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July 1, 2009

Question of the Month: Connecting Two Amps to the Same Speakers

Filed under: Question of the Month — Tags: — Alan @ 12:47 pm

Q. So here may be the dumbest idea you have heard all week. I am a confirmed 2-channel guy. I recently bought an Axiom subwoofer that I plan to use only when watching movies. I have been toying with the idea of going “all the way” and setting up a 5.1 system to use separately from my 2-channel system when watching movies. I want to continue to use my existing 2-channel preamp and amp when listening to music. It seems silly to buy new main speakers when I have my existing speakers. I am trying to figure out a way to share my main speakers between a 2-channel setup and a 5.1-channel setup. It’s not possible (is it?) to run 2 amps (2-channel and 5.1- channel) into one speaker as long as only one amp is powered up at a time (two sets of speaker cables to each speaker’s binding posts)? Would this cause a short? I am trying to avoid having to switch speaker wires back and forth every time I want to switch from movies to music. If this is a really silly idea, can you think of any other workarounds? – Mike

A. It’s not all that dumb an idea, Mike, but don’t try it. And you are not the first person to ask this question. It’s not so much the possibility of a “short” but rather that if your amp/preamp were switched off when you decided to use the 5.1-channel receiver, it would be pushing the amplified audio output not only into your left and right main speakers but also into the output circuitry of your turned-off amplifier. The voltages would likely damage the circuitry of your 2-channel amplifier and possibly the AV receiver. Or the reverse would happen if you used the 2-channel amp while the AV receiver was turned off but remained connected to your main left and right speakers.

The electrons from your 2-channel amp don’t know that you just want them to drive your main speakers and not travel up the other speaker wires to your AV receiver. They just go wherever there is a copper wire connection. It’s also possible to set up a switching system that would remove the 2-channel amplifier connections to your speakers when you wanted to run the 5.1 system. There is a manual switch but I won’t recommend it because you might forget about the switch and end up turning on either the 5.1 receiver or the 2-channel amp/preamp when one or the other wasn’t connected to your speakers. Amplifiers don’t like operating without a load on the speaker outputs; they may just shut down. However, Niles makes an audio-triggered automatic amplifier switch that should do exactly what you want. The Niles ABS-1 costs about $100 and automatically switches the speakers to whichever amplifier you turn on. If both amps are turned on, it defaults to the “A” amplifier to avoid damage to the other amp.

Alternatively, consider using the 5.1-channel AV receiver in the 2-channel mode, with or without the subwoofer, and in the surround sound mode for watching movies. All AV receivers offer you that option. Or you could sell the AV receiver and the 2-channel preamp and upgrade to a separate AV preamp/processor with impeccable specs and use your existing 2-channel power amp to power the main speakers for 2-channel listening with or without the subwoofer. You’d need to add a 3-channel power amp for the surrounds and center channel that would function when you want to use the 5.1-channel mode. The pre-outs from the AV preamp/processor would remain connected to all the amplifiers, so it’s as simple as you choosing the operating mode of your AV preamp—either 2-channel or 5.1 channel.

And though you say you are a confirmed 2-channel guy, you just might discover that you actually like some stereo recordings processed through Dolby Pro LogicIIx (or dts Neo:6 or H/K’s Logic7) in 5.1 channels. Sometimes the difference in realism can be a revelation, especially with good jazz or classical recordings and most live rock/pop recordings. If you don’t like the effect, you can simply switch it off and go back to 2-channel listening!

Alan Lofft was, for 13 years, Editor in Chief of Sound & Vision, Canada’s largest and most respected audio/video magazine. He edited Sound & Vision (Canada) until 1996, when he moved from Toronto to New York to become Senior Editor at Audio magazine.
Lofft has been writing about hi-fi and video professionally for over 20 years, ever since his first syndicated newspaper column, “Sound Advice”, began appearing weekly in The Toronto Star, Canada’s largest-circulation daily newspaper. In the late 1970s, he became a contributing editor, columnist, and equipment reviewer at AudioScene Canada, the leading national consumer electronics magazine at the time. Find out more about Alan in his bio.

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