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Phono Cartridge IMD Measurements, Observations and a Question

I've just recently added an IMD test to my cartridge alignment procedure. I use the REW RTA feature as a spectrum analyzer. It will display enough resolution to pick up the +/- 60Hz IMD sidebands at 4KHz using the AP Ultimate Analog Teat LP. (And it will go down to 2Hz, so the turntable rumble can be seen!). I convert the signal from A->D using a Benchmark ADC1.

I have an SL-1200G. I'm using two arms, a Hana ML on the stock arm and a Shelter 901 MKIII on an outboard pod mounted Micro Seiki MA-505 arm.

Signal comes from the cartridge to a switch that selects the arm, then into an SUT with Blue Cinemag treansformers., then on to a Parasound XRM phono pre operating in MM mode (40dB gain). Balanced cables come out and go to the input of a Levinson no. 38s line level preamp (oldie, but sounds and measures great). I then input the 38s signal to the A/D (balanced cable)

I measure about 0.5% IMD with the ML and about 3% with the Shelter. I have moved both arms up and down. Total vertical movement for each arm is 6mm. I cannot measure any change in IMD with either arm at any height when I change the height. Both arms have VTA on the fly, so I can observe the measurement as I adjust.

Why can't I see any changes in the IMD? Am I measuring the wrong thing? Should I be adjusting the rotational alignment of the stylus to the groove (offset angle)? Or some other parameter?

 

128x128Ag insider logo xs@2xkevemaher
lewm
13,337 posts
 

Ralph, Are you suggesting that the platter mat contributes to IMD, or what? I could imagine that it would, actually.

Kevemaher, I am still hoping you will switch tonearm/cartridge combinations to see whether that (energy dissipation in the pod vs on the plinth) makes a difference.  Another thing you could try is to deliberately screw up alignment.  Twist the cartridge in its headshell, or deliberately introduce an error in overhang.  Along those lines, we know that many cartridges have the stylus misaligned with respect to the long axis of the cantilever (some call this "zenith").  That's possibly another cause of IMD and can be mimicked by deliberately twisting the cartridge, even assuming that either or both of your cartridges bear perfectly aligned cartridges to begin with.  Any of these things would be more likely to alter IMD than simply adjustment of VTA, in my guesstimation.

atmasphere's avatar
atmasphere
11,967 posts
 

Are you suggesting that the platter mat contributes to IMD

@lewm Yes.

The role of the platter pad is still poorly understood in high end audio which is a shame. But its effects are easily measured and heard.

Most platter pads are just plain wrong. To be right, it must have the same durometer as the LP. It must then be able to absorb vibration- it must be damped. Bonus points if it also damps the platter.

I’ve had access to such platter pads for nearly 35 years. One thing I’ve noticed is that when I set the stylus down on the LP, it makes no sound. By this I mean that if the volume control is all the way down, the sound of the stylus tracking the LP can’t be heard. I’ve seen a lot of platter pads where you can hear the stylus tracking from several feet away.

Put another way the LP must be damped so it does not talk back to the cartridge.

I’ve seen many ’studies’ of LP distortion which don’t take this into account. When a large variable like that isn’t nailed down it becomes junk science.

kevemaher's avatar
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kevemaher OP
106 posts
 

Today, I am measuring IMD with the CBS STR 112 Test LP. I will post results when I've finished analyzing..

I have replaced the Shelter 901 with a brand new Benz LP-S. First look is that IMD from the LP-S and the difficulty of making adjustments by raising or lowering the tonearm are about the same.

@atmasphere I have heard what you're describing. I am using a "Platter Pad" which is some orangey rubbery material (maybe rubber). It is 6mm thick. I bought it about 1980. WIth this pad, I hear absolutely nothing, even with my ear close to the stylus. I have used it continuoously on one table or another since 1980.

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