for easy development and copy).
I want to have feedback from the platter, but use it only at startup and if I push a button.
I plan to use Manfred Huber's analog circuits (wrote him a mail, if not allowed than not) with a 16 bit dac and debug capable controller.

Regards,
JG
 
well, I'm on the way now with the DC motor control, the PCB is getting made now.
In the other hand, I did not gave up completly with 3 phase motors :)

I'm looking for motors, bigger than a HDD motor, multi phase, and designed for less fast rotation. Like an FDD motor, but better made. The magnetic field should turn at least 5-10 times until the motor turns one. More is better.
Low inertia preferred.

Do you have ideas for such a motor ?

Thanks,

JG
 
No, I'm really just coming up to speed, if you'll excuse the bad pun. So, I'm not sure which direction I'm going to go in, though I'm leaning in the direction of a Maxon DC. Unless I decide I really need to keep the cost down in which case I'll look into using 7.5 degree steppers or other inexpensive solutions.

By the way, there's a Premotec 9904 on ebay, if you're interested:

PREMOTEC dc servo motor for Hi-Q Turntable | eBay

Did Manfred allow you to use his circuit? I have one of the early DIY Teres turntables, so that is what I am using. But, I will probably want something cheaper and simpler for the turntables under consideration.

Do you know how important it is to have some sort of feedback loop? I'm wondering how detrimental it would be for these simple turntables to just have a good solid DC value going into the DC motor (or a good AC signal for an AC motor, for that matter).

I've thought about using an Arduino based controller, quick and easy, but it adds around $50 to $100.
 
AC synchronous motors...

I managed to stumble across a Japan Servo Company 27 Watt induction motor( 27 Watt output,120 VAC @60Hz, 3450 RPM). It has a built in capacitor network and thermal protection. Using a stepped driven (idler) pulley to run the output at a 300 RPM equivalent, this should allow it to be used. Or a variable frequency AC input. Unfortunately the frequency required would end up being something in the order of 5.25 Hz I'm not that sure it can be done accurately and without other effects. Perhaps a combination of an idler wheel and variable frequency generator could be used. This seems to be something that can be considered: large"ish" industrial induction motors (as long as the motor is very quiet. The one I ended up with is absolutely silent at 3450 RPM), with variable frequency generator and a means to gear down the speed using a multi-step idler.

Paul: a quick and dirty speed controller for the Altmann style drive is an MP3 player or CD-R with the appropriate frequency recorded on an MP3 player or CD-R. Just play it back and amplify the results the output is the same value as the DC voltage requirement for the stepper motor. DC voltage (even though it now becomes "AC"). The 7.5 ° stepper can then be controlled by the frequency. This allows for some experimentation to get the speed correct. I know Mark Kelly has always frowned at this, but it is quick and dirty. What do you need to pull it off? A source for the signal and a stereo amp capable of outputting the required voltage. Cheap T-amps are good for this.
 
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Mark Kelly doesn't frown on this approach, in fact I did it myself for a lark on my Garrard motor and it worked quite well.

Maybe Nanook is confusing things a bit. I have said many times that stepper motors are pretty close to useless and I'll say it again here.

The reason is the very high variation in magnetic reluctance with position - this is the feature which creates the steps. It also causes very very high torque ripple - logically the ratio between detent torque and run torque gives a percentage ripple which can approach unity.

You only need one MP3 player for a quadrature signal, one channel for each phase.

I've used the TPA 3122 in this duty and it worked well. With synch motors almost any amplifier will do, when running at synch speed they are an almost purely resistive load. Synch motors are sometimes used in industry for load factor correction. Make sure your step up transformer has adequate secondary inductance - the amp will see it as a short circuit if it's too low.

Induction motors are another matter, they can present a very difficult load.
 
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