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RE: Output Z / Cable Capacitance

Posted by Triode_Kingdom (A) on April 2, 2015 at 18:22:04

In Reply to: RE: Output Z / Cable Capacitance posted by mobiasloop on April 2, 2015 at 07:03:42:

"Canare L4E6S 185pf/ft. Nominal impedance 44 ohms. I tie the 4 conductors into two."

44 ohm cable only looks like 44 ohms if it's terminated in its characteristic impedance at the far end. I don't think a 44 ohm load on your preamp is the best solution, even with a cathode follower. Alternately, if the cable is terminated with a high-Z load, 10K or more, the cable will appear reactive, and you'll have to drive all that capacitance, roughly 0.0055uF for 30'. You could drive that value pretty well with a 105 ohm source, but you're likely to have problems with the transformer(s) used to balance the drive. Those transformers are designed to work into a known impedance, and their performance will become increasingly skewed as the load impedance increases. Worst case, they'll not only produce a very tilted response, there may be significant peaks and dips due to series and parallel resonances that would ordinarily be dampened with the correct load.

So, my suggestion for this application would be to simply use a single-ended cable (coax) of the lowest capacitance practical. One of the best (and most economical) for this in my opinion is Belden RG-58/U, rated nominally at 24pF/ft. That would produce 720pF total over a 30' span. Driven with a 105 ohm source, the loss at 20kHz relative to 1kHz is less than 0.001 dB. Even at 100kHz, it only increases to about 0.01 dB. In practice, this means you could in fact drive it with a much higher Z source and still achieve very good response. That would likely allow you to dispense with the expense of a negative supply and/or a very low Rp tube that needs lots of current.

Edits: 04/02/15 04/02/15

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