Load capacitance on MM cartridge

Iron core coils usually show a decreasing inductance and increasing series resistance with frequency due to eddy-current losses. Your example really shows that.
That is correct and measurable. The simple inductance and resistance model quoted by cartridge manufacturers is wrong. Cartridge inductance needs to be modelled as an inductance in series with L//R. All in series with resistance, ideally also with shunt resistance and capacitance. A six-component model, not two. Later Shures (V15/III onwards) in particular show a very low proportion of inductance at high frequency (<30% of the 1kHz value), whereas recent Ortofons are nearer to 70%.
 
  • Like
Reactions: hbtaudio
Calculated from Richard Visee's measurements:

50 kohm * sin(53 degrees)/(2 pi 20 kHz) ~= 317.77 mH at 20 kHz

6 kohm * sin(72 degrees)/(2 pi 2 kHz) ~= 454.1 mH at 2 kHz

460 mH according to some LC bridge working at an unknown frequency

500 mH according to the specifications of the manufacturer

I don't know what you mean by a high frequency, but the inductance at 20 kHz was about 70 % of the value at 2 kHz for Richard's V15-III.
 
I converted the imaginary part of the impedance into an equivalent series inductance, but you can also convert the imaginary part of the admittance into an equivalent parallel inductance. If I did the math correctly, the parallel inductance doesn't change much:

20 kHz: 498.21 mH
2 kHz: 502.04 mH

The resistive part varies a lot, no matter whether you look at the resistance or the conductance, although the resistance varies more than the conductance, see post #17.
 
Here are my measured results and model. Note the nice tight fit, but beware that there's considerable sample variation between cartridges and channels, so the numbers are indicative, not definitive. Nevertheless, the trend is clear, and you can see from the way the impedance changes below electrical resonance that high frequency inductance in the V15 is much less than low frequency. For the benefit of the original poster, I've included the results for an Ortofon 2M. By and large, all moving magnet cartridges should be loaded with minimum capacitance to maximise the cut-off frequency of the resulting low-pass filter, but they then need a higher load resistance (often as high as 150k). The traditional upper midrange dip was a purely electrical phenomenon caused by non-ideal loading.
 

Attachments

  • Ortofon_2M_blue.PNG
    Ortofon_2M_blue.PNG
    49.4 KB · Views: 42
  • Shure V15_III.PNG
    Shure V15_III.PNG
    49.6 KB · Views: 44
  • Six-component model.png
    Six-component model.png
    4.4 KB · Views: 44
For the benefit of the original poster, I've included the results for an Ortofon 2M.
Thank you! But am I right in thinking that this is the electrical response only? There's also the mechanical response to be considered. Of course, I've already read a lot on this forum, and Hans Polak's work (whether correct, misunderstood, or misinterpreted) made an effort to combine both. For a correct response, I think a test record is needed (something I don't have). With the help of a previous poster, I have found this for the Ortofon 2M Red.

Ortofon 2M Red - Denon DP-35F - CA¹ - 2.png

Source: Audiosciencereview.com

210 pF seems a too high load capacitance.
 
I've considered electrical response only. Best electrical response for a 2M Blue is 100pF//68k, so that's probably correct for the Red as well.

Returning to the Shure V15/III, it's clear that its transducer mechanical response is far from flat because neither is its electrical response with recommended loading. I'm quite prepared to believe it has a fierce ultrasonic mechanical resonance.

I think it's significant that the Audio Technica VM520 gives a very nearly perfect electrical response with the manufacturer's loading of 100pF to 200pF in parallel with 47k, suggesting that their transducer's mechanical response is nearly flat. Most modern cartridges seem to give a nearly flat electrical response with manufacturer's recommended loading, but reducing capacitance always extends bandwidth.

My point is that cartridge resistance is unchanging but that its inductance needs to be split into two parts to account for core losses.
 

Attachments

  • Ortofon 2M loaded.PNG
    Ortofon 2M loaded.PNG
    27 KB · Views: 23
  • Shure V15-III loaded.PNG
    Shure V15-III loaded.PNG
    25.7 KB · Views: 24
  • Like
Reactions: rongon
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?

Ray K
 
Dear Marcel,
what is exatly your term G* ?

G is normaly 1/R in [A/V] ==> Siemens

1744295840987.png


Where is the hidden C, the capacitive effect of the wire windings? How thin and how long is the wire, how many winding layers are there, etc. pp.
 
The star is a reference to a footnote that says it is calculated from measured data. G is simply the real part of the measured admittance, so the real part of the reciprocal of the measured impedance. I don't need to know how big the wire capacitance is, I just calculate G from the measured impedance magnitude and phase. Your schematic in post #37 has the losses spread over two places, while I transfer them to either a frequency-dependent resistance R = Re(Z) or a frequency-dependent conductance G = Re(1/Z).
 
  • Like
Reactions: hbtaudio
My point is that cartridge resistance is unchanging but that its inductance needs to be split into two parts to account for core losses.
cartridge resistance
is DCR

Rdc of the entire coil.

Your replacement circuit is flawless.

What definition for cartridge resistance do you use? I simply mean the real part of the cartridge impedance, no matter what its physical cause may be.
The real component of our equivalent twopole does not change, i.e. it does not change at the point under consideration, i.e. at jOmega ... and you know all this perfectly well. The pure wire resistance does not change, it is constant if T=const.

The constant perpetuation of the /this topic is very tiring.

HBt.