You might also try deliberately twisting either cartridge a bit so that azimuth is out of alignment. If you still see no change then you know there is something amiss with your test setup.
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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@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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I've done some more measurements. I created an CD IMD test file and played it via my Oppo player through to the preamp out. There is no measurable IMD (probably below the noise floor) except at max gain where the preamp is overloaded. So there's no IMD being contributed by the Levinson no 38s at my normal listening level and 30dB higher also. I moved the SUT about 6 feet away from the turntable and the phono preamp. No change in the IMD. I completely replaced the SUT/Zphono with a Musical Surroundings phono preamp operating at 60 dB gain and 120 ohms output impedance. The IMD is there and is at the same level as the SUT/Zphono combo.. This means the phono preamp is not the source of the IMD. I didn't switch cartridges with the Hana ML that's mounted on the stock arm on the table. I have not yet played with azimuth and platter pad. Will a worn out stylus cause a problem like this or will the IMD be much higher? Right now, both cartridges show IMD. The Hana ML has less than 100 hours on it. The Shelter is older. I haven't kept track of hours. It still sounds the same, which is very good, even with the 3% IMD. So far, all measurements indicate that the cartridge is the source of the IMD. |
@kevemaher A lot of phono sections have trouble with the RFI generated by the operation of the phono cartridge combined with the capacitance of the tonearm cable. That RFI is usually blocked by an SUT. To rule out that variable you use a 'cartridge loading' resistor to damp the RF resonant peak. With sufficient damping the peak can no longer generate RFI. Because changing the preamp along with the SUT had no effect its likely the phono section isn't an issue. But if you want to be scientific then the phono section has to be ruled out. Once that is done then you can concentrate on the stuff upstream. |
@atmasphere Thanks for the insight. My system (up to and including the preamp): Cartridge (901 mik III or Hana ML), Micro Seiki MA-505L tonearm on a pod, Technics SL-1200G table, A/B switch, SUT, Parasound Zphono XRM phono preamp , Levinson no 38s preamp. By replacing both the SUT and the phono pre with a 60 dB gain phono pre, I believe I have ruled out the phono electronics. I have already ruled put the No 38s, as explained previously. So the only items left are the table, arm and cartridge. Based on this, I'm not understanding your comments. I believe I've already done what you've suggested. I may be misunderstanding your comment. What do you consider the stuff upstream?
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