I bought a B&O ambiophonic adaptor from eBay for striking price of £10.50. As far as I know this is a very simple method of creating "surround" sound from stereo by adding two rear speakers in reversed phase. All info on Beoworld and Beocentral is pretty basic. Was there any sort of manual or description available for this unit or how to use it? Do the rear speakers affect the total impedance seen by the amplifier? How well/badly did these unit sell? Are they rare? Has anybody used these and what is the general opinion in ambiophonic sound? At least this unit seems to be very simple by construction, there are only some resistors and a small transformer inside the case (with the switch, cables and plugs).

I have tested it quickly; I have a Beomaster 3000 (1960's) with Beovox CX50 speakers on my bench. I connected a pair of Tandberg Fasetts (lovely speakers!) as rear speakers (holding them by hands as I didn't have any space to put them!) and there is indeed some feeling of space added. The rear speakers can be set to limited treble, normal response or off by the switch on the adaptor unit.

And as we all love photos, here we go!

The unit itself, small and stylish. 

These plugs are connected to the speaker connectors on the amp. The front speakers connect to the rear of the plug. The rear speakers are connected to the plugs in the adaptor unit.

Switch to deactivate the rear speakers or to select using reduced treble or normal response in them.

I used to build setups like this as a kid. The basic idea is to direct the difference between the main channels to one or two rear speakers, with a number of variations on the details.

Haven't bothered in 25 years or so, but I do remember that it sounded pretty good when you first connected it up - and at least equally good when you disconnected it shortly later

That's a nice looking box though and a great addition to any collection at that price

Some B&O amplifiers have this built in. Here is a link to a forum with a nice simple explanation! Look up Hafler ambience if you want more!

Yes, that's a nice little addition to your collection.
And at a very attractive price !

A sheet came with the unit but it merely pictures what you
just explained:

Martin