loading, back emf and damping

[Rothwellaudio]

People talk a lot on this forum and others about the importance of correct loading for mc cartridges, but I've seen precious little mention of the damping effect of the back emf created when current is drawn from a cartridge. It's commonly known that the motion of a loudspeaker's voice-coil creates a current which causes a back emf, and the back emf damps the motion of the cone, and the current is maximised by having the speaker driven from an amplifier with a low output impedance (hence an amplifier's output impedance is also often specified as its damping factor). However, the equivalent damping which takes place in a cartridge (and can be manipulated by the load impedance) seems to be almost universally unrecognised or ignored. The only mention I've ever seen of it was something on the Graham Slee website. Ironically, whereas most people recommend a fairly low impedance for loading mc cartridges (usually between 30 and 100 ohms) which would increase damping, Graham Slee recommends a higher impedance (470 ohms) to minimise damping.

I presume the damping of the motion of the stylus would manifest itself as a modification of its dynamic compliance. I also presume that the damping effect of back emf would have a much wider and flatter bandwidth than the damping effect of a piece of rubber. Has anyone ever managed to measure anything or know of any such measurements? Or have I (and Graham Slee) imagined the whole phenomenon?

[Hanuman]

People talk a lot on this forum and others about the importance of correct loading for mc cartridges, but I've seen precious little mention of the damping effect of the back emf created when current is drawn from a cartridge. It's commonly known that the motion of a loudspeaker's voice-coil creates a current which causes a back emf, and the back emf damps the motion of the cone, and the current is maximised by having the speaker driven from an amplifier with a low output impedance (hence an amplifier's output impedance is also often specified as its damping factor). However, the equivalent damping which takes place in a cartridge (and can be manipulated by the load impedance) seems to be almost universally unrecognised or ignored. The only mention I've ever seen of it was something on the Graham Slee website. Ironically, whereas most people recommend a fairly low impedance for loading mc cartridges (usually between 30 and 100 ohms) which would increase damping, Graham Slee recommends a higher impedance (470 ohms) to minimise damping.

I presume the damping of the motion of the stylus would manifest itself as a modification of its dynamic compliance. I also presume that the damping effect of back emf would have a much wider and flatter bandwidth than the damping effect of a piece of rubber. Has anyone ever managed to measure anything or know of any such measurements? Or have I (and Graham Slee) imagined the whole phenomenon?

I've certainly read Art Dudley in Stereophile at least hint at the electro-magnetic-mechanical interactions of moving coils and transformers.

Is "back-EMF" the correct term for a generator? Does motor effect create some opposite kinetic energy?

[Rothwellaudio]

The situation with transformers is interesting because the impedance of a transformer drops down to the resistance of its primary winding at DC. How close it can get to DC and still maintain its nominal impedance is determined by the primary inductance, but the upshot is that cartridge will see a lower impedance at very low (hopefully subsonic) frequencies than it will at normal audio frequencies. Whether or not not that is a good, bad or indifferent thing, I don't know.
Maybe back EMF is not the correct term for a generator, but the current in the coils will definitely set up a magnetic field which opposes the motion producing it. It's Lenz's Law. Actually, I think back EMF is the correct term regardless of which type of transducer it is.

[Werner]

Years ago someone on a forum went through the figures with actual magnetic field strengths and coils and concluded that any such effect is orders of magnitude below the already present mechanical damping in a common MC.

[Ldg]

Years ago someone on a forum went through the figures with actual magnetic field strengths and coils and concluded that any such effect is orders of magnitude below the already present mechanical damping in a common MC.

Previously, I reached a similar conclusion. Mechanical impedance from cartridge suspension damping is orders of magnitudes higher than that from generator load. Consider power conversion in each of the mechanical and electrical impedances. I don't believe electrical load is ever significant to mechanical impedance, even when only limited by coil R.

[Ldg]

I found my notes for a worked example, for which there is published data : DL103, 5cm/s@1kHz rms, 0.3mV rms, 40R coil, 0.15 Ns/m @1kHz mechanical impedance.

Mechanical power : 375uW rms (0.15 Ns/m)
Electrical power : 2nW rms (40R)

You can see there are orders of magnitude difference, and mechanical power involved thoroughly swamps electrical power. Electrical loading therefore has no significant influence on mechanical loading.

[Rothwellaudio]

Thanks for doing the maths ld - it's an illuminating result. Of course, I should have delved into the maths myself but although the electrical side is easy enough for me the mechanical stuff is where I'm in unfamiliar territory.

Anyway, it makes the effects of loading mc cartridges even more mysterious. From an electrical point of view the difference between a 100 ohm load and a 500 ohm load to a 10 ohm cartridge must be miniscule, yet there are people who swear that they can hear a difference. I wonder how much of it is imagined.

Actually, having looked at the maths again, there could be quite a bit more electrical power involved. Due primarily to the RIAA curve, the signal from the cartridge can peak at about 15 times its nominal figure, and if the coil impedance is about 10 ohms there's a more power. However, it's still only about 1% of the mechanical force.

[flavio81]

Years ago someone on a forum went through the figures with actual magnetic field strengths and coils and concluded that any such effect is orders of magnitude below the already present mechanical damping in a common MC.

This.
And thus another myth was busted. Science!!

[Rothwellaudio]

Yes, I'm a big believer in science and pleased to see myths busted.
BTW, I've just been doing some cartridge/load simulations for low output MCs and the science says that the load doesn't really matter (exactly what I was expecting) - but that flies in face of the "wisdom" that we're often presented with. I'm just curious to find an explanation for the perceived differences that loading is claimed to make. To be honest, I've never found it to be critical, but I think I'll continue with some empirical tests.

[flavio81]

Yes, I'm a big believer in science and pleased to see myths busted.
BTW, I've just been doing some cartridge/load simulations for low output MCs and the science says that the load doesn't really matter (exactly what I was expecting) - but that flies in face of the "wisdom" that we're often presented with. I'm just curious to find an explanation for the perceived differences that loading is claimed to make. To be honest, I've never found it to be critical, but I think I'll continue with some empirical tests.

AFAIK more than "loading" is it a matter of doing a correct impedance matching between the cartridge and the preamp. Incorrect loading might change the sound (esp. with transformers) and this change is not happening at the pickup side but at the preamp side.

[Rothwellaudio]

Yes, I'm a big believer in science and pleased to see myths busted.
BTW, I've just been doing some cartridge/load simulations for low output MCs and the science says that the load doesn't really matter (exactly what I was expecting) - but that flies in face of the "wisdom" that we're often presented with. I'm just curious to find an explanation for the perceived differences that loading is claimed to make. To be honest, I've never found it to be critical, but I think I'll continue with some empirical tests.

AFAIK more than "loading" is it a matter of doing a correct impedance matching between the cartridge and the preamp. Incorrect loading might change the sound (esp. with transformers) and this change is not happening at the pickup side but at the preamp side.

Correct impedance matching? I think that's a common misconception. With MM cartridges the impedance of the cartridge is significant and feeding it into the correct load (both resistance and capacitance) can optimise the response, but it isn't matching in the same sense as RF transmission line matching. However, MCs are much less load-sensitive and the whole idea of matching is bogus anyway.
As for "this change is not happening at the pickup side but at the preamp side", that doesn't make any sense to me. The pickup interfaces with the pre-amp and the voltage at the cartridge's output must be the same as that at the pre-amp's input because that is where they interface.

[Larry I]

This is NOT the case of loading changing the physical motion of the stylus/cantilever assembly. Loading changes affect the frequency response of the signal--the more loading (lower valute resistor for MC), the more the high frequency peak in the cartridge response is "dampened" (reduced). That response peak exists because the cantilever/stylus assembly is approaching its natural resonant frequency. In the past, that peak would be well within the range of hearing and musical content. These days, because mass has been lowered substantially, the peak is further up in frequency.

The use of loading to remove the peak in the frequency response means finding a value that shaves off enough of the peak to improve the sound without causing too much of a reduction in the response at lower frequencies. According to one designer, I believe Jonathan Carr of Lyra, the peak for MC cartridges is so far up in frequency that there really is no need for more loading than 47k, as far as taming the response in the audible range, but, he speculates that some phonostages need more loading to tame the ultrasonic part of that peak to prevent overloading of downstream electronics. That might account for why Lyra specifies an extremely wide range for its loading recommendation.

[ripblade]

The MC phono stage in my preamp (Threshold SL-10) uses a single-ended common base topology with an input impedance of approximately 3ohms (unmeasured...this guesstimate is from a textbook example), which can be increased using resistors (22R||22R, 22R) switched in series for padding the input.

In every instance so far I've found I prefer the sound w/o the padding resistors (volume adjusted to match output). Whether this due to increased electrical damping or not I don't know, but the sound is punchier, more dynamic and more lively w/o the resistors.

This is far and away from the high impedances discussed here so I'm not sure what relevance it has.

[Werner]

Previously, I reached a similar conclusion.

Perhaps it was your work I was referring to.

There could be quite a bit more electrical power involved. Due primarily to the RIAA curve, the signal from the cartridge can peak at about 15 times its nominal figure,

With most music the raw output of the cartridge is spectrally flat (since music is pink-reddish). And while the peaks are 12-15dB above the cartridge's nominal (0dB = 5cm/s) output level, most decently-mastered music averages 12-15dB below these peaks, so the cartridge's nominal output voltage is a good estimate of the long-term average levels involved.

And probably not a coincidence.