Load capacitance on MM cartridge

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#46
diyrayk said:
Questions for the group:

AFAIK the Shure type 3, 4, 5, Ultra, etc. have laminated cores which should theoretically reduce core losses. Have any of you observed/measured/calculated an effect from this design detail? Have any other mfr's used laminated cores? What are MC coil forms and step-up transformer cores made of?
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Shure took a very different design approach to everyone else. Everyone else has gradually reduced high frequency core losses over time (1970s to present day) and this shows up as a higher ratio of high frequency inductance to total inductance; typically >70%. But Shure went the other way and designed for a low ratio, sometimes as low as 23%. It seems they decided that a lossy core material worked better for their design.

Step-up transformers generally use mu-metal cores.
 
#49
hbtaudio said:
70 to 23 seems to me to be far too high.
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It's certainly radical. I didn't have the data in front of me when I mentioned 70% and 23% and should have typed 79% to 27%, but it doesn't alter your argument. But if you think about it, there are advantages to having a low inductance at high frequencies because it raises the cut-off frequency of the LC low-pass filter. The 70s and 80s cartridges I've measured typically ranged between 41% and 55%. The evolution of core material is nicely demonstrated by Ortofon with a VMS20E mkII having an HF proportion of total inductance of 61%, its successor the OM was 64%, a 2M Blue was 76%, and a recently manufactured Super OM5 (slit pole pieces) was 79%. So Ortofon have reduced core losses over time. Conversely, Shure M75 was 53%, V15/III was 36%, and V15/IV was 27%, so Shure clearly decided to make the losses useful.

Given that a cartridge contains a tiny amount of core material, I suspect cartridge manufacturers have to use what is available and can't specify custom alloys, so I suspect the general trend to a higher HF proportion in the remaining cartridge manufacturers may reflect material availability rather than deliberate choice. Also, although a reduced high frequency inductance gives a higher cut-off frequency, I can't help feel that it must compromise the transducer. I have no evidence or theory to back that thought up, just a gut feeling.
 
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#50
@EC8010

What do you mean by high frequency? How high is high?

(The reason I ask is that your high frequency inductance for the V15-III is very different from what Richard Visee measured at 20 kHz.)

I also still don't understand what you mean by 'cartridge resistance is unchanging' in post #33. Unchanging as a function of what?
 
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#51
In my plots, a pure inductance produces a rising straight line, but splitting the inductance in two puts a step in that line (you can see the step in the two graphs). Thus, "low frequency" is below that step and "high frequency" is above it. For the V15/III, that step is centred on 22kHz, so you wouldn't see the high frequency inductance dominate until 100kHz or so. It's worth remembering that unless you do a multi-frequency measurement and fit to your own model of what you think you are investigating, you are limited by the test equipment manufacturer's model, which is usually two-component because it allows a single frequency measurement to determine both components (RL, RC). But that traditional two-component model is inappropriate for an iron-cored inductor, so simply asking an LCR tester what the inductance is at a spot frequency is unlikely to give the right answer. I use a variety of different models ranging from three-component through to seven-component, depending on what I'm looking at. I'd probably need even more complex models if I had an instrument that measured to 1MHz, but 200kHz is good enough for most audio. Obviously, the disadvantage of multi-frequency measurement is that it is much slower, but it's very revealing. When I first saw the inductance step, I thought it was down to the fact that a cartridge is a transducer, but I've found that most wound components having a magnetic core need the six-component model, and some need an even more complex model.

You commented in post #28 that the resistance varies a lot. In retrospect, I suspect you meant between cartridges, but at the time I took it to mean as a function of frequency. Although I have observed skin effect causing resistance of a wire to rise with frequency (I include it in correction data when measuring uH inductances) I doubt it is significant in a cartridge. More to the point, I didn't find it necessary to include it when experimenting to find a model that fitted the measured results.
 
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#52
Thanks for answering. Some 30 years ago, Richard Visee measured the impedance magnitude and phase of a Shure V15 III from 100 Hz to 100 kHz using an HP4194A impedance gain and phase analyser:

Screenshot_20250412-003607~2.png


I calculated Im(Z)/(2πf) at 20 kHz and found it is some 70 % of the value at 2 kHz, but that makes sense now that I know that your 30 % is for f >> 22 kHz. I never bothered calculating it above 20 kHz.

I did mean that the resistance is a function of frequency. When you calculate Re(Z) from this graph, you see it increase a lot over frequency. I'm quite sure the real part of the impedance of the model of post #29 also increases with frequency, at least I don't see how it could possibly be constant.
 
#55
Yes, I do mean that I can accurately model the impedance with ordinary frequency-independent resistors and ordinary inductors and capacitors. What I showed in the model is what I used. Nothing more. Of course, on a logarithmic plot, almost anything looks like a good fit, but the < 2% standard error tells you it's a decent fit.
 
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#57
When you say Re(Z), do you mean the A part of A + jB that describes impedance in Cartesian form? If so, it certainly changes with frequency.

@stocktrader200: I'm afraid I haven't been able to measure a VM95 so I can't tell you what the optimum loading is. On that note, if anyone has lots of MM cartridges that they'd be willing to risk to Royal Mail, I'd be delighted to measure more cartridges.
 
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#59
Then we're agreed on all points. As a general comment, I suspect that the ratio of high frequency to low frequency inductance is determined by core composition, whereas the frequency of the transition is down to how the core material is physically implemented (laminations, slits, dust).
 
#60
Now that the two seafaring nations are in agreement, allow me to shed some light on the six-component model.



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Everything divided linearly. Frequency, x-axis, from 100Hz to 20kHz in 100Hz steps. Y-axis in each case, |Z| in ohms, phi in degrees, real component in ohms, imaginary component in ohms.

The present model or the component values do not correspond to Marcel's angle specifications ...
 
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