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

The first major oversight is that 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.
Second, 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.
Third, replacing 9 of the 10 the expensive LM3914N’s by multiplexing the different frequency signals.

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