etheory Microphone pre-amp custom project – part 1

Since I needed to gather some more funds to finish the API Compressor project, I’ve decided to go into customized audio design for friends and also a studio I’ve just teamed up with.

The idea is that I take a bunch of information into account, then design (or more commonly re-purpose existing designs to save time), test, produce, improve, and finalize pieces of one-off bespoke customized gear for clients.

My first project is a set of 2 microphone pre-amplifiers for a private home-studio customer.  Since this client is after a warm old sound, I’ve opted to build a transformer in-out balanced 2-stage design, with between 34 and 80 dB of gain in microphone mode, and at line input mode between 16 and 60dB of gain.  I’ve very excited about the design I’ve come up with, which includes phantom power, 2-stages of balanced gain and seriously high-specification components throughout.

The gain stages are discrete, and are based on a design from member Tamas.  The design can be referenced here:

Everyone who has build that circuit comments on it’s sound quality and it’s power and high-current and voltage handling capacity and ability to resolve detail, so it seemed like the perfect fit.  My extended mic-preamp specification also includes Sprague-Vishay coupling capacitors and ClassicAPI 2622 input and 2623-4 output transformers.  It’ll be an absolute corker of a preamp to be sure.

Simulated frequency response is about 10Hz to 100kHz to within 0.5dB of a perfectly straight-line.

I’ve already started to build the 4 discrete operational amplifiers that are required (DOA’s), and wanted to share some photos of them here:









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