GoodSoundClub - Romy the Cat's Audio Site

In the Forum: Analog Playback
In the Thread: Turntable speed analysis.

Post Subject: My deckPosted by N-set on: 6/11/2012

So as an example let me present here Paul's analysis of my EMT930 (all the credits go to Paul R).

The objective of this particalr run was to test an apparent audiopedophile(tm) legend that a 78rpmwarmup or preforming of an idler deck helps to smooth out the idler errors (eccentricity I guess).The deck was heroically spinning 78rpm for 4 days, the test LP is Ultimate Analog and the samples were taken rightbefore the warum and right afterwards.

Here is the polar plot (ignore the spikes--dust or static) for a quick glance:

First of all the warmed up and cold plots do not differ much and I even like the cold one morefor being more consecutive from revolution to revolution, so the audiophile rule did not work for me...what a surprise!
Then, one immediately sees that the shape of both cold and wrmedup samplesis pentagonal and the same for all the runs (there are approx. 5consecutive revolutions plotted)--there is some regular speed error 5x per revolution.This corresponds to (approx) 5x0.555Hz=2.775Hz.

Let's look at the demodulated speed spectrum

The strongest component is exactly those 2.7somethingHz (Paul gave the exact number from the datafiles).

Then there is a dent in the polar plot once per rev. I have little idea as to where it comes from,it may be disc eccentricity together with idler errors. This once per rev. error is visilble as (much smaller due tome having a lot of luck positioning well the test LP on the spindle)0.555Hz peak. Actually the deck was serviced by one of the reputable EMT serviceman in Lahr and he did tell me that when measuring WF he saw an increase of it once per rev, but since it was within the 0.075%spec, he has ok'ed the deck (he is using custom made test LP with a brass insert for an excellent concentricityand one of the EMT WF meters, 420 IIRC).

Then there is a 12.35Hz peak in the "cold" spectrum and it's 2nd harmonic in the "warmed up" one.This is most probably the motor (Paul tried to explain it by 3130Hz(the actual freq.)/3150Hz(the master of the LP) *25Hz=24.8Hzbut I did not get this explanation)

So what are the conclusions at this moment? The idler is the source of the biggest error. Right now there is a new original EMT idlerset up by the mentioned serviceman and said to be within the specs, all tip-top. My plan is to restore the old idler (at a fractionof the EMT price), e.g. with this guy

http://www.terrysrubberrollers.com/

and see how it performs compared to the EMT one. Beyond that I have no intent to mess up with the deck alignment again.Another thing is a dedicated power synthesiser to supply the 3phase power to the motor. Then new experimental possibilities open.
Cheers,N-set

Rerurn to Romy the Cat's Site