I've been reading about how rear fill should be mono.
I want to try it.

How do I wire it?

L+R isn't an option on my amp unless it's bridged.

Do I need to wire them in series for 8 ohm and bridge them or could they be run from a single channel.

If so how would I sum the signal?

It is unwise to attempt to sum stereo channels via a crude RCA splitter-reversed way, as this effectively causes a short. The simplest route is, as you've suggested, to bridge a stereo amplifier and connect the speakers in series.

there is nothing wrong with doing this, unless there are other speakers being driven off the same channels, which would now also be getting mono.

I'm also not sure how it would effect the other channels on a headunit.

in other words, I wouldn't do it.

Because a lot is recorded in mono and then panned or centered, a summed mono will add +3db to all center information. You really need a processor to get mono with levels correct. Might be easier to make your rear fill a L-R difference signal, and add some delay to it (20ms is ideal, but beats me how you can get that much without some silly integration of pro audio gear)

you have an amp with 2 channels, one left, one right which uses all four speaker terminals.

leave those in there, and add another pair of wires on the 'bridge' terminals. The speakers attached to this will get a mono signal, which feeds to the two rear speakers. wire the speakers in series, out of phase, to get a surround effect.

this is much the same way that the old 'matrix surround' systems worked.

if you don't want to go off the same amp, then split the RCA's to another pair of amp channels, and do the same, but don't switch the 'bridge' switch on the amp, it must be stereo.

I'll do a pic if you like

L+R is a bad idea for rears, and will only cause you problems.

L-R is a great idea, but needs T/A to work (for the reverb). Look up 'Hafler circuit' or 'Haas effect'.

And if anyone cares, the Alpine H701 does 20ms per channel.......

i remember from old home theatre amps from before the multichannel craze! that you could get a mono or fake center channel signal from a stereo pair buy bridging the two + chanels

So the hafler circuit is passive.
I think this is what Pulse-R described except in the hafler circuit the rears are bridged in series and out of phase between the positive terminals of the front pair.
Would this work if the rears were bridged like this to the rear channel positives?

NO it doesnt...what you talkin about fool ??? please explain ?

The Hafler is passive, yes. I'm not sure i fully understand your question though?

You need to ensure (either via a bridge switch, or simply using a DMM) that both channels of the stereo amp are in phase with each other. Then you simply hook up the positive channels to the positives of the speakers, and the negative coil of the speakers to each other.

Then add T/A.

What I meant was:
The hafler circuit I googled ran fronts and rears off two channels.
Will it still work properly if I hook the rears up this way to two channels by themselves?
But i think you just confirmed it will.

The problem is I don't have time alignment in my current setup.
I'm not sure I can justify buying a pxa h701
How much delay would be available if I update my eclipse hu to a cd7200 mk11 which I was considering anyway?

Do you think a pot between the speaker neg and amp neg to adjust the balance between difference and full signal would work in a car?

Ok I tried this wiring setup today without any time delay.
I was surprised how much seemingly common info is actually reproduced ie. voices that centre in the front stage are still clearly audible through the rear speakers.

My guess would be that the common component would only be 3-6 db quieter than the difference content.
Could this be a phase issue?
I'm unsure how to check phase with a DMM.

With this setup should the rears be full range or band pass?

I assume the delay is to utilise the Haas effect.
I downloaded the operation manual for the cd7200 and max delay is 10ms.
Does it really need 20ms delay to work properly?

Add a 4k7 resistor in series with left + (low level signal), a 4k7 resistor in series with right + (low level signal), join the two shields together and to signal ground of one RCA, then join the other ends of the two resistors onto the center of the same one RCA (have fun knocking together some custom RCAs). If I remember correctly.

If the centre channel information is still audible, then you'll need to flip the phase on one of the channels (channels, not speakers!). You want the opposite of what you're getting.