Thursday, July 16, 2015

Discrete solid-state microphone preamp with low fixed gain

Proposed schematic
When I removed the microphone input from the 10LS/1LB Preamp, I was left with a spare Edcor MX8cs transformer.

When Radio Shack closed its retail stores and got out of the DIY component buisiness, I picked up a bunch of assorted "jellybean" transistors, as well as some 1N4733 5.1-volt zener diodes (along with a ton of other things).

All these solutions were in need of a problem, and I found it with my occasional need to record a drum kit on an audio interface with only two microphone preamps (The now-discontinued Echo AudioFire 4).

Even with a low-sensitivity microphone (like a SM57), a loud source (like a snare) will do just fine with the 18 dB of voltage gain inherent in the transformer.  All that's needed is to bring the impedance down (after it is increased by a factor of (8.2)² in the transformer) and drive a balanced line.

Here's the thought:

  • Drive the line from a two emitter followers using Sziklai pairs.  Biasing is through dc coupling to the transformer.
  • Use the 5.1V zener to derive a virtual ground point that's one VBE drop away from half of a 9V battery.
  • Input impedance can be selected with resistor R10.
More to come when I get around to building it...

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