Many of today’s hi-fi amplifiers feature a “clicking” digital volume control circuit, but this is only rarely a real stepped attenuator based on a wafer switch. In nearly all cases, this expensive system is based on a normal potentiometer, whose spindle is fitted with a mechanical construction to simulate the stepping movement.
A normal rotary switch is not suitable for adjusting the volume of an amplifier because it briefly disconnects the input from the signal source when operated, and so readily gives rise to clicks and contact noise.
Different problems crop up when designing an electronic digital volume control circuit. Of these, distortion is probably the hardest to master, but reasonable results are still obtainable, as will be shown here.
Digital Volume Control Circuit Diagram
Basically, there are two methods for making an electronic potentiometer. One is to create a tapped resistor ladder (which is not much different from a normal potentiometer), the other is so change the resistance of the two “track sections” such that the total resistance remains constant.
The circuit proposed here is based on the second, and features 16 steps in its basic form. The number of steps can be increased to, say, 64 by adding four switches and resistors.
The electronic potentiometer for digital volume control circuit is composed of two equal sections, which have a total resistance of 15 kΩ each. The electronic switches in each section are controlled by binary counter IC5. Since switches in section ES1-ES4 and those in ES5-ES8 are controlled in complementary fashion, the total resistance of the potentiometer remains constant.
Resistors R1-R2 and R7-R8 serve to keep the potential at the input and output at 0V so as to preclude clicks when the step switch, S2, is operated. Switch S1 is the up/down selector. GatesN5-N6 form a bistable to ensure that the counter is clocked with debounced step pulses.
The number of steps can be increased by adding a counter and the required number of electronic switches, divided over the two “track sections”. These switches are then connected in parallel with resistors whose values correspond to binary order 1-2-4-8, etc., as shown in the circuit diagram.
Fortunately, precise binary rations are not required here, since adequate results are obtainable with approximations of the theoretical resistance values, and as long as the actual resistors are kept equal in both sections.
This digital volume control circuit article reprinted from 303 circuits book, page 17.