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Small switching amplifier up in smoke. Twice.

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John0922:
Hi-

I’m working through the book “Learning the Art of Electronics.” The project I’m having trouble with is attached.

I carefully wired this and double-checked to make sure there were no mistakes. I omitted the decoupling capacitors (on the supply line) as the instructions stated— they said to install them afterwards and listen for the effect.

I never got to do this. I switched on the power, and heard some hiss noise coming from the speaker, as expected. I set the function generator for a sine wave, 300 hz, .5 volts amplitude, and switched it on. I heard the tone on the speaker for about 3 seconds, then silence and a small poof of smoke.

Thinking I might’ve made a mistake, I checked everything carefully, then used the 2nd amplifier that I had and tried it again, with the exact same result.

I’m not sure this is relevant, but the bookshelf speaker that I’m using has an auto-shutoff to protect it from being over-driven. Although the volume wasn’t particularly loud, I’ve seen it shut off in the past from a quiet volume because it didn’t like the home-brewed signal I was sending it. Keeping in mind that I don’t know what I’m talking about, I was thinking that maybe the speaker shutting off caused some kind of inductive spike that overloaded the amp?

Here’s a link to the amp’s data sheet:    https://www.ti.com/lit/ds/symlink/lm4667.pdf

Thanks so much for any help you can provide.

-John Adler

soldar:
My telepathic ability is sort of weak today. Maybe you could post a schematic of what you did annotated with relevant data?

John0922:
Hi Soldar-

Thanks for your reply. The schematic is part of the project instructions that I posted.

rstofer:
With the 'gain' select pulled high, you are using low gain.  However, the + input should be tied to the incoming signal ground according to Figure 28 in the datasheet.
I don't know if it matters.

Why not just buy a cheap 8 Ohm speaker for your testing?  PC speakers are not usually 8 Ohms because they have some electronic amplification internal to the assembly.  I don't know if this is true for your speaker but the device wants to see 8 Ohms, not some magical high impedance from another amplifier.  The fact that your speaker has 'auto off' tells me that it isn't just an ordinary 8 Ohm speaker.

soldar:
If you did exactly what is in the schematic and connected it correctly my guess is it would not smoke.

So you are doing something wrong. Maybe the power supply. Maybe the way your measuring and testing equipment is connected. Maybe the input.  Maybe the output.

Really. You are obviously doing something wrong. Don't tell us what the book says you should be doing. Tell us what you are, in fact, doing.

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