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
Together with the client (our teacher) and the team, the following system requirements were decided:
- 3 stereo channels
- individual preamp with gain control
- volume sliders on each channel
- logarithmic master volume control
- high sound fidelity
- active(powered) signal mixing
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.
After the theoretical performance of the circuit was validated, the circuit was ported to Autodesk Eagle.
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.
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.
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.