An ultra Low Shunt Capacitance way for audio interconnections (LSC configuration) - Page 2 - diyAudio
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Old 28th June 2010, 10:53 AM   #11
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Quote:
Originally Posted by neazoi View Post
[snip]On the other side twisted pairs carying +/- sig still introduce some capacitance, each conductor is the plate and the insulation being the dielectric. The capacitance is much lower than coaxial cables though.[snip].
I think for reasonable interconnects the cap is a non-issue. For 3 feet, at say 250pF/feet you're looking at 750pF. If the Zout of your driving side is 100Ohms, a common value, your freq response has a -3dB point at f=1/(2*pi*R*C)= about 200kHz. I for one wouldn't lose sleep over that.

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Old 28th June 2010, 11:22 AM   #12
AndrewT is offline AndrewT  Scotland
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I would not lose any sleep over that.

But, cascade three of these cable filters over three different interconnects and you have ~-9dB at 200kHz and a low Q of ~0.4 and that extends the attenuation down into the accepted audio bandwidth.

If one of the Rs were 500r then the output would be 3dB down @ ~ 40kHz due to this single pole filter and a further 2dB to 3dB down due the the other two interconnects, i.e. ~-5dB at 40kHz and again of a low Q extending well down into the audio bandwidth.
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Old 28th June 2010, 11:51 AM   #13
neazoi is offline neazoi  Greece
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Originally Posted by janneman View Post
I think for reasonable interconnects the cap is a non-issue. For 3 feet, at say 250pF/feet you're looking at 750pF. If the Zout of your driving side is 100Ohms, a common value, your freq response has a -3dB point at f=1/(2*pi*R*C)= about 200kHz. I for one wouldn't lose sleep over that.

jd
If I get it right you are talking about LPF here?
I was talking about HPF formed by the capacitor between the UNCONNECTED shield and the central conductor, which cuts-off the lower RF frequencies. For microwaves I would not worry.
The audio is direct coupled srom the source to the sink.
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Old 28th June 2010, 11:54 AM   #14
AndrewT is offline AndrewT  Scotland
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Originally Posted by neazoi View Post
The audio is direct coupled from the source to the sink.
when I looked at the proposed interconnect, I got the very distinct impression that the signal circuit was deliberately broken. That cannot be equated to direct coupled.
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Old 28th June 2010, 12:08 PM   #15
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I would not lose any sleep over that.

But, cascade three of these cable filters over three different interconnects and you have ~-9dB at 200kHz and a low Q of ~0.4 and that extends the attenuation down into the accepted audio bandwidth.

If one of the Rs were 500r then the output would be 3dB down @ ~ 40kHz due to this single pole filter and a further 2dB to 3dB down due the the other two interconnects, i.e. ~-5dB at 40kHz and again of a low Q extending well down into the audio bandwidth.
True. But 500 ohms Zout is already pathological in my book
Although some tube preamps are guilty here with very (relatively) high Zout. Interestingly, there is some indication that the earlier hi-freq roll off does give subjectively better mid range because the ear-brain will more focus on that range and pick up more details. That in turn *could* be a reason why some like tube preamps. But I have no hard proof for it.

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Old 28th June 2010, 12:15 PM   #16
neazoi is offline neazoi  Greece
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when I looked at the proposed interconnect, I got the very distinct impression that the signal circuit was deliberately broken. That cannot be equated to direct coupled.
Maybe this is not clear enough.
The amplifier is direct coupled to the speakers and it is this design: http://www.audio-constructions.com/m...-schematic.png

The central conductor of one coaxial is the positive going from the amplifier to the speakers.
The central conductor of the other coaxial is the negative going from the amplifier to the speakers.
Both shields of the coaxials are left unconnected.

So we have no shunt capacitance from positive to groung (let's ignore the air dielectric etc)

But as a previous member states, the unconnected ground acts as an antenna. It collects RF and couples it to the central conductor through the dielectric of the coaxial. This is ANOTHER capacitor (not shunt)

If we use the standard coaxial way we have capacitor A
If we use the proposed coaxial way we do not have A but we have only capacitor B

But at microwave frequencies capacitor B is not formed because the shield reflects the RF even if it is not grounded, like a dish reflects microwaves.
At lower RF frequencies capacitor B couples RF energy to the cable, but It also behaves like a HPF, if the dielectric of the cable has low capacitance and the length of the cable is kept low. What is the cut off, I do not know..
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Old 28th June 2010, 12:24 PM   #17
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Hi,
are you saying his diagram that shows + signal and -signal are actually the signal flow and signal return of the signal circuit?

What about the loop area of the flow and return pair?

Surely, he is not proposing that you separate the signal flow from the signal return.

Look at microphone cable, twisted flow and return inside the common screen.
Look at balanced interconnects, the hot and cold (flow and return) are twisted together, sometimes without a screen.
Look at standard Coax, the signal flow and signal return are together concentrically.
Look at HF and a ground plane. The return route mimics the flow route to minimise impedance even though the return route is longer than the direct route straight across the conductive plane.

Why would all these standard methods go out the window, if they could be improved by separating the flow and return conductors resulting in big loop area?
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Old 28th June 2010, 12:28 PM   #18
neazoi is offline neazoi  Greece
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Why not separating these signals? It is the same length in both cases.
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Old 28th June 2010, 12:31 PM   #19
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Hi,
could you accept the simple answer of:- "because no one else does it"?
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Old 28th June 2010, 12:37 PM   #20
neazoi is offline neazoi  Greece
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Originally Posted by AndrewT View Post
Hi,
could you accept the simple answer of:- "because no one else does it"?
This is not a techical answer, we are trying to improve things or at least to explore the possibilities.

Back to the technical, the cable lengths are the same, inductance is kept the same, large shunt capacitance does not exist (like two wires appart from each other, not twisted). So the point is the RF interference and how much the unconnected shield will reflect or couple RF energy to the internal conductor or not.
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