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MM cartridge capacitance loading
MM cartridge capacitance loading
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Old 13th March 2011, 01:59 PM   #11
davidsrsb is offline davidsrsb  Malaysia
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At audio frequencies your lead is almost a pure capacitor. Unfortunately most meters expect the capacitance between the leads to be much larger than from either of the leads to ground. The lead has substantial capacitance from the shield to ground
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Old 13th March 2011, 02:41 PM   #12
Speedskater is offline Speedskater  United States
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Can you remove the test leads from the meter?
Can you somehow insert the smaller capacitor directly into the meter's sockets?
If so you can subtract this reading from the reading with the test leads to find the capacitance of the test leads.
But this difference reading will change with how you dress the leads.
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Old 13th March 2011, 11:31 PM   #13
stevomaso is offline stevomaso  Australia
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I may have found my problem by reading through forum posts. I believe that I need to connect the positive and negative of one end on the cable to my DMM. I was trying to measure positive to positive ends of the cable.
I will measure the cable unconnected and then connected to the turntable (Project Expression) and then connected to the phono preamp.
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Old 14th March 2011, 10:48 PM   #14
stevomaso is offline stevomaso  Australia
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Finally I can measure the capacitance of my connecting leads. My 30cm shielded gold plated economy cable measured at 160pf. However the basic cheap 1 meter dual cable measured at 110pf. My Ortofon 2M red requires 150 to 300 pf. So when you add in the capacitance of the tonearm cable, I should not need any additional capacitance in my phono preamp.
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Old 15th March 2011, 06:13 AM   #15
davidsrsb is offline davidsrsb  Malaysia
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Are you sure? Relatively high capacitance coax like RG-58 (solid dielectric 50R) is 100pF/m
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Old 15th March 2011, 06:58 AM   #16
ashok is offline ashok
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MM cartridge capacitance loading
Some current cartridges from the Shure range need just 250pF ! That is really tough with ordinary cables and connectors unless you have the shortest possible leads or install the phono pre in the turntable case.

I have some Denon test discs that have frequency sweep etc. on them. I tried using them long ago but used to have issues with the LF resonsnance of the arm/cartridge causing the output to fluctuate quite a bit.

The Jap magazines used to have lots of information on cartridge and capacitance/resistance loading with response plots. I had those articles for a very long time and used to look at them ocassionally. I tried to find them but have been unsuccessful. I don't throw away such things and am hoping that it hasn't got lost somewhere. They would come in very handy right now. There was a great one on the effect of fluid damping on tone arms too.

Wouldn't some of our Japanese members have access to their old mags ? ( these must have been from the mid or late 70's )
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Old 15th March 2011, 10:20 AM   #17
stevomaso is offline stevomaso  Australia
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Originally Posted by davidsrsb View Post
Are you sure? Relatively high capacitance coax like RG-58 (solid dielectric 50R) is 100pF/m
Yes according to my DMM the capacitance is 160 pf for the 45 cm cable (not 30). DMM is Digitech QM-1324 with two short wires inserted into Cx to allow measurement of cables and capacitors with large widths.
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Old 10th February 2012, 08:18 AM   #18
someEngineerGuy is offline someEngineerGuy
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Default Inter-windiing capacitance in MM cartridges

This is an old post -- hope somebody notices this reply :-)

I wanted to point out one thing fact that I don't think I've ever
seen mentioned about cartridge loading. All those turns of wire in
the MM cartridge coils create what is called inter-winding capacitance.
This is (in MM carts) a significant capacitance, effectively connected
across the coil and it is in parallel with the tonearm wiring and
cable capacitance.

For example, I've measured the impedance (for those engineers out there,
complex impedance -- resitance and reactance) of my Ortofon OM Super 20
MM cartridge. This measurement included tonearm wiring but no other cables
were connected. I don't think the tonearm wiring would have added more than
10pF of capacitance to the results.

The advertised resistance is 1000 ohms and in fact it is 1040 ohms. Very
close. The advertised inductance is 580mH and I came up with 510mH. Still
not that far off.

Manufacturers don't advertise inter-winding capacitance;
I measured about 108pF of it (this includes tonearm wiring,
so perhaps the actual value is closer to 100pF or so). This creates a
self resonant frequency around 22-23kHz -- and that's with no capacitive
loading from cables or preamps. When the manufacturers suggest a load
capacitance, this is in addition to inter-winding capacitance.

Another thing they don't tell you about is losses that occur in the magnetic
structures of the cartridge. This appears as a "ghost" resistor in parallel
with the cartridge -- 191k-ohms in my case for the OM Super 20.

A very accurate electrical model of my Ortofon cartridge impedance
turns out to contain three components in parallel with each other:

1) A 510mH inductor in series with a 1040 ohm resistor
2) A 108pF capacitor (includes tonearm wiring, probably < 10pF)
3) A 191k-ohm resistor

I'm sure at this point, you non-technical folks' eyes are starting to
glaze over. Here's the bottom line -- it is difficult at best to
figure out proper MM cartridge loading with only the data published by
manufacturers. You either need a test record or some amount of trial and
error with subjective listening to get it right.

In the end, this process lead me to mount
my home-made preamp under the turn-table -- soldered directly to the
tonearm wiring to minimize capacitance. I used a 90k-ohm load resistance and
zero added capacitance. This results in a very good theoretical response
to 20kHz.

There's still something missing from this -- I don't own any test records
and there could still be problems in the stylus/cantilever system that
would cause a non-flat frequency response. On the other hand, I like the
resulting sound so that's going to have to be good enough for me.

Hope you find this useful!
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