Quadraphonic SQ Decoder Testing

At first, I tested it with signal from a cassette tape deck, playing a 1980s Madonna cassette and the back channels sounded extremely clear. I also tested it with signals from a CD player, playing my favourite Girls Aloud CD and the sound was sharp and clear. One thing I noticed was that the back channels tend to have slightly lower signal level most of the time and occasionally they kick in when the sound is meant to be there.

Since the back channels were not as loud as the front, one option was to reduce the gain of the front channels, alternatively, add another gain stage for the back channels. However whilst testing, it occurred to me to feed the back channels into a separate bass and tone circuit to listen to how it sounded. The extra headroom enabled me to boost the bass and treble to maximum without signal distortions, and feed that to another separate power amplifier. This had the effect of making the back channels sound drastically different from the front channels thereby giving greater perceived separation between the back and front speakers. Consequently, after much testing, I decided to leave the circuit as it was.

As you can see, the quadraphonic decoder circuits were installed into a CD player case. The unit was beyond repair as it required a new laser pickup, thereby making repairs uneconomical, however the case was exactly what I was looking for. It has a nice metal chassis for mounting the boards and power supply, and it is a slim design so it will not take up too much space.

With quadraphonic records you need very high-end high-quality equipment for playback because if your turntable has rumble, and the stylus or record is worn giving fuzzy sound, then you will hear all of that through the back channels. At first I used a non-SQ encoded record and a standard cheap cartridge with stylus, and the back channels sounded average. However when playing SQ encoded records with a high-end cartridge, the back channels instantly came to life and I could hear the music that was meant to be there. I played the Mike Oldfield Exposed record which has SQ encoded music and it sounded great. The Mike Oldfield record is probably the best to try out first as it has many different musical instruments, and if you are an electric guitar fanatic then you will like it even more. You can pick these records up for as little as a fiver from charity shops, so it should not break the bank. With this record, instruments on the back speakers could be heard clearly giving the surround sound experience.

The circuits in this design produce phase shifting within a narrow audio band, however it occurred to me that multiple cascaded stages with RC networks could theoretically produce phase shifts over a wider band, which could improve the surround sound quality greatly. It would also be necessary to have multiple gain stages to compensate for the signal attenuation, inherent in these types of networks. Therefore, keep your eyes peeled on this site because if I have the time and money, then I might design and make another circuit.

In this design, I felt that it would be useful to place the mixer resistors on a separate board, this way; it would be possible to experiment with different mixing schemes. Hence, using male and female riser pins, it is possible to create a system whereby the mixer board could be plugged into another main board. Unfortunately, I was unable to afford riser pins, therefore I decided to break apart a chip socket and use the pins from that. It worked much better than I had hoped and much cheaper!

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