active shielded speaker cables

[pinkmouse]

Personally, I can't see it making a difference, but "What if?" is a fine and noble sentiment, so go for it!

[sunrise]

thank You, thank You...
i begun to think i am crazy with this thing and almost said that i will leave it behind ....

i will quote here a post that a very kind guy called Nemestra has written on diyhifi.org forum only because of the papers he quoted there that are talking abotu this thematic - i hope he will not mind me doing that ..... also i hope that people here will not have anything against that.......

thanks Nemestra again...
=======================

Hi Sunrise and all,

I'm concerned that a simple and straightforward question from Sunrise should be met with unhelpful comments. It may be a bad idea for many reasons. Sunrise has indicated that he's aware of that. He would still like to try it out.

Sunrise, I haven't tried this with audio designs but am familiar with driven or guarded shield designs from front ends for physiological measurement, specifically electrocardiograms. The following are references on the subject which may or may not be helpful. They are all low bandwidth designs - ECG sampling / bandwidth rarely gets above 1kHz or so even for diagnostic quality 12 lead signals. As the bandwidth of the driven shield is increased system stability will be harder to maintain.

'High-quality recording of bioelectric events - Part 1 Interference reduction, theory and practice', A.C. Metting van Rijn, A. Peper, C.A. Grimbergen, Medical and Biological Engineering and Computing, September 1990

Chapter 6, Grounding and Shielding Techniques, 4th Edition, Ralph Morrison, John Wiley & Sons, 1998, ISBN 0-471-24518-6

Biophysical Measurements, Peter Strong, Tektronix Inc., 1970

Good luck,

James
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the thread can be found on this link, post 23
http://www.diyhifi.org/forums/viewto...?p=26643#26643

[darkfenriz]

Sorry for off topic, but what's the problem with cable capacitance?
Spread inductance should be high enough in most cases to make spread capacitance not important, all in all at high enough frequencies characteristic impedance is in tens of ohms range.
What do I miss?

regards
Adam

[analog_sa]

It's not so much the capacitance as getting DA out of the equation.

[FastEddy]

flg: " ... I have worked with equipment, from a different field, That had to drive analog signals 20-30' down a cable. We used a double shielded cable. The inner shield was driven by a low power amp driving the same signal as the signal line. The outer shield was a gnd'ed shield. ... The theory here is that the signal line is in effect "cascoded" by the inner shield. ... "

This is the principle behind both USB and FireWire cabling, grounded outer shield (-), active power (+) on an inner wire or other shielding. This methodology is also related to those inexpensive TV coaxial cable boosters / amplifiers, shield as (-), center conductor as (+) = Electrons flowing down the outer shield (usually negative or grounded = 'cause that's where the electrons are). Cable capacitance / impedence is reduced, significantly in some cases ...

(This is, in principle anyway, the reason computer bus speeds have increased beyond "inactive" possibilities, like the PCI and PCI-x bus, etc.)

[sunrise]

yes, they said that they used double shielding where outer shield has been grounded and inner shield is running the sam signal as hot wire inside (in center of the cable)...this inner shield is driven via buffer and this buffer has to deal with the capacitance that is between inner and outer shield - here i have only one shield so things are not good looking....

[cliffforrest]

Originally posted by FastEddy
flg: " ... I have worked with equipment, from a different field, That had to drive analog signals 20-30' down a cable. We used a double shielded cable. The inner shield was driven by a low power amp driving the same signal as the signal line. The outer shield was a gnd'ed shield. ... The theory here is that the signal line is in effect "cascoded" by the inner shield. ... "

This is the principle behind both USB and FireWire cabling, grounded outer shield (-), active power (+) on an inner wire or other shielding. This methodology is also related to those inexpensive TV coaxial cable boosters / amplifiers, shield as (-), center conductor as (+) = Electrons flowing down the outer shield (usually negative or grounded = 'cause that's where the electrons are). Cable capacitance / impedence is reduced, significantly in some cases ...

(This is, in principle anyway, the reason computer bus speeds have increased beyond "inactive" possibilities, like the PCI and PCI-x bus, etc.)

I don't think this is the same at all!

Connecting a power supply to the screen has NO effect on the signal or capacitance.

A power supply IS ground as far as the signal is concerned (or should be!)



As for "electrons flowing down the outer shield" you need to look at your signal transmission theory again, I think!

[sunrise]

inner shield = buffered signal
outer shield = ground

only i don't have that outer shield and i will use it only with buffered signal on the one and only shield i have..

[FastEddy]

Example: USB
1) + Power = +5 VDC
2) Data 0
3) Data 1
4) - Power Ground = 0 VDC
5) Shield grounded, clamshell, tied at one end or both by connection to pin 4.

Example: 6 pin FireWire 1394a
1) + Power = + 12 VDC or greater = shield of one twisted pair
2) - Power Ground = 0 VDC = shield of one twisted pair
3) Data B receive
4) Data B transmit
5) Data A receive
6) Data A transmit
*) Outer shield = 0 VDC tied to 6-pin clamshell, one or both ends and tied to chassis ground

Note that current flows in both inner 1394a shields and can flow in the outer shield, thus aiding in impedence matching, etc. ... and also reducing EMF interference, internal and from the outside. The physics of all this is quite complicated, but it does work whether there is a dynamic flow of electrons or a static charge on the shield(s).
From: http://www.interfacebus.com/Design_C..._Firewire.html

[cliffforrest]

"Note that current flows in both inner 1394a shields and can flow in the outer shield, thus aiding in impedence matching, etc. "

Can you please educate me as to what this means. I cannot see how DC current in the screen(s) has any effect on impedance.

Nothing in your link explains this, just a reference to differential signals inside a screen.

Thank you.