Digital Volume Controlled Audio Preamplifier

Overview

The circuit was designed to build a volume control  that is operated digitally on an audio preamplifier circuit.

Terminology

• Preamplifier – a device typically used along with sound equipment to help improve the overall quality of sound where it precedes another amplifier to prepare an electronic signal for further amplification or processing
• 4093 – a quad 2-input NAND with Schmitt trigger inputs integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption
• 4029 – a pre-settable binary/BCD up/down counter CMOS logic IC where its outputs change precisely together on each clock pulse because of its synchronous counter
• DIP Switch – a set/series of tiny electrical switches that is enclosed in a standard dual in-line package and typically built into circuit boards  
• TL072 – a low noise JFET input operational amplifier with features such as common-mode input voltage range, high slew rate, operation without latch up, compensated internal frequency, high input impedance at the JFET input stage, low noise, low total harmonic distortion, protected from output short circuit, low input bias and offset currents, wide common-mode and differential voltage ranges, and low power consumption
• 4067 – a 16 channel analog multiplexer/demultiplexer enclosed in a 24-pin package which is digitally controlled analog switches device having low OFF leakage current, internal address decoding, and low ON impedance
• NE5532 – an internally compensated low noise dual operational amplifier with features such as full power bandwidth up to 140 KHz, input noise voltage of 8 nV, common mode rejection ratio, 9 V/us slew rate, high DC voltage gain, 32 V peak to peak voltage swing, wide supply voltage range from 3 V to 12 V, unity gain bandwidth at 10 MHz, and internal frequency compensation

Circuit Explanation

The design of the circuit comprises of three parts. The first circuit contains a control that functions as electronic potentiometer to control the circuit. A potentiometer is a 3-terminal resistor that is used to adjust the voltage by using a sliding contact. The circuit is being controlled by using two switches S1 and S2. Switch S1 functions by increasing the intensity of the volume while switch S2 functions by decreasing it. This scenario is made possible by the presence of two CMOS 4093 oscillators IC1C and IC1D, operating as RS bistable multivibrator or better known as flip flop which causes the circuit to remain in either state indefinitely. This logic operation controls the direction of the CMOS 4029 IC3 synchronous counter. This IC avoids the glitches which occur with ripple counters by connecting the outputs to logic gates. The operation of the IC starts when the clock input becomes high which will start the counting with the direction being determined by the up/down input. A carry in should be minimum for normal operation counting. The type of counter is being selected by the binary/decade input. Placing the desired binary number on the pin 3, 4, 12, and 13, would preset the counter by making the preset input high on pin 1.

The function of IC2B is to make sure that the counter is operating within the regulation limits. In selecting the sound level, switch S4 is utilized wherein it will return after the applying power to the circuit or after the RESET. This operation is significant in selecting the void level of sound. However, using switch S3 will rapidly turn the audio into MUTE state. The power supply of the circuit is also taken into consideration a value of 8 V. Since switch S4 is a DIP switch, it enables configuration of a circuit board for a particular type of application. DIP switches are always toggle switches, which signifies that two positions, ON and OFF) are possible for them.

In the second figure, the inputs of CMOS multiplexer 4067 IC2 and IC3 drives the logic conditions coming from the outputs from point A, B, C, and D. There are two multiplexers where 16 channels of each contain a resistor that is directly attached. This circuit employs two dual analog operational amplifiers NE5532 on the output stage. The power bandwidth and the high small signal provide superior performance in telephone applications, all control circuits, and high quality amplifiers. On the input stage lies the IC1A and IC1B TL072 op-amps. They are high speed JFET input dual operational amplifiers incorporating well matched, bipolar transistors and high voltage JFET in a monolithic circuit. The value of resistors R38 to R40 modifies the gain of the IC1A and IC1B. A full qualitative preamplifier can be produced if an additional rotary switch in placed on the input for selecting the sources of input.

The third figure illustrates a conventional power supply which was separated from the digital and analog part of the whole design. It utilizes diodes 1N4002 to handle and adjust the voltage and current rectification coming from the mains.

Application

A preamplifier may be integrated into a sound system in a diverse approach by placing inside the shelter of the power amplifier that is in contact with or it may have its own container. In hi-fi entertainment system, the preamplifier is placed closely while maintaining distance with power amplifier.

The preamplifiers can be utilized with semiconductor charged particle detectors, proportional counters and ionization chambers, time and energy spectroscopy, first stage of an instrument amplifier, as masthead amplifier used with television receiver antenna or a satellite receiver dish, as part of a standalone channel strip built into an audio mixing desk, and a standalone unit for use in live music and recording studio applications.

Source:users.otenet.gr/~ATHSAM/audio_preamplifier_with_digital_volume.htm