Above is the circuit I mentioned earlier had been intending to design with a small 3W speaker driven by an LM386 op amp driven by an AD9833 frequency synthesizer chip, driven by SPI (sourced in the schematic from an Arduino Uno).
I thought designing and testing this simple circuit would only take an hour or two. But I got hung up for a lot of hours because, in the breadboard, I was getting weird spurious signals in the output when I had the the circuit set to certain volume levels, and I couldn’t figure out what the problem was. In the end, the problem was fixed by putting a large capacitor across +5V and AGND. After that, this output sounded pure and pleasant, testing with 1000Hz, 1500Hz, and 2000Hz signals. I do not know if it is really necessary to use a capacitor quite that big, but I didn’t feel like doing additional testing.
I had thoughts to turn the circuit into a PCB, though it would be through-hole (easier to assemble) and not a miniature surface-mount design like would be seen in a commercial application. This would be around 6cm x 5cm, with the speaker plugging in on the right pins and the Arduino plugging in on the left header. The GY-9833 module would slide onto the center pins and could be soldered on to them.
I can provide design files later after I finish laying out the traces. I can also post some FlashForth code later.