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69.71.1.1
In Reply to: RE: Output Z / Cable Capacitance posted by Triode_Kingdom on April 02, 2015 at 05:53:39
Cannare star quad mic cable is what I have on hand. Thanks Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Follow Ups:
Canare L4E6S 185pf/ft. Nominal impedance 44 ohms. I tie the 4 conductors into two. But I'm open to changing. I've just had this around. Thanks, Jim
http://www.canare.com/ProductItemDisplay.aspx?productItemID=53
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
"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 04/02/15
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
Holy crap, that's a lot of capacitance!
I'm using a solid state preamp now with Class A output, very low impedance. I want to try this electron tube preamp with the cathode follower output. Thanks, Jim
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Here is the Wavetek white cathode follower using the 5687.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
With the 6AU6 in place, you'll have way too much gain for the circuit to be that useful.
In the case of the schematic above, I find White Cathode Followers to be fun to study and mess with, but quite fussy and somehow not quite as "together" as a normal cathode follower. I wouldn't expect good performance with the 5 Ohm load on the top triode.
Thank you! Yes, I'm not using the 6au6 stage, just considering the cathode follower output stage using the 5687. My first stage is a 12aY7/6072A as a diff amp. 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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