Phono Cartridge IMD Measurements, Observations and a Question

@kevemaher You might get a little device called an inverse RIAA filter to put at the output of your CD player. If you can reduce the level properly, you could test the phono section too. The one at the link provides some attenuation.

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.

Today, I did a deep dive into IMD definition and measuremet methods. I discovered that the calculation that I created neglected the contribution from the low frequency peak. After that was added, the calculated IMD reduced from 3% to 0.9%, which is more acceptable. This is for the arm on the pod.

I have more IMD measurements to make. I now feel confident that I'm measuring and calculating IMD correctly.

Left unexplaind is why I can't alter the IMD very much.

Left unexplaind is why I can't alter the IMD very much.

@kevemaher   The platter pad is a variable common to both arms and cartridges.

When the platter pad is doing its job properly, if the volume is all the way down and with your head close to the tonearm tracking the LP, it should be very difficult to hear anything at all. The worse the pad, the more you'll hear the cartridge tracking.

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.