Phono Cartridge IMD Measurements, Observations and a Question

@lewm I found a website called "Audio Check". It has lots of test waveform files. including IMD test waveforms. The one I used is  60Hz/7KHz with a 4:1 ratio 60/7000. This is the SMPTE spec. I downloaded this file and burned a CD. I played that CD on my Oppo 105 through to the preamp out.. This method bypasses the entire phono chain, but keeps the preamp. The conclusion is that there is no contribution to IMD from the digital path to the preamp (included). The IMD test on the phono section involves measuring at the preamp outputs. Since the preamp was tested OK on the previous test with the CD, the source(s) for the IMD have to be in the phono path. This path comprises the cartridge, tonearm, turntable, SUT (if used), and the phono preamp. The source is one or more of these. Or it could be some combination of components.

Atmasphere has suggested an interesting effect may be causing ecxcess IMD. I have no experience with what he's saying, although I do inderstand the physics. I am thinking about how to measure this. I hope Atmasphere replies with some useful suggestions.

My original post stated that the IMD was different for each cartridge. Hana ML was 0.5%. The Shelter 901 was 3%.

Subsequently, I have discovered that adjusting VTA changes the amount of IMD and there is an optimum, but it is like 2.5% for the Shelter, not much of a change. I'm struggling to find what a "normal" amount of IMD is. Maybe these numbers are typical and I shouldn't worry.

@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.