are there any ill efects of connecting two amplifiers (systems) to the
same speakers if only one at a time is operated?  I have a 4.0 synthesized
surround system and a separate 5.1 surround system and I would
like to use the same rear speakers for parts of both, but not at the
same time.  The unused amp would be turned off.

kilboj...

I was an Electronics Technician in the US Navy, and can
confirm what spanky2k-ga has said in his comment. 

I would add that even with the second amp turned off,
there is the likelihood that the connection would still
suffer from improper impedance matching as a result of
remaining hooked up to the output circuitry of the 
other amplifier.

Speakers are "impedance matched" so that the resistance
measured across the output of the amplifier as it puts
out audio frequencies matches the impedance measured 
across the speakers. Having the tow amplifiers in 
parallel, so to speak, would likely reduce the impedance
of that connection, and require that the active amplifier
put out more power in order to achieve the same level of
audio output than it would if hooked up by itself.

Some pages that discuss impedance matching are this one
on the hyperphysics site on the Georgia State University
website:
http://hyperphysics.phy-astr.gsu.edu/hbase/audio/imped.html#c2

This Sound Advice article on the WhatHiFi Sound+Vision site
goes into more detail:
http://www.whathifi.com/newsMainTemplate.asp?storyID=34&newssectionID=3


Additionally, if you ever accidentally switched both amps
on for even a short period, you could badly damage one or
both of the amplifiers. I've seen it happen.


The simple answer, as noted, is a switch that allows you
to switch between the systems. If you only need this to
work on the rear speakers of the two systems, this IW-101
A/B Speaker Selector wall switch, from HomeTech Solutions,
will:

"...switch one pair of speakers to either of two stereo
 amplifier outputs", as you desire.
http://www.hometech.com/audio/spswitch.html#OE-IW101


Don't hesitate to ask if there's anything I missed.

Please do not rate this answer until you are satisfied that  
the answer cannot be improved upon by way of a dialog  
established through the "Request for Clarification" process. 

sublime1-ga


Additional information may be found from an exploration of
the links resulting from the Google searches outlined below.

Searches done, via Google:

"impedance matching" speakers
://www.google.com/search?q=%22impedance+matching%22+speakers

speaker switch
://www.google.com/search?q=speaker+switch

good answer, fast.

Well, theoretically there shouldn't a problem with doing this as long
as you don't have both amplifiers running at the same time. Mind you,
the amplifier that's off might not react that well to having power
coming in through a wire when it should be going out. Also you run the
risk of accidentally running both systems at the same time with loud
music and blowing your speakers.

To be honest what would probably be a safer solution would be to have
a pair of switches connected to the wires coming from the amps through
to the speakers. You'd have to make sure you have the right type of
switch since I think you'd need a switch that would switch both wires
instead of just a live wire as most light switches have.

Amp 1 >========[switch]
Amp 2 >========[      ]=======> Speaker 1

Amp 1 >========[switch]
Amp 2 >========[      ]=======> Speaker 2

With this kind of solution you wouldn't really risk blowing anything
since as far as the amps are concerned, the speakers are either
attatched or not. An the speakers see either one amp or the other.

I hope this helps!


Spanky