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Phono cartridge self resonance
Phono cartridge self resonance
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Old 26th January 2019, 05:37 PM   #21
gpapag is offline gpapag  Greece
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Phono cartridge self resonance
Quote:
Originally Posted by scott wurcer View Post
Maybe we're just missing the boat here. McIntosh MTI100 Integrated Turntable

All the bases covered, sigh. And I thought Scots were thrifty, I certainly am.
Thanks for the info Scott. And you make a good point (not that we weren't aware of it... )
At times, it is good that product design makes circles (they listen to what the broad customer base needs).
From the description, it’s not a “cheap” item (and it will sell to around $8k)

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Originally Posted by billshurv View Post
Me I'm doing really daft things. One cartridge, 3 cantilever options.
Oh Bill…

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Originally Posted by luckythedog View Post
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
Lucky, the diversion is the talk about the internals of cartridges


Quote:
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
You are welcome.
I have dissected 2-3 MM bodies.
From what I've seen, there is nothing special with the winding method.
Plain solenoid -like continuous winding in a small size. In the latest victim’s case (Stanton MK V), each of the two coils had a copper- filled cross section 4.07mmx1.97mm consisting of approx 2500 turns of AWG 39 wire (50 layers, 50 turns per layer). No bank winding, no sectioning, just continuous winding


Quote:
This would equally apply to audio transformers wouldn't it, which is a better charted water?
I’ll put some chokes on the bench for to test. With transformers, on that kind of test I have to check the effect of secondary loading.
I will test them together with Bill’s ‘fat boy’ when it will arrive.
Results on the other forum or I’ll open another one specifically for self resonance

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Originally Posted by scott wurcer View Post
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.
Scott, I think it’s not that straight.
There is capacitance between turns (Ct), capacitance between layers (Cl), stray capacitance (Cs).

Ct is in series to each other, Cl is in parallel to Ct and Cs depends on the surrounding structure

George
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Old 26th January 2019, 07:44 PM   #22
luckythedog is offline luckythedog  United Kingdom
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Originally Posted by gpapag View Post
With transformers, on that kind of test I have to check the effect of secondary loading.
Thanks that's interesting info, George. If the secondary is open circuit it should play no part - apart from the loading of its own self resonance, ironically.

LD
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Old 27th January 2019, 10:25 AM   #23
luckythedog is offline luckythedog  United Kingdom
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Swift back of envelope calc suggests two parallel touching 1m lengths of 39 gauge enamel wire have mutual capacitance of about 42pF. Seems plausible. I don't get where this disappears to in a wound coil.

I do understand transmission lines, or so I think, but just can't find the missing piece of logic or explanation to reconcile this with what is observed.

LD
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Old 27th January 2019, 01:38 PM   #24
scott wurcer is offline scott wurcer  United States
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Originally Posted by luckythedog View Post
I do understand transmission lines, or so I think, but just can't find the missing piece of logic or explanation to reconcile this with what is observed.
It is an interesting problem, I think the big difference from a T-line would be the tight mutual coupling of the inductive elements. Maybe careful measurement as a lumped element is the best we can do.
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Old 27th January 2019, 02:44 PM   #25
Pyramid is offline Pyramid  United States
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Originally Posted by scott wurcer View Post
..... Maybe careful measurement as a lumped element is the best we can do.

Coming from an RF background, I'm wondering why this wouldn't be an acceptable solution. Using a VNA, you should be able to get an accurate complex impedance, if that is what you are seeking. Whether it exactly models the internal workings of the coil and various capacitance shouldn't really matter, should it? The lumped element model should be the equivalent of the actual circuit and it should respond accordingly, correct? Even the complex impedance can be modeled as either series or parallel components yet produce the same results.

I'm still unclear what you are trying to accomplish. Are you trying to cancel the reactive component of the cartridge? Conjugate match of cart to preamp? Even a resistive mismatch can cause reflections and signal loss and it doesn't appear that the optimal loading (per the mfr) gets anywhere close to optimal matching.
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Old 27th January 2019, 03:53 PM   #26
luckythedog is offline luckythedog  United Kingdom
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Originally Posted by Pyramid View Post
I'm still unclear what you are trying to accomplish. Are you trying to cancel the reactive component of the cartridge? Conjugate match of cart to preamp?
Low Z input impedance preamps rely on audioband impedance of the coil. Whereas one might predict that could include a self-resonance of the coil, there's no evidence that it does.

Being the curious type, I'd like to understand why - that's all I'm trying to achieve.

As to conjugate matching cart to preamp, that's one small step for man...…………..

Thankfully, such a preamp stage would still be inverting so on topic

LD
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Old 27th January 2019, 06:00 PM   #27
scott wurcer is offline scott wurcer  United States
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Originally Posted by Pyramid View Post
Coming from an RF background, I'm wondering why this wouldn't be an acceptable solution.
It is, just trying to rationalize expectations vs measurements. For instance any loss mechanism in the motor has noise, Rod Elliot's suggested model that supposedly includes eddy current losses would have a noise signature when doing an unloaded open-circuit measurement.
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Old 27th January 2019, 08:04 PM   #28
PRR is offline PRR  United States
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Originally Posted by luckythedog View Post
A quick look at self-resonance of good quality mic transformers suggests self-resonance at c 250kHz.

Some reference to self C not depending proportionally on number of turns, diminishing increases with each extra turn. ....
250KHz is true of low-impedance mike-iron windings. I suspect you found Jensen's 200:800 input transformer, specifically made for the low hiss-Voltage 990 amplifier.

With amplifiers of higher hiss Voltage, we wind-up to get more signal voltage. When going to a vacuum tube grid we sometimes aim over 20K ohm reflected impedance. The inductance, and leakage inductance, goes up with Z, but the C hardly changes. It is very common to find top-resonance below 25KHz, even 15KHz.

C has nothing to do with number of turns. 50 turns of house-wire, or 5,000 turns of hair-wire, both the same outside dimensions, will have very similar capacitance.

Recall that the Pickering et al 5mV 47K interface goes back to the days of tubes. It is clearly wound-up to as high an impedance as we can get with 100pFd of cable (we didn't usually put the hot-bottle preamp IN the turntable) and 100pFd of high-gain triode grid capacitance. The cartridge winding C is in there too, but I suspect it is not-large compared to cable and tube C.
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Old 27th January 2019, 10:06 PM   #29
luckythedog is offline luckythedog  United Kingdom
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Originally Posted by PRR View Post
The inductance, and leakage inductance, goes up with Z, but the C hardly changes. It is very common to find top-resonance below 25KHz, even 15KHz.

C has nothing to do with number of turns. 50 turns of house-wire, or 5,000 turns of hair-wire, both the same outside dimensions, will have very similar capacitance.
Thank you, PRR that's interesting and I accept that empirically to be true.

What intrigues me is why ?

LD
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Old 28th January 2019, 08:42 AM   #30
luckythedog is offline luckythedog  United Kingdom
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Originally Posted by PRR View Post
The cartridge winding C is in there too, but I suspect it is not-large compared to cable and tube C.
Yes, I accept that empirically too. The reason for interest is because, in alternate topology preamps with ultra-low input Z, cable and input stage C effects are avoided. One is just left with, and relies on, coil impedance.

It's also interesting because, if one were winding a cart for low Z input preamps and 1/f response, cartridge inductance could be increased I suppose.

LD

Last edited by luckythedog; 28th January 2019 at 08:48 AM.
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