There are two common methods of producing a pseudo stereo effect from a mono signal, playing the mono signal from the two speakers in antiphase, and the use of frequency selective techniques which normally consists of directing lower frequency signals into one channel and higher fre­quency signals into the other. This circuit uses the second technique, but can additionally give antiphase signals which can give a better effect, especially when using headphones.

stereo_synthesizer

Q1 is used as an emitter fol­lower buffer stage which ensures that the two filter networks fens from its output are driven from a low impedance source. If these were driven direct from the input, it is quite possible that they would be fed from a source impedance of a few kilo ohms or more, which would be quite sufficient to alter their effective characteristics.

The two filters are formed by R4 and C3 (low pass), and C6 plus R8 (high pass). A high roll off rate is by no means essential in this application and the 6dB per octave attenuation rate of simple RC filt­ers such as these is perfectly ade­quate. The -3dB point of each filter is at approximately 800Hz and the combined output of the filters, therefore, gives a virtually flat res­ponse with no significant peaks or troughs.

Q2 is connected as an emitter follower buffer stage and this ensures that there is minimal loading on the low pass filter. Q3 similarly ensures that there is minimal load­ing on the high pass filter, but this device is also used as a phase splitter. With SW2 switched to take the output from Q3’s emitter, Q3 effectively operates as an emit­ter follower and gives no phase inversion. With SW2 switched to take the output from Q3’s col­lector, Q3 then effectively acts as a common emitter stage with 100% negative feedback (and unity voltage gain) due to R11. 1t also provides a 180° phase shift so that the two output signals are in anti-phase. An in-phase relationship is needed to give a good central ste­reo image and the use of anti-phase signals tends to give an im­pression of increased channel separation.

In a stereo orchestral recording, it is normal for the violins to come from the left hand channel, with the cellos and basses from the right hand channel. Therefore, the high frequency signals are fed to the left channel and the low fre­quency signals are fed to the right channel so that the unit provides a similar effect (although it will ob­viously function properly with the outputs connected either way).