Oktava mod FET biasing

Here's one way to bias the FET on an Oktava microphone....

Our good friends at Valley Wood Studios in Leeds had followed one of the popular projects for upgrading Oktava MK319 microphones, which involves changing a handful of resistors and caps, along with the field effect transistor (FET). After the mods one of the mics was about 3dB louder than the other.

This kind of thing is normally due to variations in the FET's characteristics, and to optimise the modification, it is necessary to adjust one or both of the bias resistors (R7 and R8 in the PDF document). Between them these set the gain of the mic, and the bias point for the FET, which gives the best noise performance.

I often do this by hacking a decade box into the circuit, and in this case we put the mic in our test chamber so that we can tweak and scan as we go along. At the end of the tweaking the pair of mics were well matched, and noise free.

We celebrated with tea and biscuits!