In my time I've tried quite a few power filters including 1:1 Transformers, ferrites, chokes, commercial capacitor networks, DIY resistor-capacitor networks, Bybee filters and numerous commercial filtration strips. From all these trials a clear pattern has emerged - active, current limiting filtration such as generic surge suppressor strips is almost always detrimental to the sound of audio equipment. 1:1 Transformers are great for front ends like CD players and digital equipment, but terrible for pre-amps and amps. RF suppressors like ferrite clamps on AC power are always detrimental to the sound of audio devices, even when placed on only the earth wire. However they do have a place on video equipment and on non-AV equipment on located on different circuits to the audio system. Usually the very mechanism used for filtering the power has an audible side effect which far outweighs any filtering effect.
Devices that I have found which are generally positive for audio systems are 'passive' filters. These don't suppress current surges, but rather use capacitor networks whose most basic function is to shunt high frequency noise away from the active lead of the AC power supply to stop it from entering your equipment. This high frequency noise, or Radio Frequency Interference (RFI) is becoming worse in its prevalence due to computer use, wireless networks and radio communications. Even worse is the prevalence of radio frequency noisy 'switching power supplies' found in almost all household appliances. All of these sources radiate RFI directly though the power line, or through the air and are picked up by power cables just like antennae. Some components are not noticeably affected by this noise, but others, such as digital audio players, preamps, phono preamps etc can be quite profoundly affected and then amplified through the audio reproduction chain. The audible results are not usually directly noticeable as white noise or gross signal breakup, but fatiguing, edgy treble or an unnatural murkiness to the sound. Often you can't consciously hear the background noise, until it is taken away, making backgrounds sound 'blacker' and the audible signal is 'brought into focus' and made clearer.
After trying my own hand at a couple of DIY passive filters, I thought I would try some of the Alan Maher Designs "Power Enhancer" filters. I liked what I heard, which was a cleaner purer sound where I could hear more resolution in the midrange and higher frequencies. It was clearly better than my DIY attempts and other commercial passive filters I have tried. Since then I have tried almost the entire range of "Power Enhancer" filters, and now the subject of this review, the "Reference II" filter.
Soundwise the Reference II seems to combine the sound qualities of the Power Enhancer I, III and IV and roll them up in a single general product - highs and midrange are cleaner and bass is more defined. It seems as though I can 'see into' and enjoy the midrange more. The Reference II also seems more neutral tonally compared to other Power Enhancers. As with all of these passive filters the Reference II seems to interact with different components in different ways, and synergizes with some components more than others. So trial and error is highly recommended which for which component it works with best and where on your audio circuit find its most effective location. In my system the Reference IIs have settled into the adjoining socket to each of the duplexes feeding each of my Channel Islands monoblock power amplifiers. I cycled through this configuration without any filter and also with other filters in their place. The result was that the Reference IIs always sounded better than anything else in this position.
Some other information about the Reference II filters. In my experience the effect the filter has on the sound does not stabilize until they are 'burned-in' which takes about 5 weeks. So don't judge them until they have been in circuit for a while. These filters come packaged in 'wall-wart' style cases with a USA plug without a ground pin. The way they plug in is very important, in fact integral to the way they function, and must be positioned the correct way around, so that the active and neutral prong goes into the correct hole in the socket. This usually means that the hanging end of the case must face ground, or be oriented towards the where the ground pin in the socket is. They should not be plugged in 'upside down'. International users must use a reasonable quality plug adapter which preserves absolute polarity. Note that European and Australian plugs swap active and neutral positions so if there is a one to one correspondence in your adapter plug then the Reference II will need to be plugged into the adapter upside down compared to a USA socket, in order to preserve the AC polarity. Since the filter is a 'wall-wart' with quite a wide case, users may also find that this becomes cramped or physically impossible to plug it into some power boards or double duplex sockets. In this case a short IEC power cable ('jug-plug') can be used with a PC burn in adapter to plug the filter into tight spots. Also note that the Reference II uses X-rated capacitors which are safety compliant for 110-240V applications.
In conclusion, my opinion is that the Reference II is the best low cost passive filter product on the market. It is the culmination of many iterations of passive filter design based on empirical testing of how the unit sounds and not just how it measures. I recommend it as great standalone filter or for use in concert with other power treatment devices.
