Is It Safe To Connect Speakers To Two Amps??

No. The amp being used would see a load impedance equal to the speaker impedance in parallel with whatever output impedance the other amp has when it is turned off. That output impedance, in turn, is indeterminate, at least without detailed knowledge of the design. It may also vary unpredictably as a function of the signal level being applied to its output terminals at any instant of time by the amp that is being used.

Given that, both amps could conceivably be damaged -- the one being used due to excessive loading, and the one not being used due to application to its output terminals of voltages that are widely in excess of its internal dc voltage "rails," which would be zero in this situation.

And if both amps were ever inadvertently on at the same time, while one was playing music, the amp being used would be working into a near-zero ohm load impedance, corresponding to the output impedance of the other amp when it is powered up. That would result in very large amounts of current flowing, which could also damage both amps.

Regards,
-- Al

if both amps had automatic speaker disconnect relays in them, and you were extremely careful, presumably a setup like this could work. but it's an accident waiting to happen. As mentioned.. when accidentally both amps are powered on at once.

If you wanted to install a manual high quality A/B speaker selector switch... while a bit of a kludge, I guess it'd work.

Niles Audio makes a signal sensing automatic switch just for this purpose.

06-19-11: Rwwear
"Niles Audio makes a signal sensing automatic switch just for this purpose."

Here is the Niles switcher that was referred to:

http://www.nilesaudio.com/images/PDF/SPK-1_Cutsheet.pdf

http://www.onecall.com/product/Niles/SPK-1-Black/Speaker-Switching-Device/_/R-867

Bill

No, it's way to risky. I don't know how the Niles is made, so I wouldn't risk it. A lot of switches are made with solid state only in mind. And you did mention tubes. First off, with any switch, the plus and minus both should be isolated, through the whole switching device.

With tube amps involved, there should be a load resistor on a tube amp when the switch is in the other amps position. That way there would be a lot less risk. Even if your careful enough to not make a mistake. If you switch off the tube amp to turn on the SS amp for the TV, the switch needs to have a load(load resistor) on the tube amp. The reason would be the tube amp power supply would hold a lot of voltage stored in it when turned off, and any little noise could cause the output transformers to short out. This noise could happen during the switching, or a tube or other part while cooling down. It needs a dummy load, or speaker load on it all the time, just in case there is that voltage stored in it. It is quite common for tube amps to hold that reserve voltage, for unknown lengths of time. To many unknowns make this a risk. That relay system might have zero protection for this.