To understand Active Shielding, it is necessary to first understand the principles of Synergistic Research cable design. Our first step in designing a cable is by developing a geometry that complements that system at an engineering level by precisely controlling capacitance, inductance, and resistance in the cable geometry. Next, we ‘voice’ every cable in its target system by building each design out of literally dozens of different material combinations, seeking specific materials to yield natural sound.
For seven years, we worked with hundreds of passive cable geometries and over one thousand different material variations. During this process, we realized that passive differences in geometry and material selection could only take us so far, and that ultimately we were limited by reactive capacitance (signal and cable interaction) and shield performance (RFI - Radio Frequency Interference, and EMI - ElectroMagnetic Interference). During this process, we had an idea to actively suppress reactive capacitance (and RFI and EMI) to dramatically improve cable performance. So began our Active Shielding cable program.
Our first experiments with Active Shielding began early in 1996, and involved the placement of batteries in a static circuit, with the positive annode of the battery tied to a conductor running the length of a cable, and the negative annode of the battery tied to the shield. These initial prototypes subjectively improved performance in the high frequencies. However, they also increased the noise floor (especially on long runs), with the positive conductor running the length of the cable (and not being terminated to signal or ground) thus acting like an antenna picking up RFI and EMI.
We then experimented with closed circuits, where the shield carried a DC current, with a buffer circuit between shield and ground and separate conductors carrying the ground signal. This closed circuit design not only improved subjective performance, but also made our cables measureably quieter, thus improving detail with greater frequency extension from top to bottom. Since a closed circuit draws current, we could no longer use batteries, as this would drain a battery in a matter of hours.