signal propagation through audio cable. - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #11 of 29 Old 04-20-16, 09:45 PM
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Re: signal propagation through audio cable.

Quote:
chashint wrote: View Post
http://www.learningaboutelectronics....or.php#answer1

It takes a lot of capacitance to mess with audio frequencies.

Plugging the 0.1uf and 0.2 ohms would put the 3dB cutoff frequency at 7,961,783 Hz.
Probably not a noticeable difference at 20,000 Hz.
Yes 0.1uf/0.2Ohms gives a -3dB at this frequency. However if you use a typical amplifier input impedance of, say 50k, the numbers are quite different. Don't worry: interconnects are going to have C way below 0.1uF...
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post #12 of 29 Old 04-21-16, 07:05 AM Thread Starter
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Re: signal propagation through audio cable.

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RBTO wrote: View Post
I'd have to disagree.
Do you have any links that can elaborate on your idea? Electrically short or electrically long is not something I made up but very real parameters that follow principles based on physics.

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Granted in most cables, the capacitance is quite small and has almost negligible effects, any cable capacitance will react with the output impedance of the source and result in a drop-off of high frequency amplitude beyond some point. That's an effect which is not dependent on the cable being electrically long or short, though cable length directly relates to the capacitance seen by the source (doubling the length of the cable will double the capacitance seen between the two conductors).
Please reread the definition of electrically short and electrically long. These definitions are based on cable lengths and wavelength of frequency and are real.


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To demonstrate capacitance effects, add a 0.1 microfarad physical capacitor across the cable end (of a short length of microphone cable), and see what happens to high frequency behavior. The naturally occurring capacitance between the two conductors is basically the same though much smaller and distributed along the length of the cable, and because it is much smaller, it usually won't affect frequencies in the audio range, but if the cable had 0.1 microfarads of capacitance, it would be noticeable even though we're dealing with an "electrically short" case.
Adding an additional component like you proposed alters the circuit, not the cable. The cable remains invisible to the amp but the amp now sees a LP filter in parallel (with a -3db point well into the Megahertz ) with the loudspeaker. You have introduced something that isn't normally there. The amplifier will see the added capacitor but it still doesn't see the cable.

Last edited by 3dbinCanada; 04-21-16 at 07:15 AM.
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post #13 of 29 Old 04-21-16, 07:41 AM
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Quote:
dgmartin wrote: View Post
Yes 0.1uf/0.2Ohms gives a -3dB at this frequency. However if you use a typical amplifier input impedance of, say 50k, the numbers are quite different. Don't worry: interconnects are going to have C way below 0.1uF...
Yes to all.
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post #14 of 29 Old 04-21-16, 09:08 AM
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Re: signal propagation through audio cable.

Interesting indeed, my beliefs differ from these tests, however, admittedly I have no proof beyond my ears.

Good Listening

Jack

"For those who believe no proof is needed for those who don't believe no proof is possible"
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post #15 of 29 Old 04-21-16, 09:19 AM
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Re: signal propagation through audio cable.

Quote:
chashint wrote: View Post
http://www.learningaboutelectronics....or.php#answer1

It takes a lot of capacitance to mess with audio frequencies.

Plugging the 0.1uf and 0.2 ohms would put the 3dB cutoff frequency at 7,961,783 Hz.
Probably not a noticeable difference at 20,000 Hz.
That figures if you're working with a source impedance of about 0.2 Ohms and would probably hold true for the case of a speaker connection where the amplifier output impedance is that low, but in the case of a line output where the output impedance is around 10k Ohms, the cutoff drops to 159 Hz. Noticeable?

Like I said, the cable interacts with the source output impedance and that output impedance is important to consider in cable losses. Professional audio cables make use of lower output impedances (100-200 Ohms) partially for that reason (and some others), and can be run much longer distances without the capacitive losses you get with consumer cables. In any case, the longer the cable, the more capacitance one has to worry about.
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post #16 of 29 Old 04-21-16, 09:24 AM Thread Starter
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Re: signal propagation through audio cable.

Interconencts should be kept short as possible to prevent picking up induced noise. Still, the cables are extremely short compared to the wavelength being transmitted so it renders the capacitance as a non issue.
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post #17 of 29 Old 04-21-16, 09:30 AM
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Re: signal propagation through audio cable.

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3dbinCanada wrote: View Post
Do you have any links that can elaborate on your idea? Electrically short or electrically long is not something I made up but very real parameters that follow principles based on physics.
No links off the top of my head.

I completely agree with your proposition of electrically short and long but it has more to do with the requirements of the source and load impedances being a match so no energy is reflected (radio frequency applications). In audio, as you pointed out, almost all cases are electrically short but that doesn't have anything to do with load and source impedance interactions. The cable is part of the overall load.

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3dbinCanada wrote: View Post
Adding an additional component like you proposed alters the circuit, not the cable. The cable remains invisible to the amp but the amp now sees a LP filter in parallel (with a -3db point well into the Megahertz ) with the loudspeaker. You have introduced something that isn't normally there. The amplifier will see the added capacitor but it still doesn't see the cable.
I suggested adding the capacitance simply as an example that long/short doesn't come into play. Whether that capacitance is in the cable or external doesn't matter at all. That exercise was just to point out that any capacitance paralleling the conductors of the cable will have an effect on high frequencies (and yes, if the cable and source impedances are low, that effect will not be in the audio range), but in some cases (where source output impedance is high) capacitance is definitely a consideration. You always have some capacitance in an audio cable just due to the two conductors running parallel to each other. The amount of that capacitance has to do with spacing and the material between the two conductors (the insulation), as well as the length of the run.

In electrically long cases, the impedances of the source, cable, and load, all have to be considered and usually matched to prevent reflections. Not to worry about in most audio cases.

Likewise, as mentioned, most cable capacitance is pretty low (except for long runs) and won't harm audio transfer, BUT, capacitance IS a consideration and is the key contributor to high frequency loss in cables.

Last edited by RBTO; 04-21-16 at 09:48 AM.
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post #18 of 29 Old 04-21-16, 09:47 AM Thread Starter
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Re: signal propagation through audio cable.

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RBTO wrote: View Post
No links off the top of my head.

I completely agree with your proposition of electrically short and long but it has more to do with the requirements of the source and load impedances being a match so no energy is reflected (radio frequency applications). In audio, as you pointed out, almost all cases are electrically short but that doesn't have anything to do with load and source impedance interactions. The cable is part of the overall load.
I agree but its only the resistive part that's seen.

Quote:
RBTO wrote: View Post
I suggested adding the capacitance simply as an example that long/short doesn't come into play. Whether that capacitance is in the cable or external doesn't matter at all. That exercise was just to point out that any capacitance paralleling the conductors of the cable will have an effect on high frequencies (and yes, if the cable and source impedances are low, that effect will not be in the audio range), but in some cases (where source output impedance is high) capacitance is definitely a consideration.
I think noise pickup in interconnects would play a bigger role in signal degradation than the distributed capacitance exhibited by interconnects. What I'm saying if you swap out interconnects of equivalents length, the changes in capacitance by even several hundreds of pf will be inaudible. Please remember that the voltage along the entire length of the cable for a given signal in any point time is constant across the cables entire length.

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In electrically long cases, the impedances of the source, cable, and load, all have to be considered and usually matched to prevent reflections. Not to worry about in most audio cases.
Agree.
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post #19 of 29 Old 04-22-16, 06:50 AM Thread Starter
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Re: signal propagation through audio cable.

A link describing the velocity of propagation of audio signals;

http://www.computeraudiophile.com/bl...equencies-169/
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post #20 of 29 Old 04-22-16, 10:52 AM
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Good link.
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