The majority of studio hum is due to ground loops, where multiple ground paths permit low levels of mains-frequency current to circulate around the system. Disconnecting one end of a cable screen can work in some balanced systems, but it isn't a complete solution and shouldn't be applied to unbalanced systems. In fact, the most reliable way of preventing ground loops is to use an audio transformer to isolate the two problematic pieces of equipment from each other, and that's exactly the approach used in the Ebtech devices under review here.
Available in both two- and eight-channel (rackmount) versions, the Hum Eliminator provides full electrical isolation between the inputs and outputs and also provides a simple and effective means of converting between balanced and unbalanced signals. Because the circuitry uses only transformers, which are passive devices, no power is needed. Both the ins and outs are on balanced jacks, and balanced-to-unbalanced conversion, in either direction, is achieved simply by plugging in an unbalanced jack cable connected to the unbalanced equipment.
The high-quality transformers used in these boxes have a flat frequency response from 20Hz to 70kHz within plus or minus half a decibel, their distortion is very low at 0.005 percent (at 1kHz), and crosstalk between channels is better than 97dB. By way of impedance matching, the input can handle signals with an output impedance of 1kΩ or less, while the output will happily feed any line input with an impedance of 10kΩ or greater. This suits the vast majority of line-level signals and, because the transformer's headroom is limited only by magnetic saturation, all the variants of line level can be handled effortlessly.
In those instances where users also wish to convert signal levels of -10dBV to +4dBu or vice versa, a further option is available in the form of the Line Level Shifter, again available in two-channel or eight-channel formats. These are physically identical to the Hum Eliminators, but use level-matching transformers that can convert in either direction between the two common studio levels. Of course this approach still sorts out ground loop problems at the same time, and line level conversion may be useful where you need to connect pro level sources into consumer mixers and soundcards that operate at -10dBV.
The only test you need to do on these units is to see if the input and output signals sound identical, and to my ears they do. Good transformers aren't cheap, so this isn't exactly a budget solution, but the UK price is still attractive enough when you consider the havoc that hum can wreak in an otherwise well-designed system.
Hum Eliminator: HE2 two-channel £79; HE8 eight-channel £295.
Line Level Shifter: LLS2 two-channel £89; LLS8 eight-channel £345. Prices include VAT.