The note presents a low‑noise microphone preamplifier built around the Analog Devices SSM‑2017 differential input audio IC. The circuit provides programmable gain from 2 to 2000×, excellent common‑mode rejection, and an input‑referred noise below 4 nV/√Hz. Optional phantom‑power circuitry supplies 10‑48 V DC to balanced microphones while protecting the front‑end.
Design details include a dual‑stage architecture with the SSM‑2017 as a programmable gain stage followed by an OP‑275 buffer and DC‑servo. Protection against phantom‑power transients uses back‑to‑back 5.6 V Zener diodes and series resistors, limiting fault currents to about 1 A. Measured THD+N remains under 0.01 % and the output can swing up to 10 V rms into a 600 Ω load.
| Input referred noise | < 4 nV/√Hz |
|---|---|
| Common‑mode rejection | ≥ 90 dB |
| THD+N | < 0.01 % |
| Phantom‑power voltage range | 10–48 V DC |
The input‑referred noise is less than 4 nV/√Hz.