Most ribbon microphones contain simply a thin aluminum ribbon, a magnet, and a transformer. This elegant design contributes much to what we love about the technology, but there are a few drawbacks: situational vulnerability to phantom power, low output levels, and demanding impedance requirements. Triton Audio’s FetHead is a convenient inline device that does much to address both.
The FetHead is a small Class A JFET buffer that inserts via in-line XLR between mic and preamp, runs on 48V phantom power, provides 27 dB of ultra-clean gain, and presents a high 22 kΩ impedance to the microphone. While similar products exist on the market, the FetHead is distinguished by a small form factor, similar to a barrel adapter or inline pad (it essentially disappears into the cable itself).
One drawback to both of The Bunker Studio’s vintage Neve consoles is the global phantom power – it’s either “on” or “off” for all channels at once. Although the risk of applying phantom to ribbon mics is generally overstated, one condition that is quite hazardous is hot-patching via a TT or longframe patch bay (this can briefly introduce full voltage to the ribbon, causing damage as current flows). Since we use plenty of ribbons and do a lot of cross-patching, we always eschewed global phantom, favoring local 48V supplies as needed. This process brought its own disadvantages.
More recently, we acquired enough FetHeads to keep them installed on the captive cables of most of our ribbon mics. Since they block DC from reaching the mic, we’re able to leave the global phantom on. More importantly, the sound of the ribbons has, without exception, been improved by the higher input impedance. Bandwidth is extended, transients are more open, and gain-staging for lowest noise is much more easily-optimized.
Why is impedance so important? To preserve every drop of precious signal voltage, the load presented to any mic should be at least five times its source impedance. Ribbon mics exhibit rising impedance at low frequencies, so even if they show a nominal 250 Ω at 1 kHz, they might present as much as 2 kΩ below 100 Hz! While this might make a typical preamp sweat a bit too much, the FetHead takes this in stride. Moving-coil dynamic mics (such as the Shure SM57) exhibit similar behavior, though to a lesser degree. Still, the optimized impedance condition can extend the bandwidth of these mics as well, as was evident when I experimented with a 57 and a Sennheiser MD 421. For under $100 apiece, it’s hard to beat the functional and sonic advantages the FetHead has brought to our studio.