Phono Cartridge IMD Measurements, Observations and a Question

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.

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.

@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.

@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. 

@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.

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.