H   A   F   L   E   R      C   I   R   C   U   I   T
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 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.
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       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.  
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 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.
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 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
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 Have fun,
 Martin Leese
 E-mail: please@see.Web.for.e-mail.INVALID
 Web: https://members.tripod.com/martin_leese/
 19 February 1997
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