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Old 22nd January 2007, 03:11 PM   #1
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Join Date: Jul 2004
Location: Israel
Default speed control

Hi all,

I'm trying to power my DIY TT with a Maxon motor as simple as possible.
I hooked a 1.5 V battery to motor and it goes too slow,
2 batterys giving 3.0 V is too fast
Could I just add a potentiometer to lower speed with the 3.0 V ? would it be stable? what type and make of pot should i get for best performance?

As you can see I know nothing in electronics

Cheers,
IK
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Old 22nd January 2007, 03:31 PM   #2
sek is offline sek  Germany
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Normally, motors for turntables are driven by regulated speed control.
Setting the voltage (or current) in the motor will only set the speed of the platter for a certain record (mass), stylus needle and arm position (outer or inner area of the record will impose different drag on the stylus and thus different braking effect on the platter).

A simple potentiometer would work (provided it can withstand the power wasted in it, several watts peak), but a voltage- or current-regulator (optionally with feedback from the motor) works better.

For starters, you could take a look into the datasheets of National Semiconductors' LM317 or LM338.

But please keep in mind that true motor drivers for turntable platters work even more sophisticated than that - and that there is no consensus on the ultimately best way to drive a platter motor.

Cheers,
Sek.
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Old 22nd January 2007, 03:40 PM   #3
hacknet is offline hacknet  Singapore
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yeah, the 317 is a great place to start, the origin live uses a circuit similar to that to get the required voltage to spin the motor.

apparently speed will vary from the outer track to the inner track but i've heard it a couple times on a linn lp12 and it doesnt seem to be apparent.
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Old 24th January 2007, 01:45 PM   #4
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Thanks, Looks simple even for me....

Some of the big boys do use simple no regulation controls, Galibier for example.

Would you recomend the adjustable regulated circut or the one with the improved ripple rejection (pg 15)?

Also, the scheme shows input V of 28V or more. Could I use a lower V battery?

many thanks
IK
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Old 24th January 2007, 01:57 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
we are considering a DC motor drive.

I see two ways to do that
1.) fixed and absolutely steady DC voltage at extremely low source impedance. Do not use a resistor for motor speed control, that will increase the change in speed as load varies.

2.) regulated voltage with speed related negative feedback.

with 1.) the motor speed will change for changes in motor loading.

with 2.) the motor speed will constantly change as the NFB repeatedly corrects errors that are attenuated in the drive belt and hopefully held near constant at the platter. I fear this method will take a lot of development.

1.) will be much simpler and I suspect audibly acceptable. A very high inertia platter will attenuate speed variations brought about by groove modulation creating extra/less drag on the recording.
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regards Andrew T.
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Old 26th January 2007, 11:43 AM   #6
KJ42 is offline KJ42  Norway
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A ęsimpleĽ solution to the problem associated with alternative 1) could be to let the motor drive an auxiliary axle with higher friction and thus drag than the platter + pickup, this could make the variations in drag from the platter + pickup less significant than the drag from the auxiliary axle. I believe this could improve speed stability of the DC-motor, though Iím not an expert in this field.

Mvh
KJ

Ooops my time as a passive member of diYAudio.com Is over.
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Old 26th January 2007, 04:28 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

For DC motors the simplest best way of driving them is via a
variable voltage supply that has a negative output resistance
equal to the resistance of the motor windings.

Then essentially speed is fixed by the drive voltage = the back
emf generated by the motor with the winding losses compensated.
The current drawn at correct speed depends on the load.

/sreten.
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