Speed strobe recommendation.
12-19-09: Tfkaudio My KAB strobedisc is made of flexible plastic. Also my strobe light is 60Hz not the 300Hz someone mentions elsewhere in this thread. Must have been some changes. In any case I recommend the KAB system as accurate and foolproof. System Details |
Agree with Speedy9194. My partner made one using Corel Draw, but if we'd known about the one at vinylengine that might have worke too. It seems worth trying, unless $100+ doesn't matter to you. A cheap fluorescent light is all you need for illumination. (Cheaper is actually better - expensive lights often provide steadier luminosity, which reduces the strobe effect.) |
There is a caveat here: in the case of a typical, single-phase synchronous AC motor connected to the mains supply (think lower-end Thorens, Rega, VPIs, AR, some Projects, etc) its speed is derived by the incoming mains frequency - 60Hz here in the US. If the mains frequency is say .25Hz low (59.75Hz), then your table will run slightly slow but you'll never know - as the light you are pointing at the strobe disc to derive speed accuracy is also running .25Hz low. Hell, the speed could be wandering all over the place, but with the table and the fluorescent light clocked to the same frequency, you'd never know! Of course, this doesn't apply to Doug, as his deck uses a DC motor - many other folks use AC motors, but with some type of dedicated external supply (like an SDS, Lingo, SpeedBox etc) which (theoretically) ought to take care of such issues. Of course if good enough is sufficient, so be it. Have a pleasant holiday season! -Richard |
Richard, I work for a company that supports the electric power generation industry. Frequency variation on the grid should be virtually zero, certainly far less than the variation you suggested. Power generators never bring an off-speed or out-of-phase generator online. Throw a significantly out-of-phase generator online and the existing wattage on the grid would drag it into phase by brute force. That could cause millions of dollars worth of damage to both the generating plant and the grid. No utility ever makes that mistake. The speed and phase of an offline generator are computer matched to the grid, to tighter tolerances than the number you suggested. Only then is the generator connected to the grid. Voltage swings all over the place. Noise is rampant. But grid frequency is tightly controlled. If it weren't the entire grid would collapse on a regular basis. |
Doug, I believe tolerances for mains frequency delivery here in the US is specified at +/-1%, translating to a .6Hz variation. My note above merely specified a .25Hz deviation (which is a lot, but does sometimes occur). I understand the principles behind power generation, but rarely do specs match reality. Every day power grids correct frequency variations due to a wide variety of causes (equipment malfunction, load variances, etc) so that in the end things that depend on an accurate mains frequency (like clocks) are more or less accurate. It is, however, the shorter-term, briefer duration changes that would be affecting something like turntable playback (with an uncorrected AC synchronous motor). Subtle, but certainly there, and possibly enough to induce a false reading using a fluorescent lamp and a strobe disc (on any type of motor). Plug an accurate (to the hundredth of a Hz) frequency counter into a power line - you might be surprised. http://leapsecond.com/pages/mains/ -Richard |
