amp outs, which is – which is +?
Pin 2 of each connector (i.e. the ones facing the edge of the board, by the L and R labels) is positive, in the sense that the signal there will be in phase with the line output.
That said, in practice it probably doesn't matter too much as long as both speakers are connected in the same direction. And for future reference, each amp IC contains two amplifiers in a bridged configuration, so both output pins are active, with opposite phase, meaning neither pin can be connected to system ground.
The power amps expect an 8ohm load, what is the impedance of your tweeters?
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that's the weird part about it... when I measure them (resistance), they have ininite resistance...
Hmm they should normally read a few (2-8) ohms from a multimeter, unless they have a built-in series capacitor?
yepp, that's what I thought... I cannot tell about the cap without breaking one of them... which would be a shame...
but I think I'll anyhow use separate amps now, the built-in ones are quite little power...
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Yeah, that's about as much power you can get out of a 5V-powered AB amplifier on a 8ohm load.
Getting more power would require a step-up power supply.
this one's cool: https://www.adafruit.com/product/987 (Stereo 3.7W Class D Audio Amplifier - MAX98306)