Recording and Voice Processing, Volume 1

Appendix 4Tube and JFET Microphone Amplifiers

A4.1. Introduction

In an active microphone, for which a power supply is needed for the electronics to function, vacuum tubes or JFETs (Junction Field-Effect Transistors) are often used. The purpose of this appendix is to introduce them to you.

A4.2. Operating principle of tube microphones

Generally speaking, condenser microphones use vacuum tubes. They are there to ensure an impedance conversion.

Figure A4.1. A tube microphone, Neumann U47. The tube can be seen in the center, above the transformer. For a color version of this figure, see www.iste.co.uk/reveillac/recording.zip

By default, the transducers of condenser microphones provide an audio signal with a very high impedance, which has the major disadvantage of significant degradation during its transport through wires, connectors and even very short cables.

Engineers therefore had the idea of placing triode-type electronic tube behind the transducer to reduce the impedance of the signal.

Figure A4.2. Principle of a triode and a Telefunken VF14¹ tube designed specifically for microphones

In a triode tube, there is a filament, a cathode, an anode and a gate. The filament is powered and heats the cathode, which emits negatively charged electrons. These electrons are naturally ...