Wiring a Center Channel

I recall reading of a method of wiring up a center channel in phase with the mains, when using a two channel setup and dual mono amps. I think it went somethin like this:

Connect the two mains conventionally to the appropriate 8ohm tap (assuming appropriate) and the ground on each of the two mono amps. Run a low resistance conection from one ground to the other on the two amps. Wire the center channel off the two (hot) 4ohm taps, one off each of the two amps.

What confuses me is the center channel having two hot wires running into it with no ground. Have I read the diagram wrong, or does this all make sense to someone who knows better than me? And why bridge the ground on the mains?

Can anyone clear this up for me?

What this achieves is not a center channel but a difference channel which might best be placed in the rear. It is a variant of the "Hafler" circuit.
Kr4 - Can you explain that further...to a layperson? I'm not understanding quite how that works with two hots running to the "difference" channel. How does it differ from a 'true' center channel?

Thanks for the input!

For a center channel, one wants to have only those signals that are common to the left and right channels. (It's a little more complex than just summing but similar.)

However, by connecting the speaker between the two hots means that current will flow only when there is a voltage difference between the two hots. Thus, it will reproduce a difference.

Playing and placing such a speaker between the left and right will not fill the middle but, rather, increase the effective separation.
Thank you. Has anyone ever tried this? What then would be an effective means of wiring a true center channel without benefit of a third amplifier? Would one just pull both hot leads to a common hot on the center, and both grounds to the common ground?

If you connect the hots, you get no stereo. To sum, you need an additional amp with a combining network.
Shouldn't you check with manufacturers before doing this? Some receivers/amps will not like this?????????
Good point. Going back to the original post, one should not add the ground(-) to ground(-) jumper since the +to+ connection should work with common ground amps and shouldn't be used with non-common ground amps.
This is excellent information. Thank you!

Yes, you will get a difference signal, and if you have inverted one stereo channel signal, this difference signal is indeed the common mode component of the stereo signals and is what you want to drive the center speaker. Of course, you must connect the speaker of the stereo channel that is inverted backwards so that its audio output is properly phased.

I used this kind of center channel drive for many years, with the signal inversion being done by hooking up one channel of the phono pickup backwards. This is far and away the best way to derive a center channel from a two-channel amp.