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 www.emotiva.com.

  • Faraday Defcon

    As an electrician, I cant respond to all the incorrect bullcrap in this article. There’s just too much of it.

  • Boomzilla

    Hi Farady Defcon –
    I don’t claim to be an electrician, and it is possible that I’ve made technical errors. I tried to research the comments prior to posting them, but it’s possible that I got some wrong. I’d appreciate any feedback that you might be able to provide.
    Cordially – Boomzilla

  • Boomzilla

    Hi Farady Defcon –
    I don’t claim to be an electrician, and it is possible that I’ve made technical errors. I tried to research the comments prior to posting them, but it’s possible that I got some wrong. I’d appreciate any feedback that you might be able to provide.
    Cordially – Boomzilla

  • Boomzilla

    Hi again – Farady Defcon – Based on your comments, I went back and reviewed the claims of the article. Here are my findings:

    Let’s look at that review, claim by claim:

    1. “The most common cause of hum is a difference in ground resistance.” – TRUE – The most common sources of hum are magnetic fields, induction, and shared impedance. Since most audio components are designed to prevent magnetic field & induction hum, the most common cause (specifically for audio equipment) IS differing impedances.
    SOURCE: Wikipedia – “Mains Hum” – “The other major source of hum in audio equipment is shared impedances; when a heavy current is flowing through a conductor (a ground trace) that a small-signal device is also connected to. All practical conductors will have a finite, if small, resistance, and the small resistance present means that devices using different points on the conductor as a ground reference will be at slightly different potentials. This hum is usually at the
    second harmonic of the power line frequency (100 Hz or 120 Hz), since the heavy ground currents are from AC to DC converters that rectify the mains waveform.”

    2. “…connecting every component to the same electrical outlet usually works. This idea, called “star grounding” is normally sufficient to prevent hum” – DEBATABLE – This is true if every component is grounded internally to only the ground of the power cord. Now despite every component being grounded to the electrical outlet (the common ground), if interconnects also ground one component to another, then a “ground loop” is possible.
    SOURCE: analog.com – “The “star” ground philosophy builds on the theory that all voltages in a circuit are referred to a single ground point, known as the star ground point. This can be better
    understood by a visual analogy—the multiple conductors extending radially from the common schematic ground to resemble a star. The star point need not look like a star—it may be a point on a ground plane—but the key feature of the star ground system is that all voltages are measured with respect to a particular point in the ground network, not just to an undefined “ground” (wherever one can clip a probe).”

    3. “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).” – NO – Common mode is on the voltage supply & return. Differential mode is between the power and
    ground.
    SOURCE: murata.com – “One type is common mode noise that is conducted on all lines in the same direction. With an AC power supply line, for example, noise is conducted on both lines in the same direction. The other type is differential mode noise that is conducted on the signal (VCC) line and GND line in the opposite direction to each other.

    4. “The 14-ga. supply cable can pass 15 continuous amperes without heating.” – TRUE – Although ALL conductors will heat when passing current, the 14 ga. stranded power supply cable will not heat enough to damage its insulation or to significantly change its resistance at 15 amperes of current flow.
    SOURCE: homeguides.com – “The minimum wire size for those 15-amp circuits is 14-gauge.”

    5. “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.” – TRUE –
    Series capacitors DO block DC current (electronics 101). And both capacitance and inductance tend to smooth and filter AC power.

    6. “The included metal oxide varistors (MOVs) dump all voltage spikes of equal to or greater than 400 volts.” – TRUE – This is what MOVs are designed to do.

    SUMMARY: So although the common vs. differential mode noise statement was misleading, I can hardly see where the article had “all the incorrect bullcrap” as you claim. I stand by my review.

  • I have had hum sneak into audio recordings. One thing I have begun to hate is switching power supplies in the lab.

    My setup is all hobby class; iMac for edits and such. Altec Lansing powered speakers, audio-technia AT-PL50 turntable, and assorted DVD players with CRT monitors. Also, USB mic with line-in or mic in.

    I parked the old Heathkit amplifier under a box so it’s not that bad here. I tried plugging everything into my 19″ rack PDP-11 30A supplies but hum sneaks in.

    Wondering if this product will fix my headache. Thanks.

  • Boomzilla

    Hi duddits-fairuse –

    The CMX products should help your hum. Since Emotiva offers a free trial period with refund if it doesn’t work, I’d say try it & see.

  • Thanks for the clue. Not sure when I will jump on the hum problem as I’m still reading up.

  • Allan Marcus

    Only the CMX-2 does the D.C. Offset protection. The CMX-6 doesn’t have that feature.