Phono Cartridge IMD Measurements, Observations and a Question

@lewm 

I'm measuring stereo. Left and right chs for each cartridge have the same IMD. Azimuth was optimzed prior to the measurements.

I've not swapped cartridges..

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.

@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?

@atmasphere I've read the Hagerman write-up and understand it. How does this relate to the IMD test I'm reporting on? The sideband peaks at 4KHz are from the 60Hz component. How does RF cause these peaks?

How do I test whether my phono stages are stable? I would like a quantitative test. I have test gear, but not much in the way of RF gear, although I have an FM generator. Qualitative tests are difficult for me to interpret because our ears and minds can fool us. Can't fool a trace on an oscilloscope.

FYI, I am using around 100 ohms for cartridge loading.

@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.

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.

Today, I am measuring IMD with the CBS STR 112 Test LP. I will post results when I've finished analyzing..

I have replaced the Shelter 901 with a brand new Benz LP-S. First look is that IMD from the LP-S and the difficulty of making adjustments by raising or lowering the tonearm are about the same.

@atmasphere I have heard what you're describing. I am using a "Platter Pad" which is some orangey rubbery material (maybe rubber). It is 6mm thick. I bought it about 1980. WIth this pad, I hear absolutely nothing, even with my ear close to the stylus. I have used it continuoously on one table or another since 1980.