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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
lewm
13,106 posts
 
Dfel, Far be it from me to try to amplify on the advice you've been given by either Al or JCarr. However, I don't see where anyone answered your question re the typical range of inductance for a typical MC cartridge. In my personal experience, I have never heard nor read of any MC that has inductance as high as 1 mH. Nearly all MCs seem to have inductance in the low micro-Henry range, less than 100uH. The DL103 may be an outlier as regards MC inductance (don't know the numbers), because it also has a relatively high internal resistance, a sign that there is a large coil inside.

Another source for information, if you are so interested in the math, is to be found on the Jensen Transformer website, in the form of a white paper on how to load their SUTs for flattest frequency response.

Personally, I would just concern myself with impedance loading and be done with it, if I were to be using an MC with a SUT. In other words, let the capacitance and inductance take care of themselves, which will happen if you do nothing crazy, e.g., 10-foot ICs or weird phono input stages.
dfel OP
62 posts
 
Thanks Almarg, for you insight.
as to the items:

1) In the K statement I have modeled it as a perfect SUT, this can be seen the by the 1 in the " K L2 L4 1". I can use numbers of less than one to make up for losses due to the SUT, like .6 .7 .9 etc..I figured lets see what happens with a perfect transformer first to isolate the cable effects.

2) Good point, I will sort that once I have the rest of the model making sense. I am concerned that the whole network of connections is completely modeled incorrectly. Can you please give some input on the basic structure, thanks.

3)Derived to have a 1:10 ratio, no other "genius" there (Strong emphasis on the quotation marks). The values I chose were arbitrary, so long as they had at square of quotient to give 10. Maybe that is incorrect.

4) I hear you, but I want to make sure that model itself I.E/ series/parallel resistors ground inductors and everything else are placed correctly in the circuit to model that part of a stereo system. I tried to follow Jcarr's model, but in the end I am still not sure why when modeling a wire connecting devices you would model the RCL in series or in parallel, and in what order you place the RCL for the wire connectors. I am a little lost on it and hoping someone can look at the images of the model circuit I have done and help me tweak it to the one that is correct.

dfel OP
62 posts
 
Typo: Under item "4) ...place the RCL for the wire connectors.."

I do not mean wire connectors, I mean wires or interconnects.
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almarg
9,670 posts
 
... Can you please give some input on the basic structure.... I tried to follow Jcarr's model, but in the end I am still not sure why when modeling a wire connecting devices you would model the RCL in series or in parallel, and in what order you place the RCL for the wire....
As I indicated near the end of my last post, your model looks good to me aside from the SUT issues.

For the cables, R and L are in series, and C is in parallel. Representing these parameters as "lumped" elements, with the capacitance first in the chain, as you have done, rather than as a great many separate elements representing their distribution along the length of the cable, I believe is a reasonable approximation at frequencies of interest.

Regarding the SUT model, the inductances of each of the two windings will probably not be in proportion to the square of the turns ratio, and may be directly proportional to the turns ratio itself (depending on a number of variables). And more significantly, if it is not clear, the inductances that should be represented are not the inductances that each winding would have if it were divorced from the other (i.e., their self-inductance). What should be represented, as I mentioned, is "leakage inductance," in series, and perhaps also a parallel inductance (and resistance). I don't know what values would be reasonably typical for those parameters for typical SUTs.

Also, Lew's comments are good ones IMO. Jonathan's emphasis on minimizing cable capacitance on the secondary side of the SUT should also be kept in mind, of course.

Regards,
-- Al
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almarg
9,670 posts
 
P.S. to my previous post: Regarding "leakage inductance," see this Wikipedia writeup.

Regards,
-- Al
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