Moduler PreamplifierCircuit

Posted on Feb 7, 2014

To complement the 60 Watt MosFet Audio Amplifier a High Quality Preamplifier design was necessary. A discrete components topology, using + and - 24V supply rails was chosen, keeping the transistor count to the minimum, but still allowing low noise, very low distortion and high input overload margin. Obviously, the modules forming this preamplifier

Moduler PreamplifierCircuit
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can be used in different combinations and drive different power amplifiers, provided the following stages present a reasonably high input impedance (i. e. higher than 10KOhm). If a Tone Control facility is not needed, the Preamplifier will be formed by the Main Module only. Its input will be connected to some sort of changeover switch, in order to allow several audio reproduction devices to be connected, e. g. CD player, Tuner, Tape Recorder, iPod, MiniDisc etc. The total amount and type of inputs is left to the choice of the home constructor. The output of the Main Module will be connected to a 22K Log. potentiometer (dual gang if a stereo preamp was planned). The central and ground leads of this potentiometer must be connected to the power amplifier input. This Module employs an unusual topology, still maintaining the basic op-amp circuitry of the Main Module with a few changes in resistor values. A special feature of this circuit is the use of six ways switches instead of the more common potentiometers: in this way, precise "tone flat" setting, or preset dB steps in bass and treble boost or cut can be obtained. Tone Control switches also allow a more precise channel matching when a stereo configuration is used, avoiding the frequent poor alignment accuracy presented by common ganged potentiometers. Six ways (two poles for stereo) rotary switches were chosen for this purpose as easily available. This dictated the unusual...

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