That's interesting Andrew. I found out about the Hafler system in the seventies when Sony was failing with their "4 Channel" surround sound product. New equipment and compatible new records required
I could never see the attraction
. Lots of people seemed to be trying to create their own solid state equivalent for "ordinary vinyl" but the theory baffled me, never mind the construction!
The basic Hafler [I think I saw it in Wireless World] was so simple by comparison and 'cheep', as you were mainly recovering out of phase mid range frequencies, so Hi Fi Speakers were not required. My Heath Robinson stereo was a Garrard SP25 Deck, one channel through the Tape Recorder Audio section [it had a PA input] and the other via one of Mr Sinclair's IC boards into home made boxes containing surviving speakers from a burnt out stack. For surround sound, I re-cycled two car Spkrs [the ones encased in a plastic moulding]-not even proper cabinets. They were easily concealed behind the sofa and single wired [in series] from the L+ to R+ terminals. Apparently you could insert a pot as a balance control but I didn't bother with that.
It seemed to work very well but was somewhat random as it depended entirely on the Studio Microphone layout and the amount of 180 degree info they'd picked picked up on the day! My best result was with the Beach Boys LP "Surf's Up" which was a very early warning about the environment. You could hear water lapping behind the sofa at the back of the room. I was told that the Beatles Abbey Road album was very effective as well but I didn't have that on vinyl-only R to R.
I say this because I understood that the system only worked with vinyl records but maybe that's not true? I notice that a vinyl source is the only one that Chris didn't mention but that's probably a coincidence. Vinyl or not, it's so very easy to experiment with this simple but rewarding circuit. I plan to go back to it soon.
Dave W