Digital volume control

Digital volume control, Two ladder networks and a buffer each from a volume control with a range of 63 dB. Network R3-R17 provides fine control in steps of 1 dB, whereas network R20-R34 provides coarse control in steps of 8 dB. The desired attenuation is set with aid of multiplexers IC1 and IC3, each of which is controlled via three digital inputs. The design is such that the binary code on the six-bit wide overall control input accords with the set attenuation.

Resistor R1 ensures that C1  can discharge, even if K1 is open circuited. This resistor and network R3—R17 form an input impedance of 46.3, kS2. The resistor also determines the maximum permissible input voltage. That voltage depends, in the first instance, on the supply voltage to IC1 and IC3 (±8.2 V). Resistor R1 plus R3—R17 attenuate the input signal x2.4 (7.6 dB). This means that the maximum input level must not exceed 20 Vpeak, that is, 14 V r.m.s. That means also that ICS must not amplify to prevent too high an input to the second ladder network and buffer.

The amplification of the two opamps is determined by R18-R19 and R35-R36 respectively. As stated before, that of ICI. should be-unity, in which case R18=0 cl and R19 are omitted. If the amplification of IC2b is also unity, the overall control range is -7.6 dB to -70.6 dB. To obtain a control range of 0-63 dB (when the binary code on the control inputs accords with the actual attenuation), IC2b should provide an amplification of x2.4.

The current drawn by the circuit is determined primarily by the dual opamp and amounts to about 10 mA.

The overall distortion is <0.003% Over the range 20 Hz to 20 kHz and an input signal of 1 V.

The control has one small drawback: when the volume is set, weak clicks occur (which are typical of all normal CMOS switches). That makes it less suitable for super-de-luxe applications, although many listeners will not even notice the clicks. And, in any case, the volume is not varied constantly.
Digital volume control Schematic diagram

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