This isn’t exactly new. I started this preamp project when I was just getting into digital recording in 2001 and still very active in Ashly Audio but I’m still using these preamps unmodified to this day. I wanted a very clean, neutral preamp with absolutely no unnecessary bells and whistles.
A Simple, Clean Microphone Preamplifier:
Features::
- Very low distortion
- Reasonable noise performance
- Accepts high levels without overload
- Simple enough to package 8 preamps in a 1u rack enclosure
- Reliable and easy to service
- Wide range gain adjustment without complexity
With all this in mind the design took the following form:
A high quality 1:3 Jensen input transformer was used. This provides a bit of impedance matching from the mic to the first amplification stage helping to minimize noise without specialized electronics and it also gives great inherent RF and ground loop immunity,
A dual op-amp provides two amplification stages with the gain control between the stages. A linear taper control increases or reduces the gain of both stages simultaneously giving wide adjustment range and good repeatability for all channels.
A DPDT switch and 4 resistors are used to produce a switchable constant impedance 20dB pad to accommodate very high input levels.
The power supply is external so there’s no AC in the preamp chassis. This way there’s no chance of any hum.
