Nick SayerI'm going to have to take the clock back to the workbench and check some stuff out on the scope. It just doesn't seem completely reasonable that given input levels that are ~3Vpp that the LM4871 can't generate enough output. At the same time, given that the specifications of the speaker are something like 98 dBA 1W/0.1M I can't quite square that with "hold it next to your ear" levels of audio. The input levels ought to be making easily 1W of output, and 1W of output ought to be making far more sound unless I'm misunderstanding something badly.
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Hi, I really like this project!
I took a quick look at your schematic. C20 may be the problem. At 330nF, it presents only 482 ohms of impedance at 1kHz, cutting your gain considerably - it completely swamps the 20k feedback resistor. It makes a nice lowpass filter (4.82k at 100Hz vs 48.2R at 10kHz), but absolutely kills your gain.
If you need the rolloff, you might be better off with a lowpass between the op-amp and the power amp.
Feel free to point out where I'm wrong :-)
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C20 is designed to be an LPF, but by my calculations the combination ought to have a roll-off frequency of.. oh.. doing the math now, I get 24 Hz. It was supposed to be more like 2.4 kHz. That would be 0.0033µF.
I'll try removing it to see what it sounds like.
But unless the audio jumps up super high (like, it starts clipping), I think the preamp might still be necessary.
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Don't put your ear too close :-)
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Wow. You were totally right. See new log entry.
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Sorry you found it after ordering the new boards. I usually find stuff just after the new PCBs ship...
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