Eduard Cazacu

Active, 3 Channel, Stereo Audio Mixer Design

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Purpose

This audio mixer was designed and built as a first semester electrical engineering project. The project theme was sound engineering. Thus, as a team, the decision was made to attempt a modular Eurorrack-style modular signal generator. The other modules were a square wave generator, a sine wave generator, a triangle wave generator and a speaker module. Having multiple signal shapes to choose from, the need for mixing -and the module discussed here- arose.

The design process follows the Waterfall Model and its stages can be seen in the chapters bellow.

Table of Contents

Figure 1 - Finished audio mixer top view

Requirements

Together with the client (our teacher) and the team, the following system requirements were decided:

Design

The design stage started with researching existing circuits and designs. The following resource proved essential for designing this mixer:

Audio Mixing by Elliott Sound Products

Circuit and Simulation

Once the circuit was adapted and scaled to fit the requirements, it constructed in NI Multisim and simulated. The results are shown bellow.

Figure 2 - Mixer Circuit Simulation

Schematic

After the theoretical performance of the circuit was validated, the circuit was ported to Autodesk Eagle.

Figure 3 - Mixer Eagle Schematic

PCB

Because the PCB was to be manufactured in-house, and the physical size of the board allowed it, a single-sided design was chosen. Figure 4 shows the pcb layout used.

Figure 4 - PCB Stencil

System implementation

The PCB design was printed on translucent tracing paper, transfered on a copper blank PCB covered with a photosensitive layer and then etched.

The finished board can be seen in figure 5 and figure 6 . To the front, an aluminium plate was attached using machine screws.

Figure 5 - Assembled board back
Figure 6 - Assembled board front

Conclusion

The finished product performed as expected however no tests were conducted to measure audio characteristics such as THD or phase shift. There was a noticeable low-frequency roll-off which could have been mitigated by using larger coupling capacitors.

This project was a valuable learning experience in the matter of analogue electronics and audio circuits, additionally, it proved to be a great introduction to operational amplifiers and active filters. Furthermore, it confirmed my suspicions that I am more interested in the digital and embedded side of electronics and not so much the analogue side. Future projects also featured on this website reinforce this.