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Stereo preamplifier with active tone control for bass and treble

This is a tone control unit for use in higher quality stereo devices. The bass and treble setting circuit is in the negative feedback branch of the middle transistor stage. This arrangement achieves a linear gain when pots with center taps are used, so in middle position bass and treble are neither lowered nor raised. The adjustment range is in the range of just under ± 20dB.

The input stage with bootstrap arrangement provides a high input impedance of over 100kΩ. The voltage is amplified here only by a factor of two, whereby the losses of the balance adjuster are removed, so that the assembly has no total voltage gain. If all the controls are in the middle position, this results in a total gain of 0dB. In the extreme settings of the adjustment pot, the balance setting each causes one channel to be raised by 6 dB, while the other is then lowered by just under 15 dB. As a common collector amplifier the output stage works as impedance converter. The properties of the tone control network and the balance setting therefore remains unaffected by the connected output load. So power amplifiers with input impedances of less than 5kΩ can be drived. The maximum output voltage is about 2 volts.

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