Try the comparison with each cartridge mounted on the 1200G tonearm (separate measurements of course). Also are you doing this in mono or stereo? If stereo, are you getting the same result in each channel for both cartridges? Does azimuth have an effect?
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?
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am I correct in thinking that the source of your apparent dismay is that vertical tracking angle makes no difference to your measurement? Otherwise, if you are doing this to compare the two cartridges, obviously you need to test the two cartridges in the same tone arm under the exact same conditions so that all other elements are constant. I am sure you appreciate that. The absolute numbers you get are probably affected by the intermodulation distortion characteristics of the electronics that are in your signal path before the measurement is made. Perhaps IMD of those electronics are dominating the result, and that is why VTA makes no difference. |
@kevemaher That's a pretty good bet. By placing the Shelter on an outboard pod you put it at a disadvantage. Any vibration that might be affecting the platter will not be affecting the pod the same way so distortion will be higher. The SUTs will ring (distort) if improperly loaded. Since the correct load varies with the source impedance, you have to have the correct loading at the output of the transformer for each cartridge. The distortion of the phono section, whatever that is, will be compounded by the line section. I think you'll have more accurate numbers if you simply don't use the line stage. The different arm is a variable too. You'll want to know that both cartridges are aligned correctly and when that is done are able to track properly too. Resonance in the arm tube can play a role as well as any chatter the bearings might have. I think you can see now why so many of the 'LP distortion studies' in the past were bad science as there are a lot of variables at play!! FWIW I think you'll find the platter pad affects the distortion as well. This is caused by resonance in the LP which can 'talk back' to the cartridge. If the platter pad is damping the LP correctly you'll see the distortion go down.
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