Triac's appear to be cheap enough, I could buy some and try it out. Actually I may already have some from when I was harvesting transistors and thermistors in old pc power supplies.
That is an interesting project!
Triacs like SCRs require a minimum current flowing through them in order to continue to conduct after the gate pulse ends. They cease to conduct when that condition is not met. So, a triac probably will not work.
I was expecting the circuit to supply gate current throughout the complete cycle, just to keep it on.
Yes that is exactly how the triac in my application works. Remember you are not giving a pulse on each zero crossing of an audio signal, that would be silly, it is just shorting out or not.
Does this mean that a digital pin HIGH/LOW on the nano would work?
Probably, you might need a resistor to limit the trigger current or boost the voltage if the voltage is not enough. You will get these numbers from the data sheet of the Triac. But how do you propose to wire them up?
Seeing as what a triac does is apply a short then I am not sure if using one in series that is going to have the right biasing. Wiring them across a speaker is going to short out all speakers if you drive the speakers in parallel.
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Please let me review this. The suggestion is that a triac can be used similar to an NPN low side switch where MT1 is grounded, MT2 is connected to the speaker (load) and the gate is connected through a suitable resistor to a digital output pin. Setting that pin HIGH will activate the triac to pass an audio signal through the speaker. Setting that pin low will mute the speaker. Is that correct?
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I don’t think that will work because the speaker connection is a floating signal not connected to the ground of the Arduino.
I think I will experiment using my old trusty relay. But out of curiosity, what would happen if I connect - of the speaker signal to Arduino ground?
Depends on how the speakers are hooked up. If they're already GND referenced, no problems.
If Speaker- is the ONLY place in the speaker circuit the Arduino GND is connected too and you're only connected to one speaker, probably no problems.
But if you're shorting together multiple Speaker- lines and they aren't already ground referenced (like if they're Class D bridge-tied load, which is likely), that's bad. You'll be shorting two outputs together, and could overload them.
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Hi,
What is your audio amplifier?
Tom... 
At the moment just a random hifi amp, for testing. I imagine something small and not so powerful in the end. And then a separate full range PA that delivers whole soundscape.
Hi,
Thanks, you will have to make a circuit that does not rely on one terminal of the speaker at gnd potential.
Most high efficiency types use differential output.
A relay would be best, you don't have to have the relay anywhere near the listener, infact if you have your controller in a cabinet or some sort of enclosure, your relays may not be heard.
Tom...
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