advertisement


Turntable speed analysis

Status
Not open for further replies.
If I put the pictures inline then they may annoy casual browsers by being wide. Anyway I've regained access to my web space so here they are again,

First LP12 45rpm comparison,
http://www.kilmory.demon.co.uk/pics/fmresults/45rpm_compare.jpg

Broader bandwith EMT/LP12 comparison,
http://www.kilmory.demon.co.uk/pics/fmresults/EMT_LP12_a.jpg

It's not surprising to me that an LP12/Lingo is quite steady in a steady state. What happens as the load changes is a whole other matter. Threads elsewhere in the audio room link to recordings that make it clear that the LP12 (for one) has audible dynamic rotation issues.

Paul
 
I should be asking if the 24 poles in an LP12 motor are part of the rotor or the static outer section, and what/how many magnets/coils are facing the poles in the other bit?
Click to expand...
The LP12 motor goes at 250rpm when fed with 50Hz. Which is 1/12 of a rev per cycle of input.

I expected (using the Lingo) to find a 50Hz artifact at 33rpm and a 67.5Hz peak when running at 45. However I don't (clearly) see these. The variations due to motor pulley eccentricity appear though.

Paul
 
EMT at 33 versus EMT at 45.

GS_EMT_33v45.jpg


The 25 and 100 Hz peaks have moved up proportionately. What that means, who knows...

Paul
 
could they be on the record? The EMT has gone now. I'll try with the SP10 tomorrow.

Did you get your Technics running yet?
 
I think the artifacts being on the recording are quite likely. It implies that your test record might be quite old? Using a nice old analogue tone generator with some PSU dependent frequency component.

Seeing the same stuff from the SP10 will confirm it.

I've dusted my Technics. Which is serious progress.

Paul
 
This would probably all be very interesting if I understood it. I've got a couple of records with a 3khz tone (HFS69 & 75), but neither seem perfectly flat or concentric so would probably generate meaningless results, though I guess that my using an idler deck will generate different types of distortion as the motor is very high speed and the drive mechanism so different. I'll try recording the better of the two disks some time

Tony.
 
Please do.

Everything is eccentric. Especially us. But in the FM spectral plots it gives a big peak at 0.55Hz(33rpm) and 0.75Hz (45rpm), so is separable from the other artifacts. Warps are at small multiples of those frequencies, so again should be identifiable.

I think in essence it's quite simple. If you replay a tone then the exact frequency of that tone is related to the rotation of the deck. It should be constant, it isn't. If we use an FM demodulator on the tone what comes out is a signal that directly represents the rotational speed. If we do this digitally then for every sample of the waveform input we output a number proportional to the rotational speed. At the moment what I've done is convert this sequence of numbers into a spectrum. So the 25Hz peak in the plot above implies that the frequency of the tone is varying up and down at 25Hz. The height of the peak relates to the depth of the modulation, how far from the nominal the variation is. In the LP12 plots for example there was a small variation at the motor rotation rate, which is probably a pulley eccentricity.

I wonder if we'll see something related to motor speed in the 301, given it's quite a high rpm and should be damped out by the platter mass. Data at 33 and 45 rpm would help, since the motor speed is nominally constant regardless of platter speed.

Paul
 
What about the method used to turn the cutting lathe of the test record?
I use the Cardas test disc 1K tone to set up DC motors with my DMM but it is just to get the speed right, any wow from the cutting lathe will be part of your measurement imo so what are you measuring?
 
Wow from the tone generator and lathe are obviously included.

But they're a constant. So if you want to compare turntables, or perhaps more pertinently, turntable upgrades, the method has validity. The implementation is obviously immature. But it exists and it's free. I'd like to compare an LP12 with Basik/Valhalla/Mose/Lingo/Radikal. I think relevant differences would appear, and that might allow a relationship to be established with audible reality.

Anyway why not post a recording?

Paul
 
Paul Ranson said:
Anyway why not post a recording?

Paul
Click to expand...

Of a 1 Khz tone?

If I am bored in the next days i can do valhalla, Geddon clone, Hercules, all external. Maybe Rega cct also if I have not pulled it apart which I have done with the airpower.

I have posted recordings using music of all of these before.
 
Here's the SP10 against the EMT. The 25Hz spikes are similar height on both, so I think that confirms the record is the source. There are a couple of SP10 spikes at 16.6Hz and 22.2Hz which it would be interesting to resolve.

GS_EMTvSP10.jpg


Paul
 
The SP10 motor is driven at 5.55Hz/three phase. Which means 16.67 pulses per second. Which might explain the 16.6Hz in the spectrum. 22.2 is 4*5.55 so maybe that's connected too?

Paul
 
It's a very tired (charity shop) record which I haven't bothered to clean. I imagine it would be noisy on pretty much anything.

The recording of test tones unaccompanied by some other non-static modulation isn't going to reveal much other than perhaps some electro-mechanical correction taking place. (see above)

A constant tone in one channel & perhaps the beginning of Blue Monday in the other channel would be more useful.
 
I'm not sure what this is supposed to tell us other than confirming the belief (to me) that recording test tones isn't of much real use?
Click to expand...
We can see the motor going round.

In the absence of a BlueMonday/test tone track it's still quite interesting.

Paul
 
Status
Not open for further replies.


advertisement


Back
Top