| GlidingDutchman |
Hi there,
I am looking for a relative simple circuit to precisly control a small 12v DC motor. It must also have 2 speed presets - and yes, it is for a turntable... ;)
The conversion kits comercially available is rediciliously expensive! I wont pay what they ask - rather make my own unit.
Thank you in advance for any advice on where to start. I am a bit dumb when it comes to designing circuits - usually a lot of smoke and cursing...
Dewald aka GD
Attached is the pic of my DEWAUDIO Credence turntable. At the moment it uses an AC Synchronous motor. 240 VAC with a Linn Valhalla PSU. |
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| PigletsDad |
I have a circuit for controlling a 12V motor with tachogenerator. It works pretty well. Does your chosen motor have a tacho? If so, I will draw the circuit up for you and post it.
If not, there are basically two approaches.
1) Simplest - just generate a low noise, adjustable voltage. Simple, can work well, but for a DC motor, the speed at constant voltage drops as the mechanical load, the torque, increases. Variations in torque requirements (for example due to loud passage on a record) cause minor wow. For two speeds, you just switch between two voltage settings.
2) Slightly more complex - current sensing circuits. These attempt to correct for the slowing due to increased torque by sensing the current drawn, and raising the voltage to compensate. If you over-do the compensation you get instability. There are several variants in this class. I've not heard one of these myself, but
they have a good reputation. |
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| GlidingDutchman |
PD,
Thank you for the response.
I dont have a motor yet. Any recommendations on Tachogenerator motors?
Option #1 sounds very easy at the moment... but auto-compensation will be favourable...
GD |
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| PigletsDad |
Primotec offer tacho options on some of their motors, but they are very expensive (£100 to £120) to buy in small quantities through the UK supplier.
I have one that I got 15 years ago, back when the motors were made by Phillips, before they sold the motor unit. I think they then traded as Impex, and later became Primotec.
There are probably other good options from different manufacturers.
See what options are available to you through your suppliers, and we can discuss further.
A tacho based circuit is more complex than the other options, and some people prefer simple constant voltage, or current sensing systems.
But you can usually find somebody who holds any opinion! |
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| error401 |
| Does anyone use stepper motors for this? They seem ideally suited - but there may be audio quality issues I'm not considering. |
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| GlidingDutchman |
Jeepers!! I am shocked!!
RS Components is the only supplier I can buy from without hassle and the DC motors they stock are hecktic... looks like I am better off sticking to AC Synchronous motors. Runs quiet, cheap and reliable.
What hassle will it be designing a Valhalla based PSU but with frequency adjustment? The Vahalla is fixed to produce 100 V @ 50Hz (+/- 15% adj on voltage output)
The Linn Valhalla schematic can be viewed here: http://www.vinylengine.com/download...a_schematic.pdf
GD
@ Error: Stepper motors can be used but I suspect they will mechanically induce a whine noise into the system. There is a guy however that converted a floppy drive's motor to run at 300 rpm. |
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| hermanv |
Stepper motors are generally a poor choice. They move in steps even when they "seem" to be rotating smoothly. As you can image driving a turntable platter with a series of incremental movements is far from ideal.
Things can be done to minimize this problem, but they require instrumentation I'm guessing you don't have.
Like so many other things, a motor controller can be designed and built, it should not be assumed that this is a trivial task. Wow, flutter and speed stability suitable for good quality sound reproduction may not be quite as easy as you presume.
Using high speed control pulses is a good way to control a motor, but you need to be careful that those same pulses can not find their way into your cartridge. This is why many turntable manufacturers use a 60 Hz sine wave controller and then adjust the sine frequency.
Like I said it can be done, but IMHO if you can find a conversion kit for under $200 you'll probably end up money ahead.
$50 for a decent motor (no commutator)
$35 for motor power DC supply
$40 for control electronics
$40 for chassis, cables connectors
I'm sure much of this could be done for less. I also know you can spend hundreds for a really first class DC motor.
If you must, perhaps some time spent on the electric powered model airplane forums would be educational. |
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| Algar_emi |
Read that thread and included documentation. Turntable drive and speed control is a complex subject and there is no easy solution.
Search and read Mark Kelly excellent website. He explains all the tests he did to get the performance he wanted.
http://www.diyaudio.com/forums/show...threadid=109934 |
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| GlidingDutchman |
[QUOTE="HermanV"]
If you must, perhaps some time spent on the electric powered model airplane forums would be educational.
[/QUOTE]
Herman!! Thank you for awakening an idea I had a while ago whilst visiting a hobby shoppe... yes! The aeroplane DC brushless motors may be just the ticket! Controllers are available for them and they have tremendous torque - a possibility there to drive a flywheel...
GD |
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| Bobken |
Hi,
You may find some of the comments in this thread of some interest, and I posted some opinions (based on my experiences) on the merits of servo-circuits compared with more simple regulator circuits.
As hermanv has said, these motors/controllers are not 'trivial' when attempting to get the best sonic results with belt-drive turntables, and always bear in mind that it is what happens at the turntable which determines the overall results here, and not simply at the motor.
Post #142 in that thread indicates that a lot of theoretical arguments and opinions don't seem to count for much, but that good results are obtainable without excessive complication.
http://www.diyaudio.com/forums/show...25&pagenumber=1
Regards, |
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| GlidingDutchman |
| quote: | Originally posted by Algar_emi
Read that thread and included documentation. Turntable drive and speed control is a complex subject and there is no easy solution.
Search and read Mark Kelly excellent website. He explains all the tests he did to get the performance he wanted.
http://www.diyaudio.com/forums/show...threadid=109934 |
Thank you for the link - very informative indeed!
GD |
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| GlidingDutchman |
Also got another idea...
Gonna try to get me hands on a B&O Beogram 1102 turntable (owned one 8 years ago)... it uses a rather precise DC motor drive system.
Want to see how they did it :smash: .
I know the motors were MATSUSHITA ELECTRIC (National Panasonic).
Even were fitted with a toroid tranny...
Sooooo .... will let you guys know....
Thanx for all the advice so far! :cool: |
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| error401 |
| quote: | Originally posted by GlidingDutchman
@ Error: Stepper motors can be used but I suspect they will mechanically induce a whine noise into the system. There is a guy however that converted a floppy drive's motor to run at 300 rpm. |
Ahh, that's what I wasn't sure about. It seemed to me if they can be used to rotate a platter smoothly enough to extract a data stream at 15krpm, they should be able to do so at a few hundred rpm. As far as I understand, stepper motors don't actually move in steps, but have multiple coils and only a few stable states. Driven with the proper phases of sine waves, they should produce smooth motion.
Understood if the complications make it unworthy of driving a TT of course ;) |
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| hermanv |
To get smooth motion out of a stepper motor a technique called micro-stepping is used. In this idea not only are there sequenced drive pulses but each pulse has a current ramp up and down so the drive signal ends up looking more like a trapezoid.
Floppy disks are digital devices some cogging in the rotational speed is not important.
Most direct drive turntables were abandoned because of current spikes getting into the cartridge coils, they still tended to have micro instabilities, but most of all because they were hardly the simplest answer to the problem.
The only thing wrong with the model airplane motor idea is that no one in that industry is the least bit concerned about accurate rotational speeds. I think a musician can probably hear 0.1% error. |
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| GlidingDutchman |
| quote: | Originally posted by hermanv
The only thing wrong with the model airplane motor idea is that no one in that industry is the least bit concerned about accurate rotational speeds. I think a musician can probably hear 0.1% error.
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I hear you regarding the airplane motors... but who knows? Maybe they do run stable? Maybe the controllers are tweakable... one thing for sure is those motors are very well made and dont cost an arm and leg.
0.1% Error can be irritating to any listener. My one prototype turntable's belt had a thickness diff and that caused a very sutle but audible "WOW" effect.
GD |
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