On Fri, 7 Jun 2013 13:00:10 +1000, "Phil Allison"
wrote:

Does my schem show input switching ?

Oops. Nope.

And what about this bit ?

" Make "Y" leads to connect the left and right input signals to both
amplifiers."

Switching amp inputs was un-necessary and would create some audible
transients, but simply using a change-over relay to select between two
amplifiers that are producing virtually identical output signals results in
no audible transients on music or speech.

Guilty as charged. I misread the web page. Sorry(tm).

Another possibility when setting up is to connect the speakers across the
hot outputs of each amp - then adjust input levels to get the least sound.
With identical amplifiers, this should result in silence from both
peakers - which only proves the signal gains are equal as neither amp is
driving any load.

While attending college in the 1960's, I worked for a repair shop that
did warranty repairs for several manufacturers of audio hardware. One
of the chronic problems was missing, or backwards electrolytics in the
audio sections. A gain and freq response check would catch these, but
the owner wanted something that could quickly test the amps and tape
recorders. So, I devised a comparison scheme similar to yours, where
an identical and known working unit was compared with the test amp by
placing a speaker between the outputs of the amps. If anything was
heard out of the speaker, the test amp had a problem. The only
difference was that I applied a 10 ohm load to each amplifier output
so that I could catch a backwards output coupling capacitor. It was
also somewhat useful for checking left-right channel balance on amps
that had ungrounded transformer driven speaker outputs. I later
replaced the speaker with a differential input scope to catch high
frequency differences that couldn't be easily heard. Since this was
essentially a production line affair, such quick tests yielded good
returns.

Unfortunately, my efforts on behalf of the shop were rewarded by being
replaced by a less experienced and lower paid tech. The owner
realized that he didn't need a high priced tech to do such simple
tests, so I was transferred to the camera department and did much the
same work, except on cameras.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558