Figure 1 shows a low-power, low-distortion circuit that can be used to balance the left and right headphones or speakers in battery-powered automotive or computer applications. The circuit contains a low-cost voltage-feedback dual amplifier with four resistors and two potentiometers (RPOT1, RPOT2). The op amps are connected as noninverters with variable gains. The gain of amplifier A1 is 1 + [RF/(RG + RPOT1A)], and the gain of A2 is 1 + [RF/(RG + RPOT1B)].
Both amplifiers share the input signal. Adjusting RPOT1 controls the output balance. When RPOT1A equals RPOT1B, the output levels are the same, which corresponds to a system gain of 1.16. With RPOT1A = 0 and RPOT1B = 100 kΩ, the respective gain levels are 1.09 and 2. This system configuration keeps one speaker on at all times, and the other speaker output can be increased by up to 2↔. Adjusting RPOT2 controls the volume.
With a gain of 2, the circuit in Figure 1 has total harmonic-distortion levels below −80 dBc at audio frequencies. The circuit can be run off of ±1.5-V batteries or from a 3-V single power supply. For single power-supply operation, either a voltage divider or an external source must supply the ground of the system (mid-supply).
Figure 2 shows the circuit's performance at 10 kHz using a 1-V p-p input signal with RPOT1 adjusted to the midpoint. As shown, the left- and right-channel output-signal gains are 1.16.
Figure 3 illustrates how the circuit performs at 10 kHz with a 200-mV p-p input signal and RPOT1 adjusted fully to one side. The left-channel output-signal gain is 1.09, and the right-channel output-signal gain is 2.
Chau Tran, staff engineer at Analog Devices, received an MSEE from Tufts Univ., Medford, Mass.
Maurice Triplett, product technician, received a BSEE from Northeastern Univ., Boston, Mass.
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