Normally, my system has no hum issues. But lately, the “component count” has increased to the point where different components are plugged into different outlets in my listening room. Most of the time, this isn’t a problem, but occasionally, one of my two mono block amplifiers develops a hum. The hum is moderately loud and is very distracting while listening.

Hum can be caused by a variety of issues, but the most common cause is a difference in ground resistance. Modern components use a “three prong” plug where the two flat blades are the current supplies, and the rounded third plug is an independent ground. Since different electrical outlets are different distances from the circuit breaker, and since all wire has some resistance, each socket will have a slightly different ground resistance.

Little Things: The Trouble with Hum

Little Things: The Trouble with Hum

Many components are designed for hum resistance, and won’t hum at all despite being connected to different electrical sockets, but others can (and do) hum, either occasionally or continuously. If the hum is of small amplitude, you won’t notice it unless you put your ears against the woofers of your main speakers or against your subwoofers. If the hum is of larger amplitude, it is noticeable when there is no music signal, or even in quieter parts of the music.

Little Things: The Trouble with Hum

Little Things: The Trouble with Hum

The ways to avoid hum are simple – connecting every component to the same electrical outlet usually works. This idea, called “star grounding” is normally sufficient to prevent hum. But for persistent hum problems, there is another solution. The Emotiva CMX-2 (with two outlet sockets) and CMX-6 (with six) are designed to filter the AC power, eliminate high-frequency noise, and to prevent ground loops due to DC offset.

How do they work? I’m glad you asked! First, there are filters that remove noise in both common mode (the noise is on both AC current supplies) and in differential mode (the noise is on only one of the AC current supplies).

Second, there are line status indicator lights that show whether the outlet is wired properly. If the socket is wired backwards or any of the three wires are not connected at all, the lights notify you.

And finally, the devices are designed for high (15 ampere) current, unlike many inexpensive power strips where the circuit breakers in the strips disconnect if any high current device (like a power amplifier) is started cold.

The sockets are high quality and the aluminum case can be mounted (with supplied screws) in any position. The 14-ga. supply cable can pass 15 continuous amperes without heating.

Little Things: The Trouble with Hum

In addition to serving as a high-quality power strip, the CMX offers additional benefits. An inductor/capacitor network (or “LC network”) is used to filter all frequencies above 120 Hz. Having the capacitors in the two active AC lines also removes any direct current (DC) from the system. By removing DC offset, the transformers of all components fed by the CMX strips avoid saturation. Further, the series capacitance serves to smooth the AC waveform.

The CMX products also serve as high quality surge-suppressors. The included metal oxide varistors (MOVs) dump all voltage spikes of equal to or greater than 400 volts. The MOVs are very robust, but should they ever fail, the signal status lights on the CMX will all go out. Despite the high volume of CMX products sold, the Emotiva company reports that only two MOVs have ever failed, probably due to direct lightning strikes. So if one is using a CMX, no additional surge suppression is required.

So my experience is that the CMX products work admirably, and for the quality of construction, are bargains. I no longer have hum issues with my equipment, and consider the investment in a CMX to have been instrumental in achieving that goal. One can buy less expensive, more expensive, or even FAR more expensive power conditioners and surge suppressors, but in my opinion, the CMX series from Emotiva hits the “sweet spot” where your money buys real world improvements without frivolous expense.

The Emotiva CMX-2 list price (for the two-outlet strip) is $99. The six-outlet strip is $119. Shipping is free. For additional information, visit