These devices incorporate capacitors connected in parallel across the power line, and reduce noise reaching the equipment without imposing any inductive, maximum-current choking coils or inductors in the line.

Correctly applied, this major system modification or adjustment (really more than a “tweak”) dramatically improves digital and to a lesser extent analog playback quality. Digital processing, especially the serial digital, is extremely sensitive to power line noise, probably due to the effect of noise on digital timing jitter.

There are two alternate approaches for this technique: either use commercially available plug-in filters, or build superior-sounding units yourself.

Commercially available parallel-type plug-in AC filters:
Recommended: Audioprism Quiet Line plug-in filters (kit of 8). Audioprism, Inc. Tel. (425) 869-8482, Enacom plug-in power line filter, available from The Tweak Shop (707) 575-8626. The Quiet Line and Enacom filters plug into the A/C socket. The Quiet Line filters produce a large reduction in background noise level, edginess, hardness, etc. The Enacom filter is more expensive, but also improves clarity especially in the midrange, much more than the Quiet Line unit.

These filter types employ small value (0.1 microfarad) capacitors. The Quiet Line units also use resistors in an RC filter configuration with inexpensive, nonaudio-grade capacitors. Neither type filters noise from the neutral line.

Ideally, most of the Quiet Lines should be plugged only into other AC lines than the one used by the system, in the listening room and in other rooms in the house. The Enacom filters incorporate a much higher quality, better sounding capacitor and should be reserved for the outlet(s) powering the system. You need at least 2 to 3 Enacoms for the best performance. A single Enacom improves clarity and resolution over a relatively narrow band from the high frequencies through the upper midrange. Adding one, two, three, etc. Enacoms extends the improvement successively lower in audio frequency. Three together (total .3 microfarad) seem to extend fully through the midrange.

We find that the greatest sonic improvement in our system results from installing a combination of both commercially available types:

• At least 7 Quiet Line filters plugged into other AC lines than the one used for the system, covering all the AC lines in the house if possible.
• Three Enacom line filters plugged into the power conditioner supplying the signal source component (CD player, phono/line stage preamp), or into the wall outlet used if there is no power conditioner.

This may seem excessive, but the effect of all the filters is cumulative. This one combined “tweak” accomplished a wonderful improvement to digital playback quality unobtainable in any other way than AC filtering. Of course, power line quality varies from location to location and the optimum number of filters certainly also varies with location and with the AC line noise susceptibility of the particular components.

We suggest the common sense approach of starting out with one of each type, increasing numbers of filters until the point of diminishing returns is reached.