Audio Equalizer Using Combinational Logic Circuit

A few circuits for audio amplifiers were covered in earlier articles. We are going to construct an audio equaliser circuit today in order to equalise the audio signal. In addition to some passive and active parts, this circuit has two potentiometers and four integrated circuits.

The tune or melody of an input audio signal with varying pitch levels can be altered by this circuit. At different times, the signal’s qualities could change. The melody generated by the melody generator IC can be delayed, and the tune’s pitch can be varied at a different point in time. This circuit may generate diverse sounds from a single a tune.

Circuit Schematic

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CD4046 IC (x2)
CD4051 IC
555 Timer IC
Resistor (2.2KΩ, 10KΩ, 220KΩ, 56KΩ)
Variable Resistance (5KΩ, 10KΩ)
470uF/25V Electrolyte Capacitor
0.1uF Ceramic Capacitor
1N4148 Zener Diode
SL100 Transistor
4Ω Speaker

Circuit Connection for Audio Equalizer

Phase-Locked Loop

Pin 12 is used to connect the CD4046 integrated circuit (IC) for the phase-locked loop to the 220KΩ resistor (R2 and R4). The offsets are mostly given by these resistors. A significant value for the R2 or R4 resistor suggests that the offset is quite small. By offsetting the frequency range, the R2 resistor eliminates complete silence when the input voltage is 0V.

When the voltage at pin 9 is half of the voltage, the frequency at the VCO’s output (pin 4) is provided.

f=1/ (R1×C1) = 1/(2.2KΩ×4.7uF) = 96.71Hz.

The potentiometer modifies the voltage at the VCO’s input, which changes the frequency.

555 Timer IC

When the capacitor is being discharged, the 1N4148 Zener diode is attached to block the path during the off-time. Having comparable ON and OFF times (50 percent duty cycle) is the basic idea. A 0.1uF ceramic capacitor is used (not required) to eliminate any unwanted noise in the circuit.

The 555 Timer outputs multiplexes of both of the VCO’s outputs by acting as a selection line for the multiplexer.

2:1 Multiplexer

Here, IC CD4051 functions as a 2:1 multiplexer. A selection line is used to pick the output from each of these from the PLL having the same time as the output pins of the VCO, which function as inputs for pins 13 and 14 of the MUX.

A range of effects can be multiplexed and placed into the audio by adding more VCOs, with the output being sent to the speaker. There are numerous UM66 variants on the market that generate different musical tones. Consider the song “Jingle Bells.”

Working Principle of Audio Equalizer

This circuit uses a UM66 melody generator to alter the tone that it produces. Phase-locked loop integrated circuits (ICs) IC1 and IC2 are utilised in the circuit to create a phase comparator and a VCO.

The voltage applied to VCOs can be adjusted with the potentiometer’s assistance. The 5KΩ and 10KΩ potentiometers in this circuit are used to adjust the VCO input voltage. Additionally, ICs 1 and 2 receive this voltage. A CD4051 multiplexer time-multiplexes the output of the VCOs at pin 4 of the CD4046.

The 555 timer IC, acting as an astable multivibrator, generates the chosen signal for the 2:1 multiplexer. Both the high and low pulses from the 555 timer IC measure the same amount of time, 4.7 microseconds. By using a 1N4148 Zener diode, the duty cycle is guaranteed to be 50%. Next, a 56KΩ resistor is used to apply the output to the base of the SL100 NPN transistor.

The tone generator IC UM66 receives input from the collector terminal. One terminal of a 4Ω speaker is connected to the tone generator’s output terminal, while another terminal is connected to a 5V power source.

Tune and volume can be adjusted for the same song to be heard in a variety of ways using a potentiometer. Similarly, a range of tunes can be altered simply by swapping out that specific melody IC for UM66.

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