On some smaller mixers, such as Mackie’s 802-VLZ3, phantom power is switchable only globally or in banks. Thankfully, moving-coil dynamics and even most ribbon mics won’t even ‘realise’ phantom power is present!
I have a question about Shure SM57 and SM58 mics and phantom power. My Mackie 802‑VLZ3 mixer’s phantom switch enables phantom power for mic inputs 1‑3 — it can’t be switched individually for each input. If I used my Rode NT1 [a capacitor microphone that requires phantom power to operate] on channel 1 and, say, an SM57 [a moving‑coil dynamic mic that doesn’t require phantom power] on channel 2, would the SM57 be affected by the phantom power it would have available to it? I have read the manual, and apparently a ribbon mic would be upset!
SOS Forum Post
SOS Technical Editor Hugh Robjohns replies: It’s not an issue. The two ends of the moving‑coil element in the SM57/58 are wired to XLR pins 2 and 3, and the body is wired to pin 1 (screen). Phantom power puts 48V on both pins 2 and 3, returning to pin 1. So as far as the moving‑coil mic is concerned, it’s got no voltage difference across the coil (because there’s 48V at both ends) and as there’s no connection between the coil and pin 1, no phantom current can flow. Essentially, the SM57/58 has no idea phantom is present at all.
...the tie‑lines between studio floor and control rooms at the old BBC Television Centre had 48V permanently enabled...
It would be a pretty poor ribbon mic if it were so easily ‘upset’ but there are some (mostly very ancient) ribbons that genuinely don’t like phantom power. It’s not something I’ve ever worried about, though, and I happily use Coles and AEA ribbons with phantom on the lines. In fact, the tie‑lines between studio floor and control rooms at the old BBC Television Centre had 48V permanently enabled on the floor boxes, to allow mic patching on a standard (PO316 B‑gauge) patchbay without having phantom across the patchbay’s sockets. It also meant that all mic plugging/unplugging on the studio floor was ‘hot plugging’ with phantom permanently on the lines.