AudioPreAmp

Analog Audio Signal Preamplifier PCB

This board solves the problem of using salvaged electret mics with a microcontroller. The mics need a DC bias to operate and incoming sound fluctuates a tiny AC voltage on top of that line. The 0.1uF capacitor AC-couples the signal, stopping the DC bias from passing while only allowing the AC signal to pass. Using 1M and 10K ohm resistors to form a voltage divider to bias the base of the 2N2222A transistor, we can set the transistor into it's linear region ensuring that both sides of the wave get amplified and don't get clipped. The transistor now being biased in its linear region will allow the audio signal to bounce up and down around a new DC centerline in the MCU's safe voltage range and loud enough for it's ADC to read accurately.

PCB Schematic

This was my the first PCB I ever designed so I wanted to use this section to go over what I could've done better and what i've now learned is correct practice.

1. No Ground Plane

I've since learned that for analog audio work, a ground pour on an unused layer or around components is commonplace for lower impedance and noise regulation.

2. Sharp Corners

Sharp corners generally aren't recommended because they can create impedance discontinuities and are more prone to manufacturing problems. Ideally, you should opt for 45 degree angles.

3. Long Traces

Some of my traces are longer than they need to be and for board working with analog signals, trace length can affect outcomes.

Prototyping

Finished Board