Only for the EXPERIENCED HOBBYIST who is familiar with a soldering iron and basic house electrical wiring practices. Inexpensive and simple AC filter adapters can be made using large-value metallized polypropylene film capacitors. There are no commercially available units equivalent to the designs suggested here. If correctly utilized, such hand-made plug-in filters have a huge, almost unbelievable effect in improving the sound of digital, an order of magnitude more than the commercially available units mentioned previously. Some of the capacitors should be connected from AC line to neutral, and others should be connected from neutral to earth ground. The 120 VAC line to neutral capacitor adapters should of course be carefully insulated using tape and/or shrink tubing to thoroughly insulate the caps and wiring to the 2-prong plugs.

There is no specific optimum number of power line filter capacitors. Even small to moderate amounts of filter capacitance significantly improve the sound. Beyond this, the more capacitance on the more AC receptacles and separate AC lines the better. We have not yet found a point of diminishing sonic returns in increasing total capacitance. Ultimately, increases in total AC to neutral filter capacitance would be limited by buildup of excessive capacitive 60HZ charge/discharge currents in the AC and neutral lines. However, this point is well beyond any practical concern.

A moderate initial filter configuration could consist of 50 microfarads total (AC to neutral) at the system outlet, another 50 microfarads total (AC to neutral) distributed around the house, and 300 microfarads total (neutral to earth ground) distributed around the house. This would greatly improve transparency and lower the noise floor without massively covering every receptacle/line in the house. Later, as you look for more improvement, you can keep increasing the number of caps/adapters as your budget and spouse allow.

Care should be taken when removing one of the 120v AC line-connected capacitor plug-in units from the receptacle since the capacitors may be charged up to over 180 volts at the moment of removal. Always short out the plug prongs before handling. Alternatively, a 100,000 ohm 1/2 watt resistor can be soldered across the caps to automatically discharge the caps rapidly on removal from the receptacle.

The capacitors connected from AC line to neutral should be plugged in as close to the CD player (or DAC and transport) IEC power inlet(s) as possible. The largest improvement in smoothness, transparency, depth of image and lowered noise level is obtained using an adapter allowing the AC line to neutral plug-in caps to be placed near the CD player (DAC/transport IEC input receptacle). This short adapter has a female IEC at one end, then a multiple ungrounded (two wire) AC receptacle for the caps, then a male IEC connector for the power cord.

In addition, more plug-in AC line to neutral-connected capacitors should be placed in the same or adjoining wall receptacle that the system is plugged into. These capacitors drastically improve the sound by greatly reducing digital timing jitter through their AC line noise reducing effect, and also supplement the CD player/DAC/transport power supply capacitors during the short rectifier diode conduction periods (120 times per second) by acting as supplementary instantaneous current sources derived from their stored electrical charge.

The plug-in filter capacitors connected from neutral to earth ground reduce noise on neutral by bypassing or shunting it to earth ground, and improve the sound of digital even more than the AC line-to-neutral connected capacitors. The neutral to earth ground-connected capacitors must be wired to 3-prong AC plugs. For factory-terminated molded plugs, connection is from the white or blue wire (neutral) to green wire (earthground).

These plug-in filters should be inserted in wall AC receptacles wired to each of the AC lines in the house, except the line the stereo system is plugged into. This is necessary because large neutral to earth ground connected capacitors often cause ground loop hum and buzz if plugged into the audio system AC line. Experimentation is suggested here.

For best results, each of the non-audio system-connected power lines should have at least one capacitive filter on neutral because noise induced on the neutral lines by appliances, digital, and RF devices in the house all sums at the common neutral tie point in the breaker box to be transferred to the audio system-connected lines. As many separate receptacles on each line as possible should have plug-in filters.

Recommended capacitor type and values for a single plug-in unit use in AC line to neutral adaptors:
Four 10 microfarad 400-600 volts DC metallized polypropylene film caps, connected in parallel and bypassed by at least one .47 microfarad and one .01 microfarad 400-600 volts metallized polypropylene cap.

The neutral-to-earthground connected capacitors can be much larger (higher capacitance) and lower voltage rating than the AC caps.

Recommended for each neutral to earth ground plug-in adaptor: one 100-220 microfarad, one 10 microfarad, and one 0.47 microfarad metallized polypropylene film cap, connected in parallel (all caps 100 VDC or greater, no resistor needed).

Similar to when connected directly in the audio signal path, the powerline capacitors have different sounds or sonic signatures when connected one way versus reversed. For the absolutely best results, the capacitor types used should be tested to determine the best sounding orientation or polarity, before wiring into the connectors.

Recommended capacitor brands and sources (inexpensive and good-sounding): Xicon .47 and .01/630V metallized polypropylene film capacitors. Source: Mouser (800) 346-6873, part numbers 1429-6474 (.47), 1429-6103 (.01).

.47-220 microfarad/250 VDC and 400 VDC metallized polypropylene film caps manufactured by Solen. Sources: Handmade electronics (610) 432-5732, Michael Percy 530-470-8650 (www.percyaudio.com), Solen, Inc. (Canada) (450) 656-2759.

A number of other capacitor manufacturers also make as good or even better-sounding units in the same values. Examples are Reliable and Axon. Reliable: (PPMF type) (562) 946-8577; Axon: recommend only the 20 microfarad/1200v: (602) 272-6696.