Is It Safe To Connect Speakers To Two Amps??

i've got a combined HT and 2-Channel rig whereby my mains and subs function in both. i'm currently using the traditional connection scheme whereby the HT processor sends signals to the mains via the pre outs to my preamp (in bypass mode) then on to my main power amps, then on to the speakers.

if i wanted to bypass my preamp and main power amps althogther (it's a shame to burn tube hours on movies), is it safe to simply connect my main speakers to the HT processor via speaker level connections if they are also connected to my main power amps concurrently? only one amp - HT processor OR main amps - would be running at a time.

any inputs would be greatly appreciated.
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.

-- 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:

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.
if my primary reason was to protect a quality tube amp in the first place, I definitely would not risk it.
The relay system I'm referring to would be in the switch. I should have said the protection system used in the switch, might offer zero protection for the amp, in this case.
I've been using the Niles unit for nearly 10 years without issues, it does exactly what you want.
Tklp, what tube amp are you using?

Niles themselves have concerns about keeping a load on the tube amp at all times, in this speaker switch(link below). They recommend an extra resistor on the tube amp at all times on some products. I don't know if the 150ohm,5 watt resistor would be safe for all amps. I would not try it, unless the company that built the amp says ok. The other switches do not even mention tube amps, and whether it would be safe. Maybe the manufacturer of your amp could recommend what to do. This PDF link is for a speaker switch, that expresses the load at all times. Pages 4-7. []
One other possibility might be to shut your tube amp down (its own power switch) while the music is still playing. I don't know if this this would be safe for your tube amp. If not, let the tube amp at least cool down enough so the tubes can't amplify anymore (few minutes?). Then switch it over with a good switch that doesn't use resistors while playing. This might be one to ask your amp company to see if it's safe to do. Others please chime in, if this is not a good idea. I'm not sure.
Then switch it over with a good switch that doesn't use resistors while playing.
Let me clarify. I don't mean change the switch over with music playing, just no unneeded resistors a switch might put in the signal path. Definitely the amps should be off to change over. Sorry.
Hifihvn raises an important point about tube amps. They should indeed never be operated without a speaker load, or a reasonably equivalent high power resistor as a load. I too have my doubts that 150 ohms is low enough to be suitable in all cases. And a substantially lower value would, with most amps, mean that the power rating of the resistor would have to increase significantly.

The reason a load is needed is that with no load, abrupt changes in the amount of current that is fed into the primary of the output transformer can produce extremely large voltage spikes, due to a phenomenon called "inductive kickback." The result can be a ruined transformer or output tubes.

The concern is most significant if music is playing through the amp. However, I would not rule out the possibility that even with no signal present, turn-on or turn-off transients might cause a similar effect, perhaps cumulatively over time if not immediately.

-- Al
The tube amp would not be on if the other amp is on. The switch can be set for the speakers to go to the tube amp when it is turned on and when turned off to switch to the other amp. There is a delay to allow the amps to shut down completely.
Almarg, thanks for clarifying.
Hi everyone

I just wanted to add to this thread should anyone stumble upon it like I did. I was searching for an amplifier selector as I have 1 solid state amplifier and 1 tube amplifier that I would like to use with only 1 pair of speakers. (I am too lazy to remove and hook-up an amp periodically. I wanted both in place and the ability to switch between them.) From searching I thought the Niles Audio SAS-1 (Automated Switching System) was a viable solution.

I found this thread and saw the discussion about the potential issues with using this type of switcher with a tube amp. I contacted Niles Audio for their input as well and here is what they said:

“The same note from the speaker selectors applies to the SAS-1 in regards to tube amplifiers. Most Tube amplifiers need load connected at all times. I suggest that you connect a 150 ohm, 5 watt resistor across each channel of the tube amplifier’s output in parallel with the SAS-1.

Have a nice day.”

Niles Audio were quick to respond as well.

Obtain a 2 channel/2 amp speaker switch. That way there is no way both amps are paralled at any time. You can then switch between then amps (both amps being off when you operate the switch). I know there are several companies that offer such a switch.

Hey Minorl

I wish I could e-mail you for the companies you've come across that have these 2 channel/2 amp speaker switches.

Other than the Niles ones mentioned I found this speaker/amplifier selector.

OSD Audio ATM-7 7 Zone Speaker Selector with Remote Control

Is this a viable solution or one of the companies you know of?
I'll do some looking. I was pretty sure Niles audio had one, but I'll check.