Digital volume control has log taper

Digital potentiometers provide a compact and convenient way to attenuate audio-amplifier signals. However, most such potentiometers suffer from at least one flaw: a nonlogarithmic step increment. To avoid this problem, a user must usually step the potentiometer in a nonlinear sequence to simulate a logarithmic taper. For this reason, the potentiometer needs many taps, and you need software help to complete the design. The circuit in Figure 1 is a low-cost, digitally attenuated audio amplifier that does not use a digital potentiometer. The circuit attenuates the signal in logarithmic steps, by using an inexpensive 8-to-1 analog switch as its "potentiometer." The resistor string, R₁ through R₈, sets the gain of power amplifier IC₁. Analog multiplexer IC₂ selects one of the eight tap voltages and applies it to IC₁'s inverting feedback node. Because only one bias current flows through the nonlinear switches, the topology introduces no measurable distortion.

The amplifier changes gain in 3-dB steps, starting from a high of 6 dB, then decreasing to 3 dB, 0 dB,...–12 dB, and finally –∞dB (in other words, "mute" or "off") as you step the volume bits V₂ to V₀ from 7 to 0. If you don't want to turn off the amplifier at the minimum-volume setting, you could change resistor R₉ to a finite value for whatever end-attenuation level you desire. The assumption in Figure 1 is that your system has a way to generate the 3-bit volume codes. For those applications that do not have these bits available, you can use the circuit in Figure 2 to generate them. Pushing switches S₁ and S₂ clocks up/down counter IC₃ up and down, respectively. Transistors Q₁ and Q₂ decode the counter's zero state and disable the down clock when the count reaches zero. In this fashion, the circuit limits the counter to a value of 0 to 7. Once you attain a maximum or minimum volume, further pushes on the up and down switches, respectively, have no effect on the volume setting until you go in the opposite volume direction. Standby-current consumption of the logic is almost entirely a function of the resistors you use for Q₁ to Q₃, because the logic chips use almost no power. You can set a nonzero power-up volume by using IC₃'s load (LD) pin when you first apply power. In Figure 2, the counter powers up at a "4" volume, rather than "0" (muted). 74HCXX logic operates over 2 to 7V, but the power amplifier, IC₁, uses 2.7 to 5.5V. Therefore, IC₁ sets the operating-voltage range.