Mixing left/right on 2 channel amp

[NRG = mc²]

Hi all,

what I'm trying to do is run three speakers from a 2 channel amp. I want the third speaker to play the signal from both right and left channels. I understand I need a circuit to do this, does anyone know if it is easy to build such a circuit, what is the degradation in sound quality like, or if they are available on the market?

Thanks :)

[Howell]

How loud do you want the three channel to be on a percentage basis of how load the two channel is?

[NRG = mc²]

The same, or perhaps a little lower? I'd have to experiment though if possible. It might not sound any good regardless though :-?