THE STEREO FILTER
An interesting feature of the recent CARC Receiver Evening was the stereo filter brought along by Stewart Bryant G3YSX. This was to the original design of G4BWE and is a purely passive device for connection between the headphone socket of a receiver and a pair of stereo headphones, with just an on-off switch to bring it into circuit. Stewart had built the unit into a small box and the non-polarised capacitors were made up from two electrolytic capacitors in series, to minimise the size.
Essentially the stereo filter, or as G4BWE christened it, the Stereocode Filter, is a high pass and low pass filter combination, with high frequencies going to one ear piece and low frequencies to the other ear piece. The claimed effect is to produce, from a mono input from the speaker jack socket, a quasi-stereo output, with the position of the cw images dependent on their audio frequency. Tuning the receiver thus alters the relative position of the "stereo" image.
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Stewart had thoughtfully provided a lead so that the filter could be quickly connected to any of the receivers and transceivers present. Whilst it was only possible to make a quick subjective test on the evening, with the filter connected to the G3GRO FT990, there was a noticeable difference when the filter was switched into circuit. This resulted in a degree of stereo, or bi-naural effect, which although not so marked was similar to switching from mono to stereo on your domestic receiver or car radio. This was also evident on reception of voice signals.
For such a simple circuit the result was interesting and quite pleasing, adding some directional quality and apparent depth to the received signal. I understand that, encouraged by the results from the basic design, Stewart is now considering a more sophisticated approach using active components, as time permits.
John G3VLH can be reached at By Email