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Cable capacitance

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Cyreg View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Cyreg Quote  Post ReplyReply Direct Link To This Post Posted: 09 Mar 2008 at 10:49am
Thank you Graham for the extended explanation. 
My question cancelled .
 
Hi fellow-forum analog(ue) lovers and GSP-fans.
I remembered a link about cartridgeloading in general. Big%20smile
The mathematics are there, formulized and ready to use (if you know how)
 
 
If you're going to need MC stepups there is a lot of other important things to keep in mind.
 
Have fun.  Han
 


Edited by Cyreg - 26 May 2008 at 7:46pm
TecnoDec/RB250/MP110>GramAmp2C/PSU1; Cyrus CD8SE; Cyrus FM7 > Exposure XXXV > Harbeth C7ES-3 '35th Anniversary'
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kreyfish Quote  Post ReplyReply Direct Link To This Post Posted: 29 Mar 2008 at 11:58am
Graham, I run an Ortofon 2M Black which requires a load capacitance of 150 - 300pF. I have an RPM 6.1SB with a 9c tonearm which I cannot find the Cap rating for, so lets assume 20pF. The Reflex has an input Capacitance of 100pF which means I should be shooting for an interconnect capacitance of between 30 and 180pF, which seems a wide range.
Using 1m Belden 89207 (14.5 pF/ft) gives 47.5pF which puts my setup at the lower end of the acceptable range. Using 89259 (which I also have) would give a slightly better 56.7pF but I would lose the shielding (coax vs shielded twisted pair).
My question is that if I were to be finicky, should I shoot for dead centre of Ortofons required range at 225pF or am I fine with what I have? The capacitance on the output side of the Reflex will obviously be 47.5pF using these cables.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kreyfish Quote  Post ReplyReply Direct Link To This Post Posted: 29 Mar 2008 at 1:09pm
OK, I have been reading further in "The RFI Problem" and also a tip from John about using multiple conductors to raise capacitance, and am now wondering if using both conductors in the Belden 89207 cable for the signal and the braid for the return would give me the best result.  This would raise capacitance closer to that mid point, and by flowing a return signal through the braid, provide better shielding at higher frequencies. I currently have the shield connected at the source end and grounded via a 0.01uF ceramic disc Cap at the other.
Your thoughts Graham?
 
Maybe not the appropriate area for this comment, but in my time as an Automation Technician, we always bundled the shield drains of all signal cables and fed them seperately to earth. We never terminated them at a component input or output.
Thoughts on this?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 30 Mar 2008 at 4:11pm
Originally posted by kreyfish kreyfish wrote:

OK, I have been reading further in "The RFI Problem" and also a tip from John about using multiple conductors to raise capacitance, and am now wondering if using both conductors in the Belden 89207 cable for the signal and the braid for the return would give me the best result.  This would raise capacitance closer to that mid point, and by flowing a return signal through the braid, provide better shielding at higher frequencies. I currently have the shield connected at the source end and grounded via a 0.01uF ceramic disc Cap at the other.
Your thoughts Graham?
 
Maybe not the appropriate area for this comment, but in my time as an Automation Technician, we always bundled the shield drains of all signal cables and fed them seperately to earth. We never terminated them at a component input or output.
Thoughts on this?


Grounding the shield through a cap looks like a good answer. Leaving the shield open one end has been shown to be beneficial from the standpoint of hum loops, but letting in RFI because there is nothing to oppose it. By grounding via a cap there is current flow at RFI, but maybe a 0.1uF cap would be more effective?

A number of amps have their "open end grounds" grounded this way. By which I mean either the input ground or output ground. The correct way of grounding within equipment is at the input, and therefore the output 0V or reference would be grounded via a small capacitor. However, over the years several manufacturers have grounded incorrectly at the output (and so did I until I learned the lesson) placing the cap at the input. In fact some ground at the volume control, input or output depending on the mood they're in...

In automation measurement and control I guess that some bandwidth narrowing may apply within the equipment? In which case it is just the ground loops that would be given importance, and by grounding in this way such loops would be avoided.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kreyfish Quote  Post ReplyReply Direct Link To This Post Posted: 31 Mar 2008 at 8:57am
Thanks Graham. So my cable orientation would be correct (shield connected at source end of cable and grounded through Cap at other)?
Regarding my previous post asking about Capacitance. Should I be aiming for the mid point of Ortofons required range? I was always of the understanding that the capacitance of cables should be kept relatively low. When dealing with audio, it seems an accurate match is more desirable. True? or not...Confused
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