Complementary symmetry amplifier with field-effect transistor driver

Claim 2

A complementary symmetry amplifier comprising: a pair of transistors of opposite conductivity type; a loudspeaker drivingly connected to the outputs of said transistors; a bias resistor interconnecting the inputs of said transistors; and signal driving means maintaining substantially constant DC current through said bias resistor; said signal driving means including, an FET having its output connected in series with said bias resistor and its input coupled to a source of audio signal, and means establishing the operating point for said FET at a point of substantially zero slope on its output current/source voltage characteristic curve.

Claim 3

The amplifier of claim 1 further comprising a volume control coupled between the input of said FET and said source of audio signal.

Claim 4

The amplifier of claim 2 wherein said last-mentioned means includes a variable resistance and signal bypass capacitor.

Claim 5

The amplifier of claim 3 wherein said output includes the drain and source terminals of said FET and said input includes the gate and source terminals.

Claim 6

A low-cost phonograph comprising: a motor-driven turntable adapted to rotatably support a phonograph record; a pickup producing an electrical signal from said record; a power winding on said motor; rectifying means producing a low voltage source of DC power from said power winding; a loudspeaker; a pair of transistors of opposite conductivity type connected in complementary symmetry fashion for driving said loudspeaker; a bias resistor connected across the inputs of said transistors; an FET including an output circuit and an input circuit; means serially connecting said output circuit, said bias resistor and said DC source; means coupling said electrical signal to said input circuit; and means for adjusting the output circuit quiescent current flow through said FET for properly biasing said transistors.

Claim 7

A low-cost phonograph comprising: a motor-driven turntable adapted to rotatably support a phonograph record; a pickup producing an electrical signal from said record; a power winding on said motor; rectifying means producing a low voltage source of DC power from said power winding; a loudspeaker; a pair of transistors of opposite conductivity type connected in complementary symmetry fashion for driving said loudspeaker; a bias resistor connected across the inputs of said transistors; an FET including an output circuit and an input circuit; means serially connecting said output circuit, said bias resistor and said DC source; means coupling said electrical signal to said input circuit; and means for adjusting the output circuit quiescent current flow through said FET for properly biasing said transistors.

Claim 8

The phonograph of claim 5 wherein said last-mentioned means comprises a variable resistance and a signal bypass capacitor.

Claim 9

The phonograph of claim 5 wherein said FET has drain, source and gate terminals and wherein said drain and source terminals comprise said output circuit said gate and source terminals comprise said input circuit.