An audio "panpot" circuit, shown in Figure 1, continuously varies the position of a monophonic audio signal between left and right stereo channels in response to a potentiometer setting. Low cost and low distortion are important considerations for audio circuits. The AD82731 dual low-distortion difference amplifier uses internal gain setting resistors to ensure excellent matching between the two channels. With no external components, each channel is configured as two high-performance amplifiers with a gain of 3. In the audio frequency range, the total harmonic distortion is less than 0.0007%.
Although this circuit can be built discretely, integrating the amplifiers and resistors on a single chip offers advantages to board designers, including improved specifications, less PCB area, and lower production cost.
In this circuit, the signal is split between the two amplifiers, using series 10-kΩ resistors. A potentiometer, with a grounded wiper, is inserted between the two noninverting inputs. The combination of the potentiometer and the 10 kΩ resistors presents a light load that can be easily driven by most sources. The amplifiers are configured for a gain of 3. When the potentiometer wiper is at either end, one input is grounded, so no signal gets through to the corresponding output. The other input sees VIN/2, so its output is 1.5 × VIN. With the wiper in the middle, the input to both amplifiers is VIN/3, so the output of each amplifier is VIN. Thus, by moving the wiper (either mechanically or electronically), the signal level varies continuously from 0 to 1.5 × VIN on one channel and from 1.5 × VIN to 0 on the other channel, so that, to a listener, the source appears to move across the sound stage from one channel to the other. This allows the image, or the apparent source of the sound, to be placed at any location between the left and right speakers.
Figure 2 shows the total harmonic distortion and noise over the audio frequency range. The error increases with frequency, but the total error is still less than 0.0007% at 20 kHz. Figure 3 shows the connections to the IC.