Gas station without pumps

2014 May 6

Audio amp lab

Filed under: Circuits course — gasstationwithoutpumps @ 20:55
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As I expected, no one came to lab today with their prelab homework done. There was a lot this week, as they needed to figure out the sensitivity of the microphone, decide how to bias it, figure out the AC voltage out of the microphone for 60dB sound input, add a DC-blocking filter (perhaps with a corner frequency above the resonant frequency of the loudspeaker), figure out when the amplifier would start clipping because it couldn’t drive the loudspeaker, determine the maximum gain for the amplifier, and determine the component values needed to implement all that.

Only one group finished  the design, built, and debugged their amplifier within the 3-hour lab time, but the rest are fairly close and will build and test their amplifiers on Thursday.  The group that finished today will probably work on adding either a volume control or a tone control for bonus points.

The MCP6004 op amps don’t really have the oomph to power the loudspeaker—with only about a 25mA output, they can’t do much more than 5mW into an 8Ω load. Even with an optimally matched load (maybe 240Ω?), they probably couldn’t output more than 150mW.

In a couple of weeks, we’ll do a more ambitious lab, in which the students will design and build a class-D power amplifier.  I’m a little worried about whether they’ll have the ability to work through the multiple stages of that design, since they were having a lot of trouble with the much simpler op-amp design. I might move the pressure-sensor instrumentation amp before the power amp, to give them more practice with multi-stage block diagrams before the power amp.

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