Audio spectrum analyzer

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 alternatively a condenser microphone. Both have individual pre-amplifiers. The individual frequency components from 32 Hz to 16kHz are displayed on ten ten-segment bar graph displays. Each bar graph is driven by a LM3914 Dot/Bar Display Driver which is fed its frequency signal by a simple peak detection circuit formed by a diode, a capacitor and a 100 k bleeding resistor. This peak detector causes the bar to sink smoothly after a deflection.

Schematics and Layout

Mistakes & Learnings

  1. 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.
  2. 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.
  3. 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.

Leave a Reply

Your email address will not be published. Required fields are marked *