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Turntable Oscilloscope Readings

After setup of my cartridge, I used an oscilloscope to verify zenith, azimuth, and anti-skate force using Analogue Productions Test LP. Phase between channels is right on (zenith), and distortion of amplitude sweep is the same for both channels (anti-skate). I minimized cross-talk for the worse channel (azimuth). All good.

The strange thing though, is when measuring channel outputs, the right channel was about twice the amplitude of the left channel as measured on the oscilloscope. However, I can hear NO difference between channels through the speakers. Balance and centering of the sound stage appears perfect.

The components involved are VPI Classic Signature turntable with VPI analogue drive system, Soundsmith Sussuro Cartridge, and Herron Audio VTPH 1-MC Plus. The oscilloscope is connected to the phono preamp via RCA cables using a BNC to RCA adapter at the oscilloscope.

Any ideas about what is going on? 
gareents
7 responses Add your response
lewm
13,113 posts
 
Do you have a SPL meter? If you can confirm with a meter that the SPLs from each speaker are about the same, measured at about 1 meter and preferably using a pure test tone, then I wouldn’t trust the oscilloscope readings.Actually, that makes me ask what was the program material that gave you the impression of 2X amplitude in one channel vs the other?
oldhvymec
5,101 posts
 
Somehow your testing is being skewed, the final test is always "Listen"

Funny how that works.. Some how the data collection is off at the point of collection.. or how it’s being transmitted to the scope. Something is amiss.. Could just be the bias in the leads or connection point more likely.. If it sound correct.. Turn around too, swap ears.. :-)

Do a left to right lead swap to verify it’s not a left or right side issue.. Sometimes you have to calibrate what you calibrate with... Leads and connection are notorious for it.. CLEAN helps for sure.. Little steal wool or scotch bright goes a long ways too.. Little shot of electronic cleaner and then plug in.. works wonders on readings..

Regards
oldears
386 posts
 
@oldheavymech is exactly right. One can't assume the O-scope is calibrated, especially, if like me you bought it used. Swap leads and then swap horizontal and vertical to see what happens. If you are using the rca out of your phono preamp, that is another variable. You can check the o-scope by using a test CD.
mijostyn's avatar
mijostyn
8,570 posts
 
Gareents, you have to look at the scale. There might not be much difference. 

oldears and Hvymec if there is a calibration error it is the same for both channels therefore it factors out. 

Cartridges can easily be off 1 dB between channels at certain frequencies. With the right scale this can look like a huge variation when in reality it is not. 
Post removed 
tablejockey
3,916 posts
 
"I can hear NO difference between channels through the speakers. Balance and centering of the sound stage appears perfect."
 
Makes sense going the distance using  test equipment with the level of gear you have. On the other hand, your ears are pleased notwithstanding scope measurements.

May be one of those things you accept, or take it to a turntable doctor.

lewm
13,113 posts
 
I would assume the OP knows how to use his oscilloscope.  Also, as Mijostyn said, it doesn't really matter whether the scope is calibrated or not, if the OP took his readings at the same scope settings for each stereo channel, using the same pair of leads and the same channel on his scope. 
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