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.
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Are you suggesting that the platter mat contributes to IMD
@lewm Yes.
The role of the platter pad is still poorly understood in high end audio which is a shame. But its effects are easily measured and heard.
Most platter pads are just plain wrong. To be right, it must have the same durometer as the LP. It must then be able to absorb vibration- it must be damped. Bonus points if it also damps the platter.
I’ve had access to such platter pads for nearly 35 years. One thing I’ve noticed is that when I set the stylus down on the LP, it makes no sound. By this I mean that if the volume control is all the way down, the sound of the stylus tracking the LP can’t be heard. I’ve seen a lot of platter pads where you can hear the stylus tracking from several feet away.
Put another way the LP must be damped so it does not talk back to the cartridge.
I’ve seen many ’studies’ of LP distortion which don’t take this into account. When a large variable like that isn’t nailed down it becomes junk science.
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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.
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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.
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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.
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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.
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@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.
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@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.
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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.
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@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.
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@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.
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@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?
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@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.
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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.
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Why can't I see any changes in the IMD? Am I measuring the wrong thing?
@kevemaher That's a pretty good bet. By placing the Shelter on an outboard pod you put it at a disadvantage. Any vibration that might be affecting the platter will not be affecting the pod the same way so distortion will be higher.
The SUTs will ring (distort) if improperly loaded. Since the correct load varies with the source impedance, you have to have the correct loading at the output of the transformer for each cartridge.
The distortion of the phono section, whatever that is, will be compounded by the line section. I think you'll have more accurate numbers if you simply don't use the line stage.
The different arm is a variable too. You'll want to know that both cartridges are aligned correctly and when that is done are able to track properly too.
Resonance in the arm tube can play a role as well as any chatter the bearings might have.
I think you can see now why so many of the 'LP distortion studies' in the past were bad science as there are a lot of variables at play!!
FWIW I think you'll find the platter pad affects the distortion as well. This is caused by resonance in the LP which can 'talk back' to the cartridge. If the platter pad is damping the LP correctly you'll see the distortion go down.
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You might also try deliberately twisting either cartridge a bit so that azimuth is out of alignment. If you still see no change then you know there is something amiss with your test setup.
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am I correct in thinking that the source of your apparent dismay is that vertical tracking angle makes no difference to your measurement? Otherwise, if you are doing this to compare the two cartridges, obviously you need to test the two cartridges in the same tone arm under the exact same conditions so that all other elements are constant. I am sure you appreciate that. The absolute numbers you get are probably affected by the intermodulation distortion characteristics of the electronics that are in your signal path before the measurement is made. Perhaps IMD of those electronics are dominating the result, and that is why VTA makes no difference.
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@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..
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Try the comparison with each cartridge mounted on the 1200G tonearm (separate measurements of course). Also are you doing this in mono or stereo? If stereo, are you getting the same result in each channel for both cartridges? Does azimuth have an effect?
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