Some ground-loop problems are very difficult to cure, and sometimes the age-old solution of using an isolating transformer to ensure total electrical separation between the source and destination is the simplest and most effective solution.
The ART Cleanbox II is a simple and affordable stereo isolation box containing two isolation transformers designed for line-level use, with quarter-inch jacks in and out. Both the inputs and outputs are unbalanced, but are wired to accommodate balanced or unbalanced connections on TRS jacks. Because transformers are passive devices, and because no other active circuitry is needed, there is no need for batteries or a power supply, but there is a small insertion loss of three or four decibels in typical applications.
I had the perfect opportunity to test this device in my own studio setup, where I use a small analogue submixer to combine the outputs from my various synths before feeding them to a spare stereo input on my audio interface. No matter what I did, there was always more hum on the mixer output than I felt there should be, even when using balanced leads. I suspected that metal-to-metal contact via the rack was causing multiple ground paths, but putting the ART Cleanbox II between the mixer and audio interface produced the best result I've had to date. There's still some hum from the modules themselves, but at least this goes away when the mixer gain channels are muted, suggesting that any remaining hum originates from within the modules themselves. Prior to fitting the ART Cleanbox II, there was always a noticeable amount of hum at the mixer output, even with the channels muted or the inputs unplugged.
After fitting the device, the level drop was inconsequential and there was no subjective difference in the sound quality of my synths, not even at the low end where the effects of low-cost transformers are often most evident. These claim to work from 10Hz to 50kHz (-5dB) and the quoted distortion is low, even at fairly hot signal levels. There are many studio applications for this little isolator, and though you should only resort to adding gadgets like this when all conventional approaches have failed, I have to say that this unit does the trick very nicely and for very little money in the UK. And of course I'm going to have to buy this one because I don't want my hum back!
£34.99 including VAT.
Sonic8 +44 (0)8701 657456.
+44 (0)8701 657458.