Audio amplifier

An amplifier for connection in an audio communication circuit between a dynamic microphone and an output line having two conductors, one of which is at a reference potential and the other is energized with direct current, comprising:

• (a) a main amplifier channel energized from said other conductor and including first and second stages connected between an output terminal of the microphone and the energized conductor of the output line;
• (b) means coupling the first stage output to the second stage input including two arms of a T-pad;
• (c) a first transistor having three electrodes including a first electrode coupled to the thrid arm of the T-pad and a second electrode connected to the conductor at the reference potential;
• (d) means energized from said other conductor for biasing the third electrode of the transistor to saturation, said transistor being effective when saturated to provide a low impedance connection between the third arm and said reference potential conductor and thereby to hold the main channel in a low gain condition, said biasing means including a resistor connected between said other conductor and the third electrode and a capacitor connected between the third electrode and the reference potential conductor and charged from said output line through said resistor;
• and
• (e) signal integrating means energized from said output line and having an input coupled to the output of said first stage and an output coupled to said third electrode, said signal integrating means including:
• (1) means effective when the signal from the microphone exceeds a predetermined noise level to discharge said capacitor at a rate varying as a function of the integrated excess of the signal over said predetermined noise level, the discharge of said capacitor being effective to shift the transistor gradually away from the saturated condition and thereby gradually to increase the gain of the main channel in response to a voice signal at the microphone;
• (2) a second transistor having a first electrode connected to the third electrode of said first transistor;
• (3) a diode connected between said first electrode and a second electrode of said second transistor and poled oppositely to the polarity of the transistor between said first and second electrodes;
• and
• (4) a resistor connecting the third electrode of the second transistor to said reference potential conductor, effective to limit the current flow through the second transistor in its active mode and thereby to limit the rate of discharge of the capacitor and hence the rate of increase of the gain in said main channel.