DC power from speaker amp for power to switch relays?

Question

Can you—or how do you—switch a relay using the power being directed to a speaker from an AV Receiver?

Here's what I'm working on:

In my kids playroom is an iMac on the East wall, and a projector on the North wall. When we want to watch on the projector we turn 90° to the right to a screen we pull across the middle of the room. This has worked great for the past couple years because my stereo system was old and basic and just filled the room with a blanket of sound.

I recently purchased a used AV receiver (Sony STR-DG720) with 7.1 Dolby Surround Sound, now I have problem because my sound system has a definite front to it. I have it set up with the iMac being in the front and the speakers mounted in the drop ceiling (this made the most sense for the shape of the room). Now I'm trying to figure out a way to rotate the surround sound 90° when we watch a show on the projector.

I went through a number of concepts and this is what I would like to make work somehow:

_{[SBR & SBL = "Surround Back Right & Left", SR & SL = "Surround Right & Left", C = "Center" (front), FR & FL (B) = Front R&L 'B', FR & FL (A) = Front 'A'. Look at the speaker labels to see which speaker is switched to which channel.]}

The AV Receiver has a speaker selector for the front R/L speakers for selecting an alternate set of stereo speakers (Front 'A' and Front 'B'). The idea is when I switch from Front 'B' to 'A' with the remote, the power from the 'A' speakers switches all the relays for the rest of the speakers.

The part I don't know is if the power for the speakers can accommodate this. It's 105 watts for each speaker, but I don't know enough to know if that's a constant voltage or not. I imagine I need at least one more component here if it can work in principle, but I don't know what that component is.

Answer 1

No, you cannot drive relays with audio outputs, in any case they will switch weirdly when high power sound is send to them, back to normal when sound is quiet. All this assuming you don't blow up anything.

I really don't know much of high end audio equipment to give more info about this.

The best solution for this scenario I could do myself is some micro controller listening to A and B channels and switching the relays to the last output which generates an audio signal.