I recently read your article answering the question: “Why do my keyboards sound thin and weedy with some preamps but not others?” and I’m having a similar issue — but with microphones. I purchased some mics from 12 Gauge Microphones and can only use them on my non‑transformer input mic preamps. These small‑diaphragm electret condenser microphones require standard phantom power, but they have unbalanced outputs. After some experimentation I found that the mics work as expected with my FMR Really Nice Preamps, as well as my old Mackie 1604 VLZ board, so I’m 90‑percent certain it’s something with the input transformer of the preamp.
Do you know of a way to make these mics work with my transformer input preamps? Is there some sort of mic cable modification that would enable them to work properly?
Anon. via email
SOS Technical Editor Hugh Robjohns replies: You’re correct that the problem you’re having is very similar to that previous keyboard issue — the circuit path to and from the mic is incomplete when using transformer‑input preamps, and that’s because the original 12 Gauge Microphones use a very non‑standard approach in their audio interfacing.
The core problem is that these microphones are wired with an unbalanced output presented on pin 2 of the XLR (to feed the preamp’s ‘hot’ input), but whereas conventional balanced inputs expect the signal reference voltage to be on the cold input (pin 3), the 12 Gauge mics actually use pin 1 (the chassis ground). Moreover, the cold side (pin 3) carries no audio at all and is used only to derive a power source for the electret capsule’s internal impedance‑converter electronics. The power regulation circuitry in the mic involves a very high impedance (over 10kΩ) between pin 3 (the preamp’s reference signal input), and pin 1 (the actual signal reference). And therein lies the problem: there isn’t a viable audio circuit path between pins 2 and 3 for the signal current to flow, so no signal current can pass through the transformer.
In contrast, the nature of an electronically balanced input is that, although it is intended to process the signal voltage between its hot and cold inputs, the amplifier circuitry is inherently ground‑referenced, so despite there being nothing useful on the cold input (pin 3), the voltage appearing on the hot input (pin 2) inherently shares the same ground reference as the amplifier itself. Therefore the preamp can still deliver a usable output.
Sadly, there is nothing that can be done with the cabling to fix the problem for transformer‑input preamps. The only practical solution is to employ a more complicated powering arrangement inside the mic itself. While researching your query, I have been in dialogue with Brad at 12 Gauge Microphones to help work out a suitable update for future products. In the meantime, I’m afraid you’ll only be able to use these mics with electronically balanced preamps!