Posted on Nov 20, 2012 4554
Under: Audio Filters Circuits
Otherwise properly mixed sounds often suffer from a predominant solo voice (which might, of course, be the intention). If
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such a voice needs to be suppressed, the present circuit will do the job admirably. The circuit is based on the fact that solo voices are invariably situated "at the center" of the stereo recordings that are to be mixed. Thus, voice levels in the left- and right-hand channels are about equal. Arithmetically, therefore, left minus right equals zero; that is, a mono signal without voice. There is, however, a problem: the sound levels of bass instruments, more particularly the double basses, are also just about the same in the two channels. On the one hand low-frequency sounds are virtu--ally nondirectional and on the other hand, the recording engineers purposely use these frequencies to give a balance between the two channels. However, the bass instruments can be recovered by adding those appearing in the left + right signal to the left-right signal. The whole procedure is easily followed in the circuit diagram. The incoming stereo signal is buffered by A1 and A2. The buffered signal is then fed to differential amplifier A3 and subsequently to summing amplifier A5. The latter is followed by a low-pass filter formed by A6. You can choose between a first-order and a second-order filter by respectively omitting or fitting C2. Listen to what sounds best. The low-frequency signal and the difference signal are applied to summing amplifier A4. The balance between the two is set by PI and P2 to individual taste. You have noticed that the circuit does not contain input or output capacitors. you wish, output capacitors can be added without detriment. However, adding input capacitors is not advisable, because the consequent phase shift would adversely affect the circuit operation.