But you can do what the topmost review on the Amazon page has done - to feed the amp trigger output + to both the trigger IN and DC+ and trigger ground to DC- (jumper to H) or trigger + to DC- and IN and trigger ground to DC+ (jumper L). On the trigger side, the typical use case is to have separate 12vDC power supply always on to power it and provide a separate trigger input (which can be less than 12VDC). The relay side is a SPST passive switch and if you use the Rpi 3.3v and GPIO pin here, it would be the same schematic as the ones posted above and you would need the resistors for protection and limiting current draw on the RPi side. It is just a more complicated version of an automotive relay providing nothing additional for your needs and more parts to fail. This is essentially what this set up does but with no risk of high voltage or high current draw anywhere. It is connected to a relay that switches on the main power in a secondary circuit. This is no different from what happens when you hook the trigger out to a device that takes the trigger in. And similarly test the relay switch working by connecting the trigger but not to the RPi to ensure the relay works.
You can first test your relay/Rpi set up with a 12v wall wart adapter before connecting it to the amp trigger. You can also use relays meant for electronics use but the pins are designed more for PCBs and so the connections aren't as easy to do as an automotive relay for a PCB-less, solder-less setup.
As I mentioned earlier, they should draw about 150-200mA and work in that range. You can check the spec of the relay to ensure that it will work with 200ma or less. But if the amp provides close to 12v and up to 200ma, most automotive relays will work. So, the worst that can happen is that an amp does not provide enough voltage or current to switch the relay (which can also happen if you connect the trigger out to multiple devices).
They are designed for the destination to draw up to that amount when on. The good ones are likely even protected against someone putting the trigger out into a trigger out of another device feeding 12v back into that circuit! People do stupid things. Click to expand.Amps limit the trigger current draw (to 200ma in your case) because they have to protect themselves against the trigger out being shorted (with a bad cable or "user error") in the worst (and not rare) case.
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