Phono cartridge self resonance

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...speaking of loose ends...…….

So to speak, I am eternally indebted to a Fr. Crowley who taught me as a young man to speak in puns whenever possible.

I figure as an intellectual exercise the entire gamut of possibilities should be as least evaluated. Condensor mics exist with both constant charge and constant voltage mode of operation the exact analog. Unfortunately in their case the virtual ground (constant voltage mode) of operation maximizes the distortion.
 
So why doesn't MM coil parasitic C show up in after dinner conversations, audible performance, and measurements affecting f response etc etc ?

After all, a comparable inductor with similar core and construction typically has self resonant f in the upper mid audioband? It does in guitar pickups, for example.

I've never measured self resonant f of an MM coil as a 2 pin network. It could be clear of the audioband I suppose, but if so how is that achieved? WInding method?

How's that for a nice loose end? ;)

LD
 
Presumably in that case the arrangement is reciprocal and different force on the diaphragm makes a difference?

Yes, I forgot the exact derivation but the charge on the diaphragm creates an electrostatic force. Passive capacitive loading both attenuates the voltage out and gives a distortion component, the virtual ground connection senses the current instead (you use a charge amplifier, capacitor feedback) but that acts like Cload in the distortion equation going to infinity.
 
I recall George did some measurements which, IIRC, initially suffered from test rig participating in the test, which is a challenge to avoid. Then, IIRC, on a balance of probabilities there wasn't anything in the audioband but perhaps there was at 50-60kHz?


The loose end question is why not?

LD
 
I honestly can't remember. I'll find the posts and see if anything can be extracted.

Thinking around a late coffee, isn't there only one component here a big distributed R/L/C. Has anyone ever modeled it this way, maybe say with the L's having weakly coupled flux as the input? The flux bridge patents must have all expired long ago as well as all the other relevant IP.
 
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Thinking around a late coffee, isn't there only one component here a big distributed R/L/C.
Life itself is a transmission line, one comes to realise. Yes, if its a single layer sequentially wound coil. After that it gets tricky because multilayer coils have cross coupled C between turns on different layers. There's not much literature on it, AFAIK.

LD
 
The main measurements from George are here Cartridge dynamic behaviour . Too many toddlers around for me to deeply re-read at the moment :)

Some of the coil measurement basics FYI.

https://link.springer.com/content/pdf/bbm:978-1-4020-9075-2/1.pdf

Thankfully the toddlers belong to my children now. I might try some measurements (a few more bins to unpack) . Lucky, I assume most carts are wound with something like #50 wire any idea on turns/layers (just enough to make a guess if I find any good references). I have a sawed open Grado somewhere but they are the exception.

EDIT - Most references, obviously, seem to focus on high Q RF applications.
 
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PRR

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So why doesn't MM coil parasitic C show up in after dinner conversations, audible performance, and measurements affecting f response etc etc ?...

It is lumped-in with the capacitance of the wiring and the tube grid capacitance.

Most of this C being unavoidable (until headshell preamps), the industry convention is to apply 47K and adjust capacitance to get resonance at "the top of the audio band".

This allows the cart designer to trim the L-R-C bump to compensate the top octave of the pickup.

While interwinding capacitance is a thing, on a simple winding like this it just builds-up and can be sufficiently approximated as a single C.
 
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This allows the cart designer to trim the L-R-C bump to compensate the top octave of the pickup.
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We tried to find evidence of this. There are several seemingly compelling models that suggest this bump is needed, but if you then measure using a preamp that is free from Capacitive loading issues you don't get a top octave roll off. At least on anything made since about 1972.



But some of the data still doesn't fully make sense....
 
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Guys we are off track...….
I thought that, when I realised the negative input impedance stage drives the non-inverting input of the op-amp. Then the simulator confirms, curiously, that the stage overall is inverting...…..(!?!) Phew so that's fair game at least :)

Thanks for the links to your coil self-resonance measurements, George. That's how I recall it. I still think that's odd though, perhaps some winding method. This would equally apply to audio transformers wouldn't it, which is a better charted water?

LD
 
A quick look at self-resonance of good quality mic transformers suggests self-resonance at c 250kHz. I think such transformers are prob magnetically similar enough to cartridges to consider it not to be an issue, and George's tests confirm this.

Some reference to self C not depending proportionally on number of turns, diminishing increases with each extra turn.

I don't understand it, but happily accept it. Besides, it's good news for low Z input impedance preamps.

LD
 
Some reference to self C not depending proportionally on number of turns, diminishing increases with each extra turn.

I originally though of building a distributed self resonance model via the L of one layer and the C to the next but that soon makes no sense since the C's are in series and as L grows the C goes down. I suspect the entire structure along with the mutual coupling is involved.
 
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