Diretta Iniezione

Abstract

I was in need to get a dynamic microphone preamplifier, but I was not satisfied by the ones I could find. So I decided to make one, using a FET input and this design proved good.

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Nothing to do with engines and injecting fuel without using a carburetor. It is an audio preamplifier for dynamic microphones. Performs a FET input stage with high linearity and large bandwidth. XLR input with gain control and mini-jack 3.5mm line output.

DI

Characteristics

  • FET input
  • input: XLR Cannon connector.
  • output: jack TRS 3.5 mm.

Found in need to record sounds with a common Shure SM58 onto a MacBookPro, I decided to build my own little preamplifier so I could plug it to the Mac’s line input jack and have a decent timbre and a low noise figure by choosing a voltage controlled amplifier using Fets (field effect transistors) well known for their tube-like sonority and their advantages over a current controller amp.

Input stage

This little AC powered preamplifiers has an input stage configuration with a single FET transistor, very linear for its simplicity giving the warm tone due to the voltage controlled feedback just like a vacuum tube.

Was easy to google up some schematics to start off with. Actually I cannot find my own schematic nor more notes for this prototype model since I gave it as a present some time ago.

By the way I started off with this blog post because I was interested in having control over the input impedance (low and high). The basic schematic, therefore, was this one below:

valve

I guess I had a little stash of 2SK30 or 2N5457 to choose from, anyway I reckon you can obtain an acceptable sound quality with almost any JFET.

cascode wave

Cascode configuration

After the first prototype and started working on a second one with a cascode configuration:

Cascode

The output of a 440Hz sine wave captured by the Shure M58 at maximum gain is showed below. It is just a dummy test image: I did not use any calibration between reference levels and, let’s say SPL, as the audio source was the Mac’s speaker itself. But the wave trace looks smooth and sounds even better!

Cascode wave

Audio Sample

If you want to listen to a recording made with Diretta Iniezione here is a promotional video: