Phono Cartridge IMD Measurements, Observations and a Question


I've just recently added an IMD test to my cartridge alignment procedure. I use the REW RTA feature as a spectrum analyzer. It will display enough resolution to pick up the +/- 60Hz IMD sidebands at 4KHz using the AP Ultimate Analog Teat LP. (And it will go down to 2Hz, so the turntable rumble can be seen!). I convert the signal from A->D using a Benchmark ADC1.

I have an SL-1200G. I'm using two arms, a Hana ML on the stock arm and a Shelter 901 MKIII on an outboard pod mounted Micro Seiki MA-505 arm.

Signal comes from the cartridge to a switch that selects the arm, then into an SUT with Blue Cinemag treansformers., then on to a Parasound XRM phono pre operating in MM mode (40dB gain). Balanced cables come out and go to the input of a Levinson no. 38s line level preamp (oldie, but sounds and measures great). I then input the 38s signal to the A/D (balanced cable)

I measure about 0.5% IMD with the ML and about 3% with the Shelter. I have moved both arms up and down. Total vertical movement for each arm is 6mm. I cannot measure any change in IMD with either arm at any height when I change the height. Both arms have VTA on the fly, so I can observe the measurement as I adjust.

Why can't I see any changes in the IMD? Am I measuring the wrong thing? Should I be adjusting the rotational alignment of the stylus to the groove (offset angle)? Or some other parameter?

 

128x128Ag insider logo xs@2xkevemaher

@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.

@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.