H A F L E R C I R C U I T ______________________________________________________________________ I used the four-speaker variant of this Hafler circuit for 18 years to play stereo records. It is roughly equivalent to a Dolby Surround decoder and works well for Dolby MP encoded sources (I am told). It does not compare with an Ambisonic Surround Sound decoder, but is a great deal cheaper. ______________________________________________________________________ Right + ----------------------------------------------- | | P + + Right front Right rear O A - - --- - Centre front + --- | | W M Common - ---| |---+--- x Pot x ---| ---- x Dummy load x ---- | | E P - - Left front Left rear R + + | | Left + ----------------------------------------------- Notes: 1) This circuit puts strange loads on your power-amp. Use at your own risk. 2) For only four speakers, replace the centre front speaker and dummy load with a short circuit. 3) The dummy load reduces the volume of the centre front speaker. Theory suggests 19 ohms for 8 ohm speakers. 4) The potentiometer (Pot) needs to be wire wound to take the power. I used 50 ohms with 8 ohm speakers. 5) To "decode" stereo sources I found a pot setting of about 25 ohms was best. There is no best setting for mono. 6) To decode Dolby MP sources set the pot to maximum, omit it, or put it in series with the rear speakers. ______________________________________________________________________ The circuit works like this. If you set the pot to zero, the rear speakers are fed the same signal as the input. If you set the pot to a high value (or omit it), the rear speakers are fed the pure difference between the two input channels. For stereo, you will want to set it to an intermediate value to blend a proportion of the input with the pure difference into the rear speakers; I set mine to about 25 ohms with 8 ohm speakers. The "correct" setting will be slightly different for different stereo recordings, but you will soon get tired of varying it and settle on a single compromise setting. For mono recordings there is no "correct" setting. The centre-front speaker is always fed the sum of the two input channels. ______________________________________________________________________ A blend pot being connected to ground was first described by the late Micheal Gerzon in 1971. You can download his article "A year of surround-sound" from the Gerzon Archive: http://www.audiosignal.co.uk/Gerzon%20archive.html ______________________________________________________________________ Have fun, Martin Leese E-mail: please@see.Web.for.e-mail.INVALID Web: http://members.tripod.com/martin_leese/ 19 February 1997 ______________________________________________________________________