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In Reply to: RE: Output Z / Cable Capacitance posted by mobiasloop on April 02, 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.
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Buy Chinese. Bury freedom.
Edits: 04/02/15 04/02/15Follow Ups:
Thank you very much! I'll look for some Belden RG58U and use the Canare for microphone use. The preamp will have both unbalanced and balanced outputs and will only use the transofmers if I have to feed my Ampex and with short cable. Did you look at the Wavetek output stage using 5687? Thanks, Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
When you buy the RG58, just be sure it has a copper braid that you can solder to, not aluminum. Nearly all of this is copper, but I do remember an off-brand some years ago that wasn't. Yes, the Wavetek output stage looks good. I installed their values into a SPICE simulator with 6SN7s, and it drives the capacitive load easily. Frequency response and distortion should both be excellent. I think this will prove to be a perfect solution for your application.
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Buy Chinese. Bury freedom.
Hey thanks! I really appreciate it. I'm not an engineer and this helps a lot! Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
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