Review of the Ebtech Hum X accessory (for linguists)
Ebtech is a US company specializing in problem-solving accessories for various kinds of audio applications. Hum X (Figure 1) filters out unwanted voltage on the ground line, which is a common cause of ground loop hum. The product has been on the market for seven years and Ebtech claims that "it is absolutely 100% effective on ground related noise." Simply place the Hum X on the end of the power cord of the equipment that has a ground loop and plug the Hum X into the wall outlet.
Figure 1. The Ebtech Hum X
I tried the Hum X with a laptop computer, plugged into one of my notoriously noisy (ground-related, hum) A/C outlet. Without the Hum X, the recording has the hum, as expected. With the Hum X plugged in, the hum is gone. As simple as that.
Figure 2 illustrates the effectiveness of the Hum X. The spectrum of the noise print with the Hum X plugged in has only a very faint trace of the hum, while the spectrum in the right panel shows ground-related hum in all its glory. I highly recommend getting the Hum X and keeping it in your gear bag at all times. Even if you do not have a ground-related hum problem now, you are likely to run into it at one point or another. Trust me, it has happened to me far too many times. (Download the audio file)
Figure 2. A comparison of noise spectra with Hum X applied (left panel) and without (right panel)
Note that hum can originate from sources other than ground loops (see this article). Ebtech is very clear and honest about the fact that there may be situations in which the Hum X is simply not the right tool for the job: "You have applied it to the wrong pieces of gear, you may need a Hum X on multiple pieces of gear in the set up, or you have a different type of noise (usually cable or connection related)"
I would like to thank Ebtech for providing the Hum X for the purposes of this review.