This design idea details how a standard volume control can be replaced by its solid-state equivalent. Digital potentiometers are often advertised as replacements for rotary volume controls. To simulate these controls, many circuits employ up/down pushbuttons, switches, or microprocessor interfaces.
In this circuit, a conventional rotary potentiometer (volume control) drives a digital potentiometer. This configuration optimizes audio volume control with a solid-state digital potentiometer, while the user enjoys the feel and action of a familiar volume control. One rotary (or slider) control adjusts any number of channels in unison. Plus, the digital potentiometer eliminates the typical scratchy noise associated with mechanical pots.
The circuit uses three CAT5113P-10 digital potentiometers from Catalyst Semiconductor (see the figure). These pots have control lines connected in parallel as well as isolated resistor elements. One resistor element forms the feedback element in the control system, with the remaining elements controlling the volume for each audio channel. The figure shows the configuration for two-channel stereo control. Additional digital potentiometers can be paralleled to control more than two channels. This forms the equivalent of one multigang control, as all pots effectively move up and down together.
An LM339 quad comparator continuously monitors the positions of the control versus the reference resistor element. The circuit implements a balanced bridge configuration. When the control setting is higher than the reference, the digital pot's Up direction is selected and a 50-kHz square wave is applied to the clock line. This causes the reference to step up at a 50-kHz rate until its setting matches the control. The remaining digital potentiometers follow along, adjusting their individual channel volumes to match. Conversely, when the control setting is lower than the reference, Up is driven low and the clock is applied to the clock line. This causes the reference to step down until its setting again matches the control.
The 2.2-mF capacitor at the control wiper forces all digital potentiometers to reset to their zero position on power up, ensuring that all pots start at the same position. After the capacitor has initially charged, the digital pots track the control position. Then the capacitor serves the secondary role of filtering out any noise from the control wiper.
The digital pots provide 100 steps of resolution, and the hysteresis around the Up and Down comparators and the deadband between them is optimized for this resolution. While the 6.8-MW resistors set the hysteresis, the 10-kW resistor at the base of the transistor sets the deadband. The dc power required by the circuit is under 7 mA at 5 V dc.
If other potentiometers are used in this circuit, the hysteresis and deadband must be altered to reflect the number of steps of the digital pots. The 50-kHz stepping frequency isn't critical. Chosen for its convenience, this level is well above the audio frequency range.
The voltage at the digital potentiometer resistor elements must stay within the power-supply voltage rails. Therefore, the 47-kW resistors and associated coupling capacitors maintain a 2.5-V dc bias on these elements. The corresponding audio voltage range is ±2.5 V, in which normal audio levels fall.
The 3.3-kW resistors at the outputs of the digital pots are optional. They convert the linear action of the digital pots to a quasi-logarithmic audio taper action similar to that obtained with an audio taper pot. Other resistance values may yield better performance in some applications, depending on the load impedance at the outputs.
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