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How to switch a speaker on and off?

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Starlord:
I have a 40W digital amplifier, in a mono BTL configuration (left and right channels both fed into a single output), powered by 12V and being fed into two 4 ohm speakers:
http://www.ti.com/lit/ds/symlink/tpa3131d2.pdf 

I would like to add a third smaller 8 ohm speaker and be able to switch it on and off, without using a relay, and without hearing a pop when it's turned on and off.

I'm not sure how to accomplish this though. 

I looked at some analog switches but didn't see anything that could handle anywhere near the amount of current required.  Also I believe the push-pull nature of the BTL setup means the voltage has to be able to swing negative, and a lot of analog switches don't support that.

I thought about sticking a couple nfets back to back like when you want to block current both ways, but I'm not sure if that will work with the negative voltage swings, and it probably won't do anything to avoid popping.  I may be able to mute the audio when switching them however to avoid a pop. 

Another thing that concerns me and that I don't know how to deal with is that the speaker is an inductive load, and will cause a voltage spike when switched off, but unlike a motor I might drive in only one direction, I can't just stick a diode across it.  I'm not sure how an h-bridge deals with this issue, but it's something that's probably going to be difficult to deal with.

Any ideas?

Pjotr:
Look for an optical coupled MOS FET, i.e. NEC PS710B. It is bi-directional and can stand 60V (off) and 2.5A (on). There are many flavours of optical coupled MOS FETs with various withstanding voltages and carrying currents. If you are afraid of inductive voltages from the speaker, simply use catch diodes from speaker to rail and ground on both sides for a BTL amp. But if it's a small speaker the fets will probably catch the small turn off energy spike easily in avalanche.

BillW50:
My Marantz 4400 (a quad from the 70's) uses a relay to switch four speakers on and off and it doesn't have a pop. You can also add another four speakers (they call remote vs main) that is also on a relay. I do know the relay has a delay to allow the amplifiers to stabilize first. That might have something to do with this. But they probably have other circuits to avoid the pop too.

retrolefty:

--- Quote from: BillW50 on July 29, 2015, 09:43:26 pm ---My Marantz 4400 (a quad from the 70's) uses a relay to switch four speakers on and off and it doesn't have a pop. You can also add another four speakers (they call remote vs main) that is also on a relay. I do know the relay has a delay to allow the amplifiers to stabilize first. That might have something to do with this. But they probably have other circuits to avoid the pop too.

--- End quote ---

 I agree, most speaker pop comes at power on and power off which was handled by most of the great vintage hi-fi amps and receivers (70s) with a delay/speaker protection relay. Once stabilized one could switch to different speakers without artifacts. Many such amps had termination for two sets of speakers that could be switched between or all on with just a standard front panel switch or button(s). You just had to insure you don't switch in too low a total speaker impedence (<4 ohms) when driving parallel switched in speakers.

Starlord:
Is it possible to do this in a solid-state manner?  I looked at a few solid state relays but I can't seem to find any which look like they'd accept a negative voltage swing.

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