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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
jcarr
382 posts
 
Dear Dfel: When using a stepup transformer, any capacitance present on the secondary side of the transformer will be reflected back to the primary side (IOW, the phono cartridge), but multiplied by the square of the primary-secondary turns ratio. It is therefore more important than ever to use super low-capacitance cables to connect a stepup transformer to the phono stage, unless the goal is to build a filter.

As an example of loading sans stepup transformer, the following thread on What's Best forum may be useful reading. The electrical models used are presented in the figures along with the response charts, are more complete than Hagerman's, and are derived from real-world measurements of cartridges, signal cables and phono input stages.

http://www.whatsbestforum.com/showthread.php?15077-Cartridge-Loading-A-Misnomer

kind regards and hth, jonathan carr
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almarg
9,670 posts
 
Jonathan, thanks very much for chiming in. Dfel, Jonathan is the designer of Lyra cartridges, so we are privileged to be receiving some exceptionally knowledgeable inputs. He is also the author of the post that I linked to earlier which explained why minimizing capacitive loading of a low inductance low output moving coil cartridge can be important, even in the absence of a SUT.

Regarding your two most recent posts, which I thought were well done summaries of some good work, I just have a couple of comments:

1)With respect to MM's, I would emphasize per my earlier comment that premature rolloff can result from too little capacitance, as well as from too much, since in the case of MM's the LC resonance will in many or most cases directly affect frequency response within the audible range. As I mentioned, in general (and perhaps always) the manufacturer's recommended range of load capacitance should be adhered to.

2)Regarding your point about inductance not being specified for many cartridges, I would expect that in general there would be a significant degree of correlation (albeit probably a very loose one) between a cartridge's inductance and its rated output voltage under the standard test conditions.

And finally, just a very minor quibble: In a couple of places in the longer of your two recent posts the word "subsonic" appears to have been substituted for "ultrasonic," although "ultrasonic" was correctly used toward the end of that post.

Regards,
-- Al
dfel OP
62 posts
 
Jcarr, thank you very much for posting I appreciate it. I will read through the forum, gather my thoughts, and post in this thread again. I am also wondering, what do you find is the typical inductance of a moving coil cartridge (low output and also high output) ? I get that several generators can be used but I am just curious what your take is on this. Are we in the realm of 5-500uH or are we more in the area of 5-500mH?

I can see what you are saying. I will put it into practical terms for anyone else who is reading so they can follow as well ( all 2 of them). Suppose that you have 100pf cable from the cart -> Sut and another identical one from the SUT->Preamp. and you are using a 1:10 SUT. Then...

The cart sees:

-100Pf + 100pf *sqrt(10)= 416pf
-The 47K load gets reflected back at 470 Ohms
-and the inductance of the wire is meaningless even after -being reflected with a Sqrt(10) multiple tied to it. The -cartdrige inductance is x * sqrt(10) and can be substantial depending.

The SUT sees ( bad phrasing, Through the SUT...):
-100pf *10 +100 pf = 1,100pf
-inductance from the cart x*10 = 10X
-and 40 Ohms from the cart *10 + 47K = 47400, or just 47K
dfel OP
62 posts
 
Al, thanks for pointing out those details. You are right. I had not looked at the too little side. On the correlation I would hope so, it would make things easier but there in solid convention for quotation that would make it easy to figure out, but I will have to think about this. I do see what you mean, X MV output and Y resistance into an open circuit figure out ballpark for inductance etc.

As for the Ultrasonic Subsonic...tomato tom-ah-to LOL, you are correct I goofed on that.
dfel OP
62 posts
 
Jcarr, Please email the white paper, you reference in the thread I am exceedingly interested in reading it!!!! Thanks. I will PM you.
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