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
Looks like you've made some progress. Good!

I'm surprised that relocating the connections of the cable capacitance elements in the model (together with the minor changes you've made in some of the capacitance values) had such a profound impact on the results. I don't know how to explain that.

Also, keep in mind that even though the response peak is way above the range of audible frequencies, its amplitude and frequency are nevertheless important considerations, which stand a good chance of being audibly significant. Again, see Jonathan's post in another thread that I linked to earlier.

Regarding the transformer model, I read through the "Using Transformers in LTspice/SwitcherCAD III" paper you linked to, and I also looked at some of the literature on LTSpice at the linear.com website. Their simulation is done differently than what I was envisioning when I provided my earlier comments. I was envisioning that the transformer would be modeled as an ideal transformer (k = 1; infinite self-inductance of the windings; zero leakage inductance, etc.) in combination with external circuit elements representing its non-ideal characteristics. They are modeling it as a single non-ideal element. So take that into account when considering my earlier comments.

I would expect either approach to yield good results, IF the parameter values are suitably chosen. And again, I have no knowledge of what the appropriate values might be for a typical SUT. Also, you might try re-running your simulation with k values of say 0.9, 0.8, etc., to see how sensitive the results are to that value. k = 1 corresponds to zero leakage inductance, which of course is not possible with a real world transformer.

Finally, a note of caution. It appears that despite statements indicating that LTspice/SwitcherCAD III (or "LTSpice IV" which is what is available for download at their website) is/are "general purpose," it appears that their program is oriented toward facilitating analysis of switching power supplies. As stated in one of their papers, "LTspice is a high performance SPICE simulator, schematic capture and waveform viewer designed to speed the process of power supply design. LTspice adds enhancements and models to SPICE, significantly reducing simulation time compared to typical SPICE simulators, allowing one to view waveforms for most switching regulators in minutes compared to hours for other SPICE simulators." And of course the designs of power supply transformers and cartridge stepup transformers are vastly different.

Regards,
-- Al
IMPORTANT P.S: I just noticed that in the second model the units of the transformer coil inductances appear to be Henries, while in the first model they appear to be microHenries (with the numerical digits being the same in the two models)!!!

That would certainly seem likely to account for the differences in the results!!!

Regards,
-- Al
For anyone's information, I believe the Jensen engineers do mention the leakage inductance values, at least for their many SUTs, on their website. From that, one could get a ballpark idea of what to expect from a "typical" SUT.
Lewm, can you please give a link. I have looked on Jensen and not seen these specs. Thanks.
AL, thanks for pointing that out!!!!

You are right I must have not typed in uh or mh and it defaulted to full henries, and I completely missed this. I will have to look at this later when I get a chance.

But now that, that is a thing I might as well figure out what the inductance is in acutal step up transformers. I emailed cinemag hopefully they respond.