Analog Audio Mixer
Collaborators: Cianna Janicke (EE) , Lam Phung (CE)
November 2021
Abstract
This project was intended to give us experience in designing practical electronic systems.
Project Requirements
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Design a stereo audio mixer system.
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The system will take no less than three input channels.
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One channel is for a microphone circuit. The microphone circuit will include an electret microphone drive circuit and a preamplifier circuit.
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Two channels can be interfaced with cell phones, laptops, or other music players using cable or bluetooth. The signals are stereo signals.
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The audio signals from these channels will be mixed, conditioned, and sent to two 4 Ohm speakers.
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The AC power delivered to the speaker should be about 1W (speaker dependent).
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The audio signal out of the speaker needs to be clear and free of noise (hum and/or buzz noise).
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The volume control should be introduced.
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The gain control should be introduced.
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Input protection for the audio amplifier (if used) should be implemented.
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Power can be supplied by batteries, USB, or a wall charger.
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A fixture must hold all components attractively and securely.
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An equalizer control needs to be included to adjust the levels in different frequency bands.
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(Optional) The two input channels can be interfaced via Bluetooth.
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(Optional) Higher power speaker (> 1W) You may choose to use high-power IC audio amplifier or discrete power amplifier.
Solution
The circuit shown below implements our 4-stage Analog Audio Mixer. This is a three-input system for one microphone (built-in), and two 3.5 mm audio jacks. From these inputs, there is a left and right signal (except for the microphone) within the input that is conditioned individually.
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Both the right and left signals are conditioned in the same manner, however they cannot be combined if stereo audio is desired. With this in mind, each signal is added during the first stage of the system to make a single signal that is comprised of all the individual frequencies that the inputs carry, which to the human ear is indistinguishable.
Then, the signal is fed through a treble (high frequency) control stage which can be adjusted via a potentiometer. This stage in particular acts as a high-pass filter (See graph in slides below), meaning that it allows frequencies higher than the set cutoff frequency and dampens anything below.
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Likewise, the next stage the signal is fed into is a Bass (low frequencies) control stage. Such stage acts as a low-pass filter (See graph in slides below), meaning that it allows frequencies lower than the set cutoff frequency and dampens anything above.
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In conjunction, these two filters cascaded one after the other causes the both to act as a single, adjustable band-pass filter (See graph in slides below). Causing the audio passed through the circuit to only allow the frequencies in the desired range.
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Once the audio circuit was designed, careful consideration was taken to implement lower power consumption within the design. Originally, this circuit was meant to be run on 9V or higher, but upon further analysis, a 5V alternative was implemented. Additionally, power conditioning was implemented as an extra-credit feature to ensure best performance out of our design. Within this power conditioning, as additional features was implemented with the use of a rechargeable Li-Ion battery and a USB-C rechargeable circuit. to make this a fully integrated audio mixer.
Once the circuit was designed, Cianna Janicke took charge of designing the PCB for this project. Her design was implemented in a two-layer board, taking careful consideration for utilizing polygon pours for power nets given that the system utilized 5V with a current draw of approximately 500-600 mA when fully engaged. See below the unpopulated PCB with its Altium counterpart.
As a way to implement bluetooth into the design without the need for a microcontroller or redesign of the circuit, a bluetooth dongle was purchased to be plugged into one of the available audio jacks. Although a non-permanent solution, it was a simple one to implement for added features.
My Role
My role within this project consisted of the design of the overall circuit. From power conditioning, to each of the stages, and added features.
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Within this role, I was also tasked to debug the circuit given the initial issues with the noise of the signal, and improving upon the first filter designs.
Challenges
Some of the challenges encountered throughout the project was at first the lack of effect by the Treble and Bass control. The issue we found was within our circuit design. After the summing stage of the circuit, the signal was split into two parallel filters that were indeed individually filtered. However, when trying to add them again via a summing amplifier, the filtered signals were recombined with the oppositely filtered components which resulted in the original signal with no change at the output. This problem was easily fixed by setting the filters in series rather than parallel.
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Another issue we encountered was the noise in the signal from the multiple stages, as well as the noise within the power supply. This issue was fixed by adjusting some of the capacitor values used throughout the circuit.
Outcomes
The outcomes for this project were exemplary. Although this was our first time designing analog circuits, there were many things learned. From a design perspective, what to look out for, and from a development perspective, what is possible.
Early Circuit Prototype
First PCB Assembled Test
Finalized Design
Team Picture on Project Day