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Volume
44 – May 2010

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Ultralow Distortion Audio Panpot Amplifier

By Chau
Tran

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.

Figure 1. Audio panpot
amplifier.

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. Total harmonic
distortion and noise vs. frequency.

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.

Figure 3. Connection
diagram.

References
1www.analog.com/en/audiovideo-products/audio-amplifiers/ad8273/products/product.html.

Author

Chau Tran
[[email-masked]]
joined Analog Devices in 1984, where he works in the
Instrumentation Amplifier Products (IAP) Group in
Wilmington, MA. In 1990, he graduated with an MSEE
degree from Tufts University. Chau holds more than 10
patents and has authored more than 10 technical articles.

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