Adjustable Three-Channel Analog Audio Equalizer Circuit with Variable Gain 

Analog Circuit Design | December 2023

Project Overview

This project involved designing, simulating, and building an analog three-band audio equalizer capable of independently adjusting the bass, midrange, and treble content of an audio signal. The circuit was designed around TL081 op-amps and operated from ±15 V supplies, with the final output driving a 7 Ω loudspeaker.

The input signal was first buffered, separated into three frequency bands, independently amplified or attenuated, and then recombined before being passed through a transistor-based current amplifier to drive the speaker.

The three channels were designed around 20–200 Hz for bass, 200–2000 Hz for mids, and above 2000 Hz for treble. The resulting circuit provided approximately ±10 dB of adjustable gain for each channel.

The design was verified through Multisim AC-sweep simulations and then constructed on a breadboard for experimental testing. The final circuit successfully demonstrated independent control of the three frequency bands and was able to drive the connected loudspeaker without observable distortion at the tested operating levels.

Objectives

• Design an analog circuit capable of independently adjusting bass, midrange, and treble frequencies of an audio signal.

• Develop filter stages that separate the input audio into three frequency ranges.

• Provide adjustable amplification and attenuation for each channel.

• Recombine the modified channels into a single audio signal.

• Provide sufficient current to drive a low-impedance loudspeaker.

• Verify the circuit through simulation and experimental frequency-response testing.

• Select electrical components to fulfill the design requirements and verify reasoning.

Constraints and Challenges 

The design was developed around several hardware and performance constraints. The circuit was required to operate from ±15 V rails and use readily avilable op-amp ICs. Ultimately, the TL081 was selected for the final design.

Initial measurements of the audio output source showed a 300 mV amplitude signal with a -2.1 V DC offset. Because this offset would shift the audio signal closer to the op-amp saturation limits, the design needed to remove it before significant amplification.

The loudspeaker also presented a more demanding load than the op-amps could drive directly. With an impedance of approximately 7 Ω, the expected output required roughly 128 mA, while the TL081 could only provide about 26 mA. This required a separate current-amplification stage at the output.

Design Methodology 

The circuit was developed as a series of stages, with each stage addressing a specific requirement of the audio signal path.

Op-Amp Selection

The TL081 was selected for the active filter and amplifier stages because its 13 V/µs slew rate and 3 MHz gain-bandwidth product provided sufficient performance for the audio-frequency range while allowing the circuit to operate at the required gains.

Voltage Follower

The audio input was first passed through a voltage follower. This stage was chosen to buffer the audio output source and reduce loading between the audio source and the equalizer. This allowed the following filter stages to process the signal without placing excessive current demand on the source. Other than the TL081 op-amp, no components were needed to be selcted.

Treble Channel: High-Pass Filter

The treble channel was implemented as a high-pass filter because its purpose was to isolate frequencies above 2 kHz. A high-pass response attenuates frequencies below the cutoff while allowing higher-frequency content to pass.

The cutoff frequency was first converted to angular frequency:

Using this angular frequency, a Bode Plot can be created:

Using this bode plot, components can be solved for and selected. Starting from the transfer function:

The RC relationship was then used to determine the component values. A 0.001 μF capacitor was selected because it produces a practical resistance value for the 2 kHz cutoff:

The calculated resistance was implemented using a series combination of fixed resistors (a 75k Ω 1% + 2k Ω 1%) and a potentiometer (5k Ω), providing a practical value near the calculated resistance while allowing the cutoff frequency to be fine-tuned during testing.

Midrange Channel: Band-Pass Filter

The midrange channel was designed as a band-pass filter because the desired frequency range was limited to 200–2000 Hz. A high-pass filter alone would allow the treble frequencies to pass, while a low-pass filter alone would also allow frequencies below the desired midrange. Combining the two responses creates the desired frequency band.

The lower and upper cutoff frequencies was first converted to angular frequencies:

Using these angular frequencies, a Bode Plot can be created:

Using this bode plot, components can be solved for and selected. Starting from the transfer function:

The resistor and capacitor values were then calculated from the two cutoff frequencies. Computation after selecting a 0.01 μF input capacitor led to a 79.55k Ω resistance and a 0.001 μF feedback capacitor. Fixed resistors and potentiometers were used to obtain practical values and allow adjustment of the filter response.

As seen with the high-pass filter, the calculated resistance was implemented using a series combination of fixed resistors (a 75k Ω 1% + 2k Ω 1%) and a potentiometer (5k Ω). This allowed for fine-tuning during testing.

Bass Channel: Band-Pass Filter

The bass channel was also designed as a band-pass filter, targeting the 20–200 Hz range. A low-pass filter would establish the 200 Hz upper cutoff, but it would also allow frequencies approaching DC to pass. Adding a lower 20 Hz cutoff defines the intended bass range and prevents the input's DC component from passing through the channel.

The lower and upper cutoff frequencies was first converted to angular frequencies:

Using these angular frequencies, a Bode Plot can be created:

Using this bode plot, components can be solved for and selected. Starting from the transfer function:

The lower cutoff required larger capacitance than the higher-frequency channels. A 0.1 μF capacitor was choosen as the input capacitance. This yielded a 79.37k Ω resistance and a 0.01 μF feedback capacitor. Fixed resistors and potentiometers were used to obtain practical values and allow adjustment of the filter response.

As seen with the previous filters, the calculated resistance was implemented using a series combination of fixed resistors (a 75k Ω 1% + 2k Ω 1%) and a potentiometer (5k Ω). This allowed for fine-tuning during testing.

Channel Amplifiers (Adjustable Gain)

After each frequency range was isolated, an adjustable amplifier was used to independently control its level.

An inverting amplifier was selected because its gain can be configured both above and below unity. This allows each channel to either boost or attenuate its corresponding frequency range.

The voltage gain is determined by:

A 3.3k Ω input resistor and a 10k Ω feedback potentiometer were selected to provide approximately ±10 dB of adjustment. This range was large enough to produce an audible change in each channel without unnecessarily increasing the risk of output saturation.

Voltage Adders (Signal Recombination)

After the three channels were independently adjusted, they were recombined using an inverting voltage adder. This configuration was selected because it allows the outputs of the three channels to be combined into a single signal while maintaining independent control of each frequency range.

Equal 10k Ω input resistors were used for the three channels so that each channel contributed consistently to the summed output.

Current Amplifier

The TL081 could only provide a limited output current, while the 7 Ω loudspeaker required significantly more current.

The simulated output from the voltage adder was approximately 900 mV peak-to-peak. The corresponding speaker current was estimated using:

This exceeded the approximately 26 mA output current available from the TL081, so a transistor-based current amplifier was added to the output.

An NPN/PNP transistor stage was used to increase the available output current without adding the voltage gain already provided by the previous stages. Several resistor values were evaluated during the design, with a 33 Ω resistor selected for the final circuit to provide the required current while avoiding saturation.

Complete Circuit

The individual stages were then combined into the complete equalizer, connecting the input buffer, three filter channels, adjustable gain stages, voltage adder, and speaker driver.

Results and Evaluation 

The controller was evaluated with both simulation and physical hardware implementation results.

Simulation Results

The circuit was first evaluated in Multisim using a 300 mV input with the measured −2.1 V DC offset. The individual filters produced responses close to their intended frequency ranges. The simulated treble cutoff was approximately 2.006 kHz, while the midrange response extended from approximately 203 Hz to 1.97 kHz. The bass response extended from approximately 20.4 Hz to 198 Hz. 

Note: To keep this section shorter, simulation plots for each individual channel were omitted. If you're intersted and want to see the full report, please reach out!

The complete circuit was also evaluated at 0 dB and +10 dB channel settings. With the channels balanced, the overall response remained approximately flat, while increasing the channel gain produced the expected increase in output level.

Experimental Hardware Results

The circuit was then constructed on a breadboard and evaluated using an oscilloscope, function generator, audio source, and loudspeaker. Experimentally, the treble response had a cutoff of approximately 1.8 kHz, the midrange response extended from approximately 178 Hz to 2.22 kHz, and the bass response had an upper cutoff of approximately 224 Hz. The measured peak gains were approximately 8.2 dB for treble, 8.71 dB for midrange, and 8.33 dB for bass.

Note: To keep this section shorter, oscilloscope plots for each individual channel were omitted. If you're intersted and want to see the full report, please reach out!

The equalizer successfully allowed the bass, midrange, and treble levels to be adjusted independently, and the output stage provided enough current to drive the 7 Ω speaker at the tested operating conditions.

Conclusions and Future Work 

The completed circuit demonstrated an analog three-channel equalizer capable of independently adjusting bass, midrange, and treble content. The design combined active filtering, adjustable amplification, signal summation, and a transistor-based current amplifier to process an audio signal and drive a low-impedance loudspeaker.

Both simulation and experimental testing produced frequency responses reasonably close to the intended design ranges. The physical circuit also demonstrated functional independent channel adjustment and successful speaker operation.

Future improvements could include refining the filter transitions to reduce overlap between channels and using a higher-current output stage if greater speaker volume were required.