Electronic volume control. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Electronic volume control. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Tone, volume controls

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The recent trend of electronic stepped volume control using switching a matrix of discrete resistors using counters, decoders and analog switches opens up wide opportunities for creating multi-channel sound reproducing devices with almost identical control characteristics. However, such regulators have insufficient smoothness of regulation, their output resistances change significantly during the adjustment process, and the power they consume often turns out to be quite significant. These shortcomings hinder the use of electronically controlled volume controls in high-quality sound-reproducing equipment.

The author of the article managed to develop a volume control that has a wide range of regulation with a resolution almost an order of magnitude less than in similar devices described in the literature, has small dimensions, and is easy to set up. The characteristic of the regulator is linear over the entire control range, which is especially important at low volume levels. The signal can be adjusted in each channel separately or in both at the same time.

There are two control modes: step-by-step (the volume changes by one step of regulation each time the control button is pressed) and automatic (the volume changes in the specified direction at a certain speed).

(click to enlarge)

Main technical characteristics:

Number of control channels.......2
Adjustment range, dB, not less.......60
Regulation step, dB, not more than ....... 0,24
Current consumed from a voltage source +15 (-15) V, mA, not more than.......15(6)

The schematic diagram of the regulator is shown in Fig.1. The volume is adjusted using the momentary buttons SB1-SB4. Their "bounce" is eliminated by the DD3 chip. The regulator also contains a clock generator on the elements DD1.1 and DD2.1 and a two-channel device, each channel of which consists of an RS-trigger on the elements DD1.2, DD1.3 (DD4.1, DD4.2), reverse counters on microcircuits DD7, DD9 (DD8, DD10), an integrated digital-to-analogue converter on DA1 (DA2) chips, an output amplifier on the op-amp DA3 (DA4), a device for blocking a random transition from maximum to minimum volume and vice versa on the elements DD2.3 (DD5.2), automatic control unit on elements DD1.4, DD2.4 (DD4.3, DD5.3) and elements DD2.2 (DD5.1), providing a step-by-step mode.

The regulator works as follows. When the power is turned on, the counters of both channels of the regulator are initially set. Rectangular clock pulses with a frequency of about 20 Hz from the output of the generator are fed to the synchronization input of the DD3 chip (pin 5). Since the rest of its inputs (pins 4, 14, 7, 13) are connected to a common wire through resistors R1-R4, logic 2 levels are recorded at the outputs of this microcircuit (pins 1, 10, 11, 0). Further operation of the regulator depends on whether which button the operator will press. To enable step-by-step volume control, it is enough to press the corresponding function button once and then release it.

When you press the "+" button of channel 1 (SB1), a logic 2 appears at pin 3 of the DD1 chip. As a result, a logical 2.2 is set at the output of the DD0 element and the pulse coming through the DD6.1 element to the pins of 15 counters DD7, DD9 increases state of the latter by 1.

When you press the "-" key of channel 1 (SB2), a logical 1 appears at pin 1 of the DD3 chip and the state of the counters DD7, DD9 is already reduced by 1, since from the output of the RS flip-flop on the elements DD1.2, DD1.3 to the pins 10 of the counters DD8, DD9 will receive a logic level 0.

Automatic mode requires the operation of two buttons. To adjust the volume level in the desired direction, first press the button with the corresponding functional action, and then press the second button of this channel. When the desired volume is reached, both buttons must be released.

So, when you press the buttons SB1, SB2, the automatic control mode is set in the first channel. At pins 2 and 1 of the DD3 chip, logic 1 levels appear, as a result of which the logic 1.4 level is set at the output of the DD0 element and the clock pulses from the generator begin to pass to the counting input of the counters DD7, DD9. Capacitor C5 (C6) increases the noise immunity of meters when switching counting modes.

The outputs of the binary reversible counters DD7, DD9 are connected directly to the control inputs of the integral keys of the digital-to-analog converter DA1. The switches switch the resistors of an integrated R-2R type matrix, the output of which is loaded onto the inverting input DA3. Thanks to the OOS, from the output of DA3 to pin 16 of the DA1 microcircuit, the voltage level at the output of the regulator changes smoothly and with high stability. The output impedance of the controller remains constant and is determined by the output impedance of the op-amp DA3.

At the output of the element DD2.3, the level of logical 0 is present as long as the outputs of the transfer counters (pins 7) have at least one level 1. The state of logical 0 is set at the outputs of the transfer when the outputs of the counters DD7, DD9 (pins 6, 11 , 14, 2) state 1111 occurs when the count increases and 0000 when it decreases. This operation of the counters ensures that the DD6.1 element is blocked and makes it impossible to switch from the maximum volume level to the minimum one and vice versa. The second channel works similarly to the first.

All parts of the regulator, except for the SB1-SB4 buttons, are placed on a printed circuit board measuring 80x70 mm: a) - view from the side of the microcircuit installation, b) - from the opposite side, c) illustrates the location of the elements on the board. The latter is attached to the front panel of the equipment, near the input of the final amplifier (to reduce the background level).

The board is designed to use MLT resistors, KM-6 and K50-16 capacitors. Buttons SB1-SB4 without fixation - PKN-150-1. Instead of these elements, you can use any small-sized close denominations - resistors VS, OMLT, capacitors K10-7V, K50-6, K53-19, P2K buttons.

It is practically not required to adjust the regulator. If necessary, the automatic control speed can be increased by reducing the resistance of the resistor R5 or the capacitance of the capacitor C1.

The regulator remains operational without deterioration of parameters when the supply voltage drops to ±5 V.

Author: V.Popov

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