Phono Cartridge IMD Measurements, Observations and a Question

@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?

@kevemaher You’ve not ruled out RFI sensitivity on the part of the Musical Surroundings preamp, which you don’t seem to mention in your list just above. RFI issues can produce quite a lot of IMD.

By 'stuff upstream' I meant the arm mounts, arms, cartridges, platter pads, etc. 

@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.

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.