A condenser microphone preamplifier is described in which an oscillator controlled by voltage-dependent capacitors (V.D.C.) is used. The V.D.C. control has an amplitude-stabilizing as well as an amplitude-modulating function. The stabilizing action maintains the right operating point for the oscillator. The modulating function establishes a modulation of oscillator amplitude with audio input signal obtained from the condenser microphone by conventional audio-frequency circuitry. Thus, in contrast to high-frequency biasing, in this circuit the condenser microphone does not form part of any tuned circuit; this guarantees easy interchangeability. An analysis of gain and noise is given and some design aspects are considered. Noise measurements have shown that the circuit is equivalent to audio-frequency circuits with tubes. The small dimensions of the elements and the low power consumption make it possible to build the whole circuit, together with the battery, in a small enclosure.
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