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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.
128x128srosenberg

Showing 2 responses by almarg

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almarg
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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
almarg's avatar
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almarg
8,673 posts
 
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.

Regards,
-- Al
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