A year or so ago I bought a few 1W mono amplifiers from a Chinese eBay seller for cheap – see the attached photos. The only marking on the PC board is YX-1667 but I’m not sure if that’s the potentiometer or the PC board itself.
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I found that if I pushed the 3.5mm plug all the way in to the input jack like it’s supposed to be, it didn’t have continuity, I had to leave the plug pushed halfway into the jack for it to work. So I just left it that way. I checked the jack and it was connected to the volume control improperly, the Ring was connected instead of the Tip.
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So now I decided to correct the problem. I also found that the input had a 0.1 uF (104) ceramic capacitor between the jack and the volume control, which isn’t needed for DC blocking because there is no DC; however it does roll off the low frequencies so I decided it wasn’t needed, but it can be left in the circuit.
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I decided to put a short insulated jumper wire between the outermost unused jack pin directly to the pin of the 100k potentiometer. I soldered it like shown in the PC side of the board, being careful that the jumper didn’t short to the grounded copper foil that is close to the jack pin.
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Another way to do this would be to cut the pad from the soldered jack pins in half. Then solder a jumper from the outermost unconnected jack pin to the capacitor half of the cut pad. This will keep the capacitor in the circuit.
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End of this blog post.