Phono Cartridge IMD Measurements, Observations and a Question

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.

Thanks for providing more detail. As I guessed, your test CD does encode a control for IMD. Also, I did already understand that you’re seeing different levels of IMD from one cartridge vs the other. That could be real or due to mounting the two cartridges in two different tonearms, one of which is on a pod. Or even u nintentional minute differences in setup accuracy. Now you’re saying that VTA does make a slight difference which is encouraging in that it suggests your method is actually reading IMD due to cartridges. I think this question arose earlier, but have you yet tried swapping the cartridges between the two tonearms? If the cartridge mounted on the pod is consistently exhibiting more IMD than the one mounted on the TT, that may suggest the pod is partly a culprit in causing the differences.

Just reading your exchange with Atmasphere, it seems that the readings are repeated under all circumstances and also regardless of VTA and that the L and R channels read the same or possibly identically. Doesn’t this suggest that the measurements might be spurious, and by "spurious" I mean the result of some factor you and we are ignoring (not necessarily RFI). It just seems unlikely to me that two channels of two different cartridges would not at least slightly differ from one another in IMD. On the other hand, you seem to know what you are doing; I am not suggesting that you don’t. I hope we can agree as a separate issue that the differences between the two cartridges might be entirely due to the fact that they are mounted in two different tonearms, one of which is on a pod.

You wrote, "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." Can you say more about the CD? In what way is it an IMD test file? (If it is to test for IMD, then ought there not to be an encoded signal that creates a control for a certain level of IMD?  If so, in your statement are you saying that you see the control amount of IND encoded on your CD, and it is not augmented by passing through the linestage and amplifier or just the linestage. I am only questioning because the topic is interesting and to me worthy of discussion, because maybe I can learn something.

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.

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?