2. analog
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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

Showing 2 responses by jcarr

jcarr
382 posts
 
Dear Dfel: Here is an alternative way to envision what a SUT does.

As a passive device, a SUT cannot create more energy. Its voltage amplification abilities are limited to converting the MC cartridge's output current into voltage. But since a SUT cannot create more energy, the cartridge signal current that gets converted into signal voltage is no longer available at the SUT output.

And since current is what charges and discharges capacitance, inserting a SUT between MC cartridge and phono stage reduces the cartridge's ability to charge and discharge whatever capacitance is present on the SUT secondaries by the square of the primary-secondary turns ratio.

hth, jonathan carr
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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