Line output circuits

Claim 2

A hum loop cancelling output circuit having a pair of nonearthy and earthy input terminals, and a pair of non-earthy and earthy output terminals and including a line amplifier having a pair of non-earthy and earthy input terminals and a pair of nonearthy and earthy output terminals, a combining means having two inputs and an output connected to the non-earthy input terminal of the line amplifier, the non-earthy input terminal of the circuit being coupled to one input of the combining means, the non-earthy output terminal of the amplifier being coupled to the non-earthy output terminal of the circuit, and the earthy output terminal of the circuit and any hum loop voltages from the earthy side of the line being coupled to the second input of the combining means to establish a positive loop of substantially unity gain from the earthy output terminal of the circuit to the non-earthy output terminal of the circuit.

Claim 3

A line output circuit according to claim 1, wherein the earthy output terminal of the circuit is connected to ground through a resistor.

Claim 4

A line output circuit according to claim 2, for use with audio equipment, wherein the said resistor has a value of the order of 100 ohms.

Claim 5

A line output circuit according to claim 2, for use with video equipment, wherein the said resistor has a value of the order of 10 ohms.

Claim 6

A line output circuit according to claim 1, further comprising a phase compensation circuit in the positive loop between the two output terminals, such as to render the net phase change around the loop at least at low frequencies substantially zero.

Claim 7

A line output circuit according to claim 5, wherein the phase compensation circuit also renders the net phase change substantially zero at high frequencies within the operating band of the circuit.

Claim 8

A line output circuit according to claim 1, wherein the gain of the positive loop is slightly greater than unity.

Claim 9

In an audio unit having a hum loop cancelling line output circuit including a line amplifier having a pair of non-earthy and earthy input teminals and a pair of non-earthy and earthy output terminals, a pair of non-earthy and earthy unit input terminals and a pair of non-earthy and earthy unit output terminals, the improvement comprising a combining means having two inputs and an output connected to the non-earthy input terminal of the line amplifier, the non-earthy unit input terminal being coupled to one input of the combining means, the non-earthy output terminal of the amplifier being coupled to the non-earthy unit output terminal, and the earthy unit output terminal and any hum loop voltages from the earthy side of the line being coupled to the second input of the combining means to establish a positive loop of substantially unity gain from the earthy unit output terminal of the circuit to the non-earthy unit output terminal.

Claim 10

In an audio unit having a hum loop cancelling line output circuit including a line amplifier having a pair of non-earthy and earthy input teminals and a pair of non-earthy and earthy output terminals, a pair of non-earthy and earthy unit input terminals and a pair of non-earthy and earthy unit output terminals, the improvement comprising a combining means having two inputs and an output connected to the non-earthy input terminal of the line amplifier, the non-earthy unit input terminal being coupled to one input of the combining means, the non-earthy output terminal of the amplifier being coupled to the non-earthy unit output terminal, and the earthy unit output terminal and any hum loop voltages from the earthy side of the line being coupled to the second input of the combining means to establish a positive loop of substantially unity gain from the earthy unit output terminal of the circuit to the non-earthy unit output terminal.

Claim 13

A method according to claim 9, and further including the step of developing the error voltage across a resistor connected between ground and the earthy line terminal at the input end of the line.

Claim 14

A method according to claim 9, and further including the step of compensating for the lowering effect of the earth return line resistance upon the error voltage by making the gain of the positive feedback loop slightly greater than unity.

Claim 15

A method according to claim 10, and further including the step of compensating for the lowering effect of the earth return line resistance and of the resistor upon the error voltage by making the gain of the positive feedback loop slightly greater than unity.