Simple Surround Sound Decoder

Posted on Feb 5, 2014

This surround-sound decoder is based on the `Hafler` principle, first discovered by David Hafler sometime in the early 1970s. The original idea was to connect a pair of speakers as shown in Figure 1, for use as the rear speakers in the surround setup. This is ok just as it stands, but problems are created if the main speakers are bi-amped or using

Simple Surround Sound Decoder
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bridging, for example, since there is no longer a full-range / full power signal available for the rear speakers. There is also no way to control the level reproduced, since it will always simply be the difference signal between left and right channels. This circuit works by allowing the rear speakers to reproduce only the difference signal between the left and right outputs. All stereo encoded material has some difference between left and right channels (if it didn`t, it would be mono), and it is this difference signal that is reproduced by the rear speakers. It is important to ensure that the connection between the rear speaker negative terminals is not earthed, or they will simply be in parallel with the main speakers. So, if you want to use separate amps for the rear speakers, basically you can`t - unless you get sneaky. The first circuit in Figure 2 is completely passive, but requires that a suitable transformer is available. A suitable transformer means a line level, 10k impedance unit with a 1:1 ratio - these are very scarce (I would suggest almost impossible to get). You might be able to get away with a 600 Ohm unit, but because of the impedances you need, its performance will be very ordinary, with an extreme lack of bass (there is not enough inductance for a 600 Ohm transformer to work satisfactorily at high impedances). Loading the transformer will give back some of the bass, but the preamp is unlikely to be...

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