Testing direct drive turntables

Content:

Initial general setup
Part I. Motor without plinth. Arm in the pod standing alone.
 - Setup A. Motor and tonearm on the table.
 - Setup B. Marble base under tonearm, rubber spheres under SP-10.
 - Setup C. 60 lb granite slab on the table.
Part II. Original Technics obsidian plinth SP-10B3, EPA-100 tonearm
Part III. My budget “universal” plinth


Turntables to be tested

Initial setup: Turntable -> Carver C-1 preamp ->Audigy 2 ZS

Software: Audacity + Creative WaveStudio 7. Recording settings:


Test of the background noise of recording track after turntable  

Image 1. No signal on open AUX input, soundcard itself:

That pick at the 60 Hz will be carried out in all spectrograms

Image 2. No signal on MC open input

Image 3. No signal on open MM input

Image 4. MM Cartridge connected.

Pretty much within Carver’s specs for phono stage to have about 80 dB noise floor in the main audible region.  


Part I. Motor without plinth. Arm in the pod standing alone.

Setup A. Technics EPA 100 tonearm.

Image 5. No signal on MM input with Stanton 681 connected (recorded with Audacity)

For comparison I had the same signal recorded by Creative. No difference in spectrogram detected.

Spectrogram on the Image 5 has to be taken as the common background for all following measurements.  

 Image 6. Needle on the table, motor is not spinning (Recorded with Audacity)

Image 7. Needle on the table, motor is spinning 33 (Audacity)

 Analysis of the spectrograms 6 -7 showed an unexpected for me results: there is big (20 dB) component of the noise in the 6 – 40 Hz region that present even when motor is not spinning. I had no idea where it came from. So I’ve decided to change my setup a little.

Setup B. Marble base under tonearm, rubber spheres under SP-10.

Note: Image 6 and following. Needle is sitting on the aluminum block on the top of the motor unit:


The same exact effect, even with another tonearm

Image 8. Just Arm (Signet):

Image 9. Arm on the motor, no spin

No needs to continue, something is wrong here. The only valuable data is existence of the free arm resonance at the 20 Hz. To check if there is something originated that signal in the motor, I’ve tested the case when needle is rested directly on the table.

The same result. The problem is not in the SP-10 unit.  Let’s try another setup.

Setup C.  To exclude possible influence of the vibrations coming from the floor, I put a 60 lb granite slab on the table.  

Arm on the marble base is sitting on the aluminum block.

Image 10. Arm in the air

Image 11. Arm’s on the table. Motor is not spinning.


Image 12. Arm’s on the motor spinning 33.

Similar curve with additional 10 dB added in the region below 100 Hz. Now, when I’ve isolated influence of the base, the only possible source of those resonances could be only air. Nothing can do about it right now.

To check it again, two tests without the SP-10. Granite slab 60 lb as a base.

Image 13. Tonearm itself, cartridge in the air

Image 14. Tonearm on the base, cartridge lowered on the aluminum block sitting on the base.

The same strange behavior. Time to test stock unit.


Part II. Original Technics obsidian plinth SP-10B3, EPA-100 tonearm

 

Image 15. Arm itself

Image 16. Arm on the motor without spin

The main difference is that resonance of the arm returned back to the expected 10 Hz. Other than that everything looks the same.

Image 17. Arm on the motor spinning 33

Conclusion: When you subtract spectrum 15 from 16, and then result from the 17, the only area that showed 2 – 5 dB increase of the noise signal (potentially associated with the rumble) is from 20 Hz to 40 Hz.

Bottom line, this method of the “in home” rumble measurement is inaccurate and not conclusive.


Part III. My budget “universal” plinth

To Be Continued .....


Just for a comparison a few spectrograms taken when playing some records:

Test record, 1 KHz test signal

Test record, “Quiet” groves

F. Sinatra recording 1.

F. Sinatra recording 2:

One can see that spectrum easy goes up to 20 kHz even without proper cart-arm adjustment in my primitive setup.