The name audio spectrum analyzer is probably a bit too high trotting. A more befitting name would be funny looking graph display that changes with the music. But this was my second electronics project in 2018. It uses only through-hole components on a two-layer PCB.
Theory of Operation
The input signal is selected by switch S1 from either a 3.5 mm audio jack or the microphone. Both have individual pre-amplifiers with adjustable gain. The selected signal is then fed into a filter bank comprised of ten bandpass filters with fixed center frequencies from 32 Hz to 16kHz. The individual frequency components are then rectified and buffered. The buffer is formed by a RC circuit with a timeconstant of 100 ms, resulting in a dischargetime of about 0.5 s which is choosen purely for aesthetic reasons, so that the bar graph drops nicely. Each signal is display on it’s own ten-segment bar graph display. Each bar graph is driven by a LM3914 Dot/Bar Display Driver. The diode D7 roghly compensates for the forward voltage drop of the rectifier in the signal path.
Schematics and Layout
Mistakes & Learnings
- IC1B is a linear amplifier, but the loudness of sound is inherently exponential. Therefore any music file contains waves with multiple different magnitudes of amplitude. A logarithmic amplifier is a must.
- The caps for the peak detection circuit, even though this isn’t a precision instrument, should be low drift and somewhat precision e.g. not electrolytic.
- Replacing 9 of the 10 expensive LM3914Ns by multiplexing the different frequency signals.
Edit (20.11.2022): Looking back four years and five Semesters EE later, I see that I was more focused on making the device look good, rather than electrically sound. I wasted a lot of time aligning vias and arranging components rather than calculating and verifying my choices.