Gas station without pumps

2014 May 9

Low-power audio amp lab completed

Filed under: Circuits course — gasstationwithoutpumps @ 07:54
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Everyone finished their audio amps yesterday and got them working, and most finished on time, though one group took a bit longer than the rest, so I was in the lab for 5 hours instead of 3. They did not keep careful notes of how they did the prelab, and could not reconstruct their thoughts, so they were very confused about why the amplifier was clipping with the loudspeaker in place, but not when it wasn’t.  I think that they eventually figured it out, with somewhat heavier hinting than I usually like to use (I was getting tired).

Each group had a different design, as they chose slightly different voltages for the power supply, hooked up the mic with either the full power supply or half the power supply for DC bias, used different pullup resistors for biasing the mic, used different combinations of R and C for their high-pass filter, and had different target gains.  I think I should point out in class how small changes in somewhat arbitrary design choices can result in rather different designs—all of which are correct.

One problem I had not anticipated, but ought to have, is that a lot students initially chose to use small resistors and large capacitors for their high-pass filters, but the filter is in parallel with the bias resistor for the microphone for AC, so if it has a low impedance, the I-to-V conversion results in a small AC signal.  This problem was noticed when one group was confused about why their AC signal was so much lower than they had expected.  It took me a while helping them debug to figure out what was going on (I generally don’t use electrolytic capacitors unless I need to, so I’d never set up a DC-blocking filter with a low impedance).  In next year’s handout, I’ll have to put in a bit more information about the need to have the RC filter be a fairly high impedance compared to the DC bias resistor, perhaps even including it in the sensitivity computation.  After seeing the problem, I did warn each group about the problem, and they all had to redo their RC filters, as they’d all chosen large C and small R.

A number of groups also used very small resistors for their feedback loops, until I suggested that using much of the current capabilities of the op amp to drive the feedback network was not leaving them much to drive the speaker.

I’ll also have to outline the steps of the sensitivity and gain computation more carefully next year, as students were not able to do it on their own without guidance.  Almost everyone ended up with too high a gain empirically, getting clipping at the loudspeaker with fairly modest sound input, but I didn’t see mistakes in their computation, and the op amps were clipping at about the expected current limit. Perhaps the input sounds were louder than we had allowed for—certainly the signal generators driving loudspeakers that we were using as sound sources were much louder than 60dBA at 1kHz, which is what the circuits were designed around.  Students did observe that without the loudspeaker the amplifiers produced nice-looking sine waves, but that adding the loudspeaker produced the clipping—some turned up the input sound to the point where they could observe the voltage clipping without the loudspeaker.  I should add a request for testing the amplifier with and without the loudspeaker next year.

Everyone did get enough gain from their amplifiers to hear sound from their loudspeakers and to get feedback squeal if they put the loudspeaker near the mic.  Not even the quickest group had the time to add a volume control or tone control circuit, so I’ll probably cut that from next year’s lab, though I’ll probably have them do adjustable gain for the class-D amplifier in 3 weeks, if only to save our ears from high-volume feedback squeal.  I’ve been wondering whether I should include a traditional large potentiometer (and not just an 18-turn trimpot) in the kits next year, so that students can have a more easily (if less precisely) adjustable resistor.  They are only about 70¢ for cheap ones, and they would make adding a gain control easier.

Students are getting fairly good at using the Tektronix TDS3054 digital oscilloscopes for making measurements, but everyone once in a while the scopes seem to get wedged in a weird mode where they don’t respond to some of the knobs (like setting the low pass filter) and the only way out we’ve found is to run “autoset”, and then reset a number of the parameters from there.  Although the scopes have some nice features, the user interface is still one of the worst I’ve encountered, with deeply layered menus covering the screen and buttons that cycle through different modes.

Measuring anything about the amplifiers was difficult, because the ambient noise in the room, particularly of other groups talking, made fairly large uncontrolled input signals.

One thing I did not expect, and still don’t have an explanation for, is that for all the groups the output seems to have been centered a little high when the loudspeaker was in the circuit, so that the op amp was at or near saturation at +150mV output even when there was no sound input.  With the gains the students were using, that would have resulted from the input voltage being around 0.1–1mV too high, which seems to be too much for any of the explanations I’ve thought of.  I did not observe this in my testing at home, but I don’t have a ±3v power supply at home, so I did not have exactly the same circuits as the students.

One group had a 50kHz triangle-wave oscillation for a while, but it went away while we were attempting to debug it, so I never found a satisfactory explanation.



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