Quiet turn-on circuit for carbon microphones

Claim 2

A preamplifier for a carbon microphone having an input circuit with a transistor device the conductivity of which is responsive to a bias voltage across said input circuit, circuit means including a microphone switch for connecting said input circuit to said microphone, a voltage divider circuit including a resistor across said input circuit across which a bias voltage is developed when said microphone switch is in ''''off'''' position causing an attenuation of the gain of said preamplifier, said circuit means having a low resistance causing a substantial decrease in said bias and restoration of said gain when said microphone switch is thrown to on position, and a delay circuit associated with said bias circuit to prevent an instantaneous change in the gain of said amplifier arising from said decrease of said bias when said switch is turned on whereby the surge of current to the microphone when the switch is turned on is not amplified.

Claim 3

The preamplifier set forth in claim 1 wherein said preamplifier has an input field effect transistor operating as a switch which in the absence of an input bias voltage has a low internal impedance and which with a preset input bias voltage has a high impedance simulating an open switch condition.

Claim 4

The amplifier set forth in claim 2 wherein said delay circuit comprises an RC parallel circuit connected between the gate of said transistor and ground, and a rectifier connecting said bias resistor to said gate for causing the bias resistor to charge said RC circuit when said switch is in off position and said RC circuit to maintain mOmentarily the bias on said gate after collapse of the bias voltage across said resistor when said switch is turned on.

Claim 5

A preamplifier for a carbon microphone having a field effect input transistor, a switch for connecting said microphone to the input of said amplifier, an isolating condenser connecting said input to the source terminal of said transistor, a voltage divider including a resistor connected across the input of said amplifier and ground, a rectifier connecting said resistor to the gate of said transistor for providing a bias on said gate to render the transistor normally non-conductive, said microphone circuit via said switch having a low resistance effectively shunting said resistor and reducing the bias across said resistor to render said transistor conducting when said switch is closed, and a parallel RC circuit connected between said gate and ground normally charged to the bias voltage across said resistor via said rectifier when said switch is off, said rectifier preventing discharge of said RC circuit through said resistor when the bias voltage across said resistor is reduced by the turning of said switch to on position whereby the RC circuit maintains the bias on said gate for the time duration of discharge of said RC circuit.

Claim 6

A preamplifier for a carbon microphone having a field effect input transistor, a switch for connecting said microphone to the input of said amplifier, an isolating condenser connecting said input to the source terminal of said transistor, a voltage divider including a resistor connected across the input of said amplifier and ground, a rectifier connecting said resistor to the gate of said transistor for providing a bias on said gate to render the transistor normally non-conductive, said microphone circuit via said switch having a low resistance effectively shunting said resistor and reducing the bias across said resistor to render said transistor conducting when said switch is closed, and a parallel RC circuit connected between said gate and ground normally charged to the bias voltage across said resistor via said rectifier when said switch is off, said rectifier preventing discharge of said RC circuit through said resistor when the bias voltage across said resistor is reduced by the turning of said switch to on position whereby the RC circuit maintains the bias on said gate for the time duration of discharge of said RC circuit.