Digital volume control has log taper
Edited by Bill Travis
Doug Farrar, Los Altos, CA -- EDN, April 11, 2002
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.
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