Active shielding. What is?

Active shielding is the practice of putting a positve DC voltage across the shield in an effort to lower the noise floor. It's really a matter of ratios. Shields have the tendency to pick up RF interference. By putting a voltage across the shield, the ratio of static power in the shield to any interference is minimized.

Please, anybody that knows more about this correct my mistakes.

Go to "synergisticresearch.com". They market Active Sheiding cables and give great explanation why this can be effective.

Roger here's some answers to your mystery. Brian H explains active shielding partially, yet there's a bit more to it. Synergistic applies 15 or 30 volts DC to the cable shield, which of course draws a small DC current through it. The shield is of course discrete (not in the signal path). This shielding current helps to prevent the coupling of stray EMI & RFI into the signal conductors. I use a dual-module Synergistic Master Control Center which has both 15V & 30V outputs; experimentation can determine which shielding voltage works best with which cable; the results can vary significantly depending upon whether it is a signal cable or a power cable. The selection of AC cable supplying power to the Master Control Center also affects sonic characteristics to a significant degree. Synergistic's Ted Denney can explain all this to you in greater technical detail than you might ever desire to know.
The shielding voltage also accomplishes another function. It saturates the insulation dielectric, which if left to its' own will actually leach electrons (audible micro-information) away from the signal conductors.
Audible effects of active shielding can vary of course from one rig to another. In my rig, I perceive improved bass extension & control, smoother midrange vocals, a greater sense of depth, staging & HF detail. The greater that the capabilities of your equipment are, the more improvement that you'll hear with the active shielding system.
For economy applications, Synergistic also offers a mini power coupler (wall wart DC power supply) in lieu of the Master Control Center. I have never tried a mini power coupler so I cannot compare them for you, but I suppose that performance wouldn't be as good since you don't have the optional output voltage selection nor the AC power cord selection capability. Still, based upon my own experiences, I definitely recommend trying it out for yourself one way or the other. I have heard no downsides when using active shielded cables. Of course there is some additional expense however.

Anyone know, since Synergistic uses a wall wart, what effect the AC ripple that is not filtered out would have on the internal conductors? Seems to me, unless it is highly filtered and regulated, the contaminated DC would be like running a low voltage power cord parallel to the cable. I know Synergistic uses the term DC but it would be expensive to have ripple free DC from the power supply. Seems a battery would be a better idea. Just a thought since this is old attenna theory.

Greetings Bigtee,

The only current flow in a circuit that the Synergistic Active draws would in the regulator(power supply) circuit it's self.
Since the Active shielding only produces a charge potential(open circuit).
Due to the fact that close to no current flows in such a circuit, the ripple (even with that scrawny supply) is almost nonexistent.
Nonetheless, an upgraded supply could not hurt.

the current is only just enough to light an LED: 10 to 20mA DC

The shielding in this cable makes a complete circuit from top to bottom. It has a wire that runs the length of the cable and comes back to source. Anytime power travels through a wire, it creates a magnetic field. I'm still back to the question of the purity of the DC since it is a complete circuit along the entire length of the cable. The AC ripple is going to have an effect.

I also meant to include this in my last blurb. Bob, you stated that the power cord would make a difference in the sonic signature of the cable. I hope this is not the case. Once you convert to DC, there should be no sonic signature. If there is, well, then my question is even more relavent.