TOPIC:

My flex is using 3 out of 4 outputs, so 1 output is free. Can I use a cable like this one in the free RCA out to send a trigger signal from the flex to the amp? It needs a 12V signal.
www.amazon.com/CableWholesale-6-Feet-3-5...6/dp/B004Y39L2W?th=1

 

No you cannot. At first the Flex outputs a AC signal. The 12V trigger needs a 12V DC continuous signal.
Also the output cannot provide the power to switch the relay in the amplifier. It needs approx 200mA IIRC.

What you could do is use an old 12V adapter you have laying around, for example from an old external harddisk and use that 12V. If you connect that to a wireless switch you could use that as a manual trigger.

Can you tell me more please? I have a 12v adapter. What is a wireless switch? Like a smart plug?

What do I connect to the trigger input on the amp? 

Indeed like a smart plug, with a RF remote control, or zigbee or whatever system you like.

In most cases you can use a mono plug with +12v at the tip. Remote the normal power connector and replace it with a mono mini jack.

Of course this is at your own risk. I would suggest to measure the correct trigger voltage and polarity yourself, or check the manual.