Quadraphonic SQ Decoder Using MOSFET Transistors

I created this quadraphonic decoder specifically for decoding SQ encoded records, using readily available and affordable 2N7000 MOSFET transistors. Since quadrature mathematics has been covered many times over, I felt that it was better to focus on the design so that one could build such a decoder at the lowest possible cost in the simplest possible manner so that almost any hobbyist could build it. The article shows a very clean and simple strip-board layout and plenty of photographs to give the constructor an idea of how it all connects together. The circuit is based on the one shown by Geoffrey Shorter in the March 1973 Wireless World article. In that circuit, they used bi-polar junction transistors (BJTs) which are no longer cheaply or readily available. I simply wanted to see how the circuit would sound utilising the latest MOSFET transistors which is of course a far more superior component compared to BJTs available in 1973. I kept the biasing simple and accurate for a 9 V power supply, and the sound quality turned out to be rather excellent to say the least. If you make this, then you will be pleasantly surprised.

In the first stage, a MOSFET transistor splits the phase so that we get two waveforms of equal amplitude, but opposite phases. One wave form will be at the drain junction of the MOSFET transistor and the other at the source junction, and they will be 180-degrees out of phase with each other. At the source junction, the signal will be in phase with the input signal, however at the drain junction, the signal will be out of phase with the input signal. The RC networks at the drain and source junctions provide the phase shift, however since they attenuate the signal as well, we need the phase splitter biased such that the amplitudes of the two output waves are moderately large. Consequently, the gate is biased at mid-rail with resistors of equal value forming a potential divider.

To produce the 90-degree phase shift, the two signals pass through RC networks and are then recombined. This particular circuit is expected to maintain 90-degree phase shift with ±10-degree deviation in the range 100 Hz to 10 kHz. Ideally we need the RC network to maintain the phase shift over a wide frequency band and not just at one particular frequency; hence many designs incorporate multiple cascaded stages. Hence if I have the time and money, then one future project will be to design a circuit with multiple phase shift stages.

At this point, I should mention that I also have another SQ Quadraphonic Decoder build utilising the Motorola chipset (MCP1312, MCP1314, and MCP1315). An advantage of that build is that the component count is fewer; however the disadvantages are that these ICs are getting expensive and difficult to source and there are fakes around as well. In addition, it requires a 20 V power supply. In qualitative comparison, I felt that the plethora of circuitry within the logic control and the gain control ICs changes the sound quite a lot and audiophiles may not like it. Furthermore, even with this complex chipset, the system still requires fixed cross-channel blending resistors. In comparison, the 2N7000 MOSFET decoder changes the sound much less. The separation between the back and front channels is not as pronounced but the sound is a faithful reproduction of the original. The build layout is also simple, and essentially the components consist of a bag of resistors and transistors that are readily and cheaply available.

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