Design of Ultra-Low Distortion Audio Panpot Amplifier

Figure 1 shows an audio panpot circuit that responds to the setting of a potentiometer by continuously changing the position of a mono audio signal between the left and right stereo channels. Low cost and low distortion are important considerations for audio circuits. The AD82731 dual-channel, low-distortion difference amplifier uses internal gain-setting resistors to ensure that the two channels are well matched. It requires no external components, and each channel is configured as two high-performance amplifiers with a gain of 3. Total harmonic distortion is less than 0.0007% over the audio range.

Although this circuit could be constructed discretely, integrating the amplifier and resistor on a single chip offers board designers many benefits, including better performance specifications, smaller PCB area, and lower production costs.

Figure 1. Audio Panpot amplifier.

In this circuit, the signal is split between two amplifiers via 10 kΩ series resistors. A potentiometer with the wiper at ground is inserted between the two noninverting inputs. The potentiometer and 10 kΩ resistor combination forms a light load that is easily driven by most signal 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 is delivered to the corresponding output. The voltage at the other input is VIN/2, so its output is 1.5 × VIN. When the wiper is in the middle position, both amplifiers have VIN/3 as their input, so the output of each amplifier is VIN. Thus, by moving the wiper (mechanically or electronically), the signal level on one channel changes continuously from 0 to 1.5 × VIN, and the signal level on the other channel changes continuously from 1.5 × VIN to 0. To the listener, the sound source appears to move from one channel to the other between the sound levels. Therefore, the image, or apparent source of the sound, can be located anywhere between the left and right speakers.

Figure 2. Total harmonic distortion and noise vs. frequency.

Figure 2 shows the total harmonic distortion and noise over the entire audio range. The error increases with frequency, but the total error is still less than 0.0007% at 20 kHz. Figure 3 shows the connection diagram of the IC.