Microphone preamplifier

I tried out the new op-amp prototyping boards by wiring up microphone pre-amplifier (a slightly more sophisticated version of Lab 7 from the book).

The new boards have just a quad op-amp chip, rather than a quad op-amp and an instrumentation amp. This allowed more room for resistors and a somewhat looser layout, making wiring and routing easier.

I made some of the mistakes I chide my students about:

• I didn’t check my wiring carefully against my layout—I had left out the wire from the microphone to the bias resistor for the microphone, resulting in 0V for the mic and no signal to amplify.  Debugging this only took a couple of minutes, but I had to get out the soldering iron again, because I had 0ver-confidently put it away.
• I hadn’t checked my layout carefully against my schematic—I had accidentally routed the microphone bias resistor to 1.65V instead of 3.3V.  This required removing 2 wires and replacing them.

After fixing those wiring mistakes, the circuit worked fine.  I also determined that it is possible to do the microphone pre-amp lab without an oscilloscope.  PteroDAQ does a reasonable job of capturing the waveforms:

Screenshot of PteroDAQ showing the sparklines for a 330Hz sine wave. Note that for steady-state signals, the RMS reported can be used for calculating the gain.  Here I had the gain for the amplifier set fairly low (2.49 in the first stage, and 6.61 in the second stage).  When I get near the resonant frequency of the loudspeaker (around 156Hz), the high-gain waveform goes rail-to-rail.