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Old 13th Dec 2018, 5:44 am   #7
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Nice Way To Measure Turntable Speed

Variations in mains frequency shift both the speeds of synchronous motors and the indications of strobe discs. These are used to command the start-up and shut down of generating capacity. They're well under 0.5% in the UK, and so should be unnoticeable to most people, yet the thought of them has sold a lot of expensive Lingo and similar inverter boxes in the UK, though they have more justification on less developed power grids. The Japanese had their quartz-locked direct drive turntables.

In the non-audiophile world, up-market turntables were expected to have a variable speed feature AND a built-in strobe disc so the user could set the speed using mains frequency as the reference, while the basic models got a synchronous motor and a fixed ratio belt drive. I found this somewhat ironic. Was there really so much inaccuracy in pulley diameters, and belt-slip?

Strobes on the turntable mat could only be seen with no record. Large diameter drop-on strobe discs got in the way of playing the record, but some turntables had strobes around their periphery (underside with mirrors on my B&O 4000) These could be used while playing records and you could check for any effect of stylus drag creating differences on loud patches, or differences between beginning inner and outer tracks if you were interested.

I've had some experience of people using phones to time things that were also timed with calibrated equipment, and had a chat with some friends in the phone development business, so I thought I'd better add a word of caution.

David
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