Cartridge loading explained

the thin end of the wedge
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Re: Cartridge loading explained

Post by Guest » 08 Feb 2012 09:00

A question :

Why doesn't cartridge coil self-capacitance feature at all ? Given the coil is typically multi layer, and apparently randomly wound ?

dlaloum
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Re: Cartridge loading explained

Post by dlaloum » 08 Feb 2012 12:07

Do we know them to be multi layer and randomly wound?

The AT25 family are torroidal coils, and given the manufacturing processes for torroids, I doubt the winding would be random... (but I certainly don't know...)

Is there a reliable way of measuring coil self-capacitance? (will a normal DMM capacitance meter do it?)

Guest

Re: Cartridge loading explained

Post by Guest » 08 Feb 2012 12:50

Multilayer yes, quasi-random typically by the look of it. Pretty obviously, most cart coils aren't toroids. Even those that are must typically be multilayer.

For sure, DMM capacitance meters won't do it, of course. Simple way would be to know the inductance and self-resonant f. But that is a bit circular and self-fulfilling, if inductance was determined by finding the self resonant f against a known external C too....!

I'm just surprised, and curious. It's a loose end.

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Re: Cartridge loading explained

Post by Guest » 08 Feb 2012 15:48

Hmmm. What's more, if a DMM (for example) measures inductance by determining magnitude of impedance |Z| at a set frequency, coil self-capacitance would cause the method to overstate measured inductance. If the set frequency is below the self resonant frequency, which it likely is.

And this seems a candidate to explain some of the oddities over on the capacifier thread, where coil inductance appeared to behave as to loading as though it were smaller than measured with a DMM.

Some very rough working backwards from observed inductance derating factors suggests coil self capacitance c 150pf-200pf, which seems plausible?

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Re: Cartridge loading explained

Post by superluca71 » 09 Feb 2012 08:00

dlaloum wrote:Which simulation program are we talking about?

....
David
I'm talking about carlosfm software B2 spice

dlaloum
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Re: Cartridge loading explained

Post by dlaloum » 11 Feb 2012 02:54

This reference came up in the famous (infamous) MM thread by Raul on audiogon....

the BAS newsletter : http://www.bostonaudiosociety.org/pdf/b ... -7710b.pdf

Has a section discussing cartridge loading - here is an extract from it:
My major disagreement with their comments stems from their assumption that what manu-facturers cite as the nominal load (e. g. , 47k Ohms, 400 pF) is actually the correct or properload.This assumption is directly contrary to my own experience. Let me give an example: theAKG P8E's recommended load of 47k Ohms, 470 pF. British reviewers have found something like25k Ohms, 200 pF preferable.When I set-up my P8E, I found the optimum load to be describedby two networks in parallel as follows: 27k Ohms in parallel with 200 pF, in parallel with a net-work of 22k Ohms, in series with 1100 pF.

The optimal network is a long way in terms of model from 47k Ohms in parallel with anything. And the AKG is by no means unique in this respect. Also, the optimal loading seems to vary fromstylus assembly to stylus assembly, as one might expect, given the nature of this kind of device.I have fitted two Shure M95 cartridges that were flatter into 70k Ohms than into 47. I therefore argue that the nominal load impedance of a cartridge is just that nominal. 47k Ohms is not a bad resistance to use, and if you do, the specified capacitance is probably the best. It does not say much about what any given cartridge will sound like when used with a phono preamp that has that characteristic.

With respect to complex impedance, those preamps (both with tubes) 1 have found with complex input impedances exhibit an additional resistance in series with the input capacitance, e.g.,47k Ohms in parallel with 150 pF in series with 10k Ohms.
If you add the usual 150 to 200 pF cable and arm capacitance, the 10k resistor is no longer very important. I know some transistor designs exhibit nastier impedances, which include diode effects, and these may be very important. The only way to know which you have and how important it is to test for it with something more sophisticated than the simple nulling device Foster and Swanbon use.

Given all of this, it would seem that if there is a "best" input impedance, it is infinity. It is certain that under present circumstances the only way to get flat response from a cartridge-cable-preamp combination is to set it up, test it, and determine its proper loading, preferably with the insertion of a test jig and networks. I will happily offer this service to BAS members, though at a fee.
I should add that I have found that many of the differences between phono cartridges disappearonce they have been properly loaded. Not all, however, and the ones that remain may be very much of interest. I use an Ortofon SL-15 and an AKG P8E. The latter is optimally loaded, while the former, because of its very low impedance (600 pF drops response 2 dB at 10 kHz), is better used with active equalization. These two are virtually identical, even though the driving prin-ciples are entirely different.

To summarize, the notion of a standard or ideal input impedance is problematical, indeed. The frequency response of any given combination can be properly ascertained only by testing,and the optimal loading, best achieved through a separate network, may differ considerably fromthe nominal.Finally, the idea of a phono preamp that exhibits "no phono interaction", as expressed by one local "expert" who called Shop Talk a few weeks ago, is absurd. [Depending on what's meant. It is, at least, possible to prevent a cartridge from disturbing a preamp's RIAA equalization simply by putting a flat buffer amp at the input. That, of course, is just whatusually happens when a moving-coil pickup is used. -- Ed.]
Interesting!
Also more third party confirmation that adjusting a cartridge for flat F-R results in most cartridges sound almost identical. (and almost is not completely of course.... )

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Re: Cartridge loading explained

Post by Guest » 11 Feb 2012 04:42

dlaloum wrote:.....Also more third party confirmation that adjusting a cartridge for flat F-R results in most cartridges sound almost identical.
Not at all. What the author means is that loading gross-outs are characteristically audible, and removing them is convergent toward a 'non grossed out' sound. Which is actually pretty obvious when you think about it. But that is quite distinct from all properly loaded cartridges then sounding alike, which is your proposition DL, and is self-evidently untrue. For good, explanable, verifiable, measurable reasons. There's far more to cartridge sound than just frequency reponse. Especially FR at a single level.
BAS newsletter wrote: Given all of this, it would seem that if there is a "best" input impedance, it is infinity.
I can see where the author comes from, but issues such as coil self-capacitance make it impossible to achieve. Even in a head-amp. And yes, phono preamp input impedance can be far from a simple or 'ideal' RC model.

I thought about this previously, and thus came up with zero input impedance as the better theoretical loading, as it shunts coil and cable capacitance. Hence the MM/MI transimpedance preamp design.

However, I disagree that the frequency response of any given preamp-cartridge combination can even necessarily be properly ascertained by testing. Well, not by testing in the conventional sense of measuring FR at a single level. It is self-evident that if FR varies with level, it doesn't work, and is arbitrary. The best one can hope for is to spot, and so avoid, the gross-outs.

Back in the day, they no more had this issue worked out than we do now.

Guest

Re: Cartridge loading explained

Post by Guest » 11 Feb 2012 15:25

The more I think about coil self capacitance, the more odd I consider that is totally neglected. It should influence cartridge coil impedance, and at minimum mess up certain attempts to measure coil inductance which are based on impedance magnitude measurents at a single frequency.

Coil self capacitance is not easy to model well. One crude way is as a lumped capacitance which shunts the coil. In which case, coil impedance can be approximated by an equation as follows :

20369

Conventional cartridge coil impedance models simply assume C = 0, then Z = R + jwL. And an inductance meter, for example might assume the jwL dominates |Z|, so measuring |Z| allows measurement of inductance L. But if C is significant, the term w^2LC can contribute. |Z|, and measured impedance will be overstated.

True coil inductance might well be overstated if determined from coil impedance, when C is ignored but significant.

In real cartridge use, coil self capacitance, self inductance, and losses, are a resonant system. Coil impedance would show at peak at resonance (frequency response dip), the Q of which is determined by losses. Unfortunately, the lumped model is not very good here. But the general form of a frequency response dip should be present. And coil inductance behave as though it is somewhat smaller than measured/spec. Such things seem plausible.

Guest

Re: Cartridge loading explained

Post by Guest » 13 Feb 2012 06:14

Still on the topic of coil self-capacitance.

Cartridge coil inductance is relatively high for such a physically small coil. There are 'standard' miniature inductors which resemble the small size, dc resistance, and inductance of cartridge coils.

Here's an extract from an example spec :

20418

On the face of it, this inductor resembles a cartridge coil. But has a self resonant frequency of only 10kHz..........?!?

Then, for this inductor, self capacitance is significant. I'm unsurprised. But, on the face of it, it resembles a MM/MI coil. So why not the same problem ?

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Re: Cartridge loading explained

Post by bullpeters » 03 Jun 2012 10:49

I never got passed the first couple of pages, but halving the shunt resistor and removing the cap has stopped my dog from hiding under the coffee table on some "bright" tracks
Thanks Carlos

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Re:

Post by ryk74 » 04 Oct 2012 14:58

Werner wrote:Here's the M97xe. Measured, not simulated.

The resistive load was 47k. The wiggle at 10kHz is on the test record.

http://www.tnt-audio.com/gif/shure_m97x ... sponse.gif
Hi,
how can i make a measurement like this (preferably with some computer program)?
I used AUDIO TEST BENCH (http://www.higherfi.com/software.htm), but it can't make such a measurement (or i can't find it!)

Thanks!

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Re: Cartridge loading explained

Post by khenegar » 19 Mar 2013 03:33

How did you every get 100pf including the tone arm cable? Most tone arm cables at least have 100pf plus and the cable to the phono is at least 80pf plus what the phono stage has. Can you explain?

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Re: Cartridge loading explained

Post by khenegar » 19 Mar 2013 03:35

Sorry wrong person for the reply

dlaloum
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Re: Cartridge loading explained

Post by dlaloum » 19 Mar 2013 04:23

Not necessarily - my own tonearm + interconnect adds up to 60pf...

But I custom wired it and kept the interconnect very short to achieve that.

bye for now

David

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Re: Cartridge loading explained

Post by khenegar » 24 Mar 2013 15:42

Dialoum. I borrowed a expensive capacitance meter and measured my tone arm wiring and it came up with 36.5 pf capicatance. So far all I can find for a 3' interconnect cable to the phono stage is around 80pf. my final total is 116.5pf. my rotel phono stgae i took out the 100pf capicator so i have no pf in the rotel. i have to use a 3' cable because of distance. Will this be ok? I am using the shure m97xe cartridge. Also is it important to use low capacitance cable from the phono stage to my lexicon mc12 processor?

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