Turntable Speed Control

Having gotten round the motor problems we now have a red herring!

I've been very fussy about getting the 45 rpm speed bang on, especially for American customers where obtaining the correct frequency for 60Hz sync motor turntables isn't that easy.

I now realise the significance of the 45.11 rpm markings on 50Hz strobe discs, which led me to research things further.

For a 50Hz sync motor turntable 45 rpm is so easy to obtain from a quartz crystal. It just requires a 67.5Hz sine wave. And if you understand how to work with divide by n counters, obtaining 67.5Hz isn't rocket science.

For a 60Hz sync motor turntable 45 rpm is not easy at all. It requires 81Hz and it is difficult to get exactly 81Hz from all the quartz crystals which run at sensible rates, and with logic which again runs at sensible rates, by which I mean frequencies which aren't going to generate EMC problems.

So for 45 rpm, if 67.5 Hz is achieved for a 50Hz sync motor turntable, and 81Hz is achieved for a 60Hz sync motor turntable, then you have 45 rpm bang on - end of story...

Not!

Because 45s made in 50Hz countries are actually 45.11 rpm, and 45s made in 60Hz countries are 45 rpm.

The most difficult to get right, in the 60Hz USA (and other 60Hz countries), is 45 rpm, which really needs to be 45 rpm.

The easiest to get right, in 50Hz countries, just happens to be of no use, because 45s in these countries were mastered at 45.112 rpm...

Why? Because the lathes were synchronised to the mains, not a crystal.

60Hz is 3600 cycles a minute and if divided by 80 gives 45 rpm.

50Hz is 3000 cycles a minute and would have to be divided by 66.66666 recurring to get 45 rpm. For some reason they could divide it by 66.5, but not 66.6 or 66.66.

So, in the UK (and Europe etc) 45 rpm is 45.112 rpm, and in the US (and Canada etc) 45 rpm is 45 rpm.

Is it still 45.112 rpm on new pressings? Nobody knows. Probably they don't give a damn.

But the point is this. If a controller for a 50Hz sync motor turntable set to 45 rpm is used with a 50Hz strobe disc, it will not look locked... because 50Hz strobe discs are made for 45.11 rpm.



Edited by Graham Slee - 27 Dec 2017 at 3:30pm

I would  agree with your logic regarding the bars on the 45rpm strobe disc. It can be done, just needs 133 bars at 2.7 degree intervals and a .9 degree space. Alternatively 400 bars at equal spacing would give the correct indication as every 3rd bar would be picked up as stationary.
Surely, the assumption that the "error" is due to the inability to divide the clock frequencies to exact integers would only apply in this digital age?
The classic Neumann vinyl cutting lathes use/d Lyrec low speed (331/3, 45 and 78 rpm) synchronous motors with 3 different rotors on a common shaft and 3 stators with windings in the same casing. They drive the very heavy turntable via a fluid coupling to reduce motor noise and cogging. Power supply to the motor was direct from the mains.
For me this does raise the question as to how accurate the speed of the orignal master is.




Edited by BackinBlack - 28 Dec 2017 at 10:06am

Thanks, it's just numbers!
No doubt someone who is able with CAD can produce a template.