OMO, back EMF brushed dc motor speed control

[DarrenSV650S]

How does the load on a turntable vary?

[Bibio]

Originally Posted by DarrenSV650S

How does the load on a turntable vary?

well ac motors which most commercial tt's have take their speed control from the mains Hz wave but as we all know this varies so therefore the speed of the ac motor varies.

place the needle on a record and you get what is called needle drag depending on the modulation of the record (loud quiet music) the motor system has to overcome this to keep a constant speed. you also have flywheel effect of the turntable platter and spindle bearing drag depending on the viscosity of the oil in the bearing, add to that coupling of the motor to the plinth/sub chassis. ac motors are noisy so you have to decouple them from the arm/platter/cartridge with dc motors you can bolt them straight onto the plinth/sub chassis which gives better speed control but only if the motor and electronics are good enough to keep up with the flywheel effect of the platter mass.

you did ask

[DarrenSV650S]

What a load of pish. You are such a dork, you do know that don't you

I bet you £50 your ears can't tell the difference



Anyway, isn't that what the strobe markings are for?

[Bibio]

lol

strobe markings on a platter are there to tell you that its running at a constant speed. strobe markings are usually associated with direct drive turntables which is a whole different ball game but still suffer from the above. the strobe has nothing to do with actually keeping the speed constant.

as for my ears.. well nowadays yes your right but at one time yes. a classic test is piano notes, if the speed is not constant and at the right speed the notes will waiver.

however you can have the most accurate speed control in the world but its all down to how well the record was produced in the first place, which is a whole subject on its own but let me just say that no two batches of records are the same as regards to speed.

[Bibio]

its alive

Supply.jpg

motorboard.jpg

motor shaft speed adjustment 55rpm to 2900rpm. that equates to any belt drive turntable ever made

[Bibio]

on and lit up.

for 33 1/3
Pink.jpg

for 45
Amber.jpg

[SIII]

That's gorgeous.

[Bibio]

Thanks SIII

its taken me all winter to do this but it will be worth it

wait till you see it all together and on the turntable. that wont be for another few weeks/month

[madcockney]

In days gone by and when vinyl was the main source of listening to music, many hifi systems and those used in places such as radio stations used strobe systems to control turntable speed. (It may still happen today with high end systems.) There was a feed back circuit using the actual rotational speed of the turntable that corrected the rotational speed to compensate for things such as mains frequency or voltage fluctuations.

Most things can handle slightly varying voltage but frequency is another matter and has to be taken into consideration where critical.

[Bibio]

Originally Posted by madcockney

In days gone by and when vinyl was the main source of listening to music, many hifi systems and those used in places such as radio stations used strobe systems to control turntable speed. (It may still happen today with high end systems.) There was a feed back circuit using the actual rotational speed of the turntable that corrected the rotational speed to compensate for things such as mains frequency or voltage fluctuations.

Most things can handle slightly varying voltage but frequency is another matter and has to be taken into consideration where critical.

the strobe system was mostly magnetic gear tooth very much like the speed sensor on the SV.

a DC motor creates voltage as a by product of spinning, its this extra voltage that you can monitor to create speed accuracy.

each of the above are dependant on how well the drive circuit has been implemented. a badly designed encoder can be worse than back emf and visa versa.

people like myself are resorting to using back emf due to the silly prices charged for gear tooth/optical encoders.

back emf can be used with pwm but its very complicated to get right.

i'm still only half way there with the deign as i still have to test under load on the turntable. so far though while using a laser speed measuring device on the spindle of the motor the speed accuracy is bang on.