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Phono rig capacitance

I have read up on LPFs (low pass filters) and corner frequencies. and found the following... this equation gives the -3db corner frequency: Fc = 1/(2*Pi*R*C), inductance is ignored but can be impleneted using the R-adjusted instead of R as SQRT(R*L), geometric average. Though the value may not be significant, which is why I usually see it omitted.

I am interested in:

1. how one computes the -0.5, or -1db or any db cut in frequency NOT just the 3db corner frequency.

2. How to compute the corner frequency for the cartridge to SUT, given the amount of capacitance in the interconnect. For the example I suppose using the all familiar cinemag 3440 makes sense and for the cart the denon 103.

3.Same as above, but to compute for the interconnect from the SUT to the preamp..

4. Same as above but compute for the interconnect from the preamp to the power amp.

5. And perhaps the same for loudspeakers as well.

The goal is to find a value that ensures there is no roll off taking place and to select a suitable wire for each interconnection in a phono based playback system using an MC cartridge->SUT->Pre->Power.

I know, less capactiance blah blah blah, buy a 4 thousand dollar cable blah blah blah is the usual answer, but I am looking for a more scientific and technical approach to selecting wires that are in the ballpark of what makes sense based on well understood engineering principles.

I know that there are several members with advanced degrees in electrical engineering or are technically apt (Almrag, Atma, Raph etc...) and I am hoping that one of you can find the time to chime in please.

Thanks guys, looking forward to hearing your take!
dfel
dfel OP
62 posts
 
Hi guys, I have been having a lot of trouble modeling this correctly with the SUT in the mix. I am sure I am making some silly mistakes but I can't find them. Here is a link to the image of the simulation + circuit model. I tried to follow the model from the snippets I could find online to the paper by J carr, I would love to read the whole paper one day..please PM me since I cant PM you Jcarr.

For the earlier simulations it was easy to lean on the Hagerman article and simply copy the circuit and play with the parameters. But, in this case it is not so simple, and I would appreciate some help. Thanks.

http://s27.postimg.org/aage8hg2/Attempt_at_Circuit_Model.png

I am assuming the same cart specs from the paper, and using the specs for the mogami 2549 wire, 0.022 Ohms/foot, 26pf/foot, and 0.72uH. I used a 1:10 Step up ratio for the SUT.

Please tell me how if I have made a mistake here ( I am sure I have), and how to correct it as I am interested in seeing this display something meaningful. Please be gentle with the commentary as I am a rookie and know nothing about Electrical Engineering. Thanks again.
dfel OP
62 posts
 
Link was not working lets try again:

http://s27.postimg.org/aage8hg2r/Attempt_at_Circuit_Model.png

http://postimg.org/image/6e32chv33

http://postimg.org/image/6e32chv33/full

Hopefully one of those links works.
john_tracy
341 posts
 
" the mathematics that is involved in analyzing RLC circuits is quite complex"

But not so complex if you use the Complex form of reactive impedance. ;~)
Seriously, the use of Complex (imaginary) numbers makes the process much more easy. The circuit can be reduced to a Complex matrix equation using Ohm's law and then solved. The Complex form keeps track of both the magnitude and the phase of the signal.
dfel OP
62 posts
 
agreed John. Once the model is set. However once it is set is easy to simulate with software which is a more flexible and visual way of observing the data.

The thing is, the basic model is not complete. Have you looked at the links above to see how it was modeled? Does this make sense ?
almarg's avatar
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almarg
9,670 posts
 
Dfel, as I indicated earlier in the thread I can't speak too knowledgeably about modeling of SUTs. But FWIW the following thoughts occur to me:

1)Depending on how the simulation program you are using works, I think you might have to model the SUT as an "ideal transformer" combined with external elements representing its non-ideal characteristics (i.e., resistance, inductance, and capacitance, to the extent that they may be significant).

2)I would expect that the resistance of its two windings would be significant enough to warrant inclusion in the model, and that it should be modeled as two resistors external to the transformer itself, one of them in series with each of the windings. Or, alternatively, the resistance of the secondary winding could be modeled as reflected into the primary circuit, multiplied by the square of the primary to secondary turns ratio.

3)I don't know how you derived the inductances shown in your model for the two windings, but I suspect that they are not what should be included in the model. I believe that what should be included are series inductances in the primary and secondary circuits representing "leakage inductances." With the secondary circuit leakage inductance alternatively being modeled on the primary side, multiplied by the square of the primary to secondary turns ratio.

4)I don't know if it would be significant enough to warrant inclusion in the model, but including a parallel combination of resistance and inductance across the primary winding MIGHT also be called for, to account for core losses and reactances. (More precisely, those elements would be modeled directly across the primary winding if the impedances in series with the secondary are modeled on the secondary side; if the secondary side impedances are modeled as reflected to the primary side the reflected secondary side impedances should be modeled closer to the primary winding than those elements).

Otherwise your model looks good to me. It assumes that the cable parameters are "lumped elements" rather than "distributed elements," but that assumption seems to me to be reasonable for present purposes.

Regards,
-- Al
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