While mics such as the Rode NT3 can be run off batteries when needed, it’s best to opt for phantom power wherever possible.
My Rode NT3 mic was producing a nasty medium‑frequency buzz, and I eventually discovered that it was caused by a cheap battery which I’d installed in the NT3 a month ago. Taking the battery out and switching on my audio interface’s phantom power solved the problem. I’m surprised that powering the mic from a battery caused buzzing, rather than the mic simply ceasing to work when the battery died. I’d also noticed more sibilance and, indeed, that I seem to have developed a slight lisp in recent recordings! Could this also be due to a battery on its last legs?
SOS Forum post
SOS Technical Editor Hugh Robjohns replies: In the digital world, we’ve become accustomed to devices which work perfectly and then just stop as the battery dies. But as the impedance converter electronics in a microphone work with analogue signals, the performance tends to degrade in myriad ways as the supply voltage decreases. Depending on the complexity of the circuitry and its design, some parts of the signal path may become unstable as the power‑rail voltage decays, and start self‑oscillating — this might explain the medium‑frequency buzzing. The circuitry’s bandwidth may also change, as might the noise floor and distortion characteristics, possibly explaining the sibilance effects.
The point is that the way in which the technical performance degrades with reducing power‑supply voltage, such as with a dying battery, is complicated and largely unpredictable. Still, it’s easy to find out if such issues are related to a failing battery by installing a new one. And the fact that these problems went away when you switched to phantom power indicates pretty conclusively that you had a low‑voltage issue when operating on battery‑power.
I don’t like leaving batteries in microphones. They tend to get forgotten about, the slowly degrading performance might go unnoticed, and serious damage can be done if the battery leaks. So, firstly, always take the battery out when it’s not in use, and secondly if the mic can be run on phantom power, choose that option where possible. That avoids any risk of battery leakage and decaying supply‑voltage issues, and the higher voltage will often mean better technical performance too, such as more headroom, lower noise and less distortion.