Simple Preamplifier

1 R3 This design answers the need for an inexpensive, yet good quality, preamplifier equipped with a tone control section. Fig. 1 shows the circuit dia- gram. The amplification of the input stage set up around opamp Al is adjustable between 10 and 20 with preset Pl. The 0 dB level at the input is 50 mV, while the input im- pedance and capacitance are 47 KQ and 47 pF, respectively to enable ready connection of most record players and cassette decks. The tone con- trol section is a standard Baxan- dall type with P3 and P4 as the respective bass and treble con- trols. The gain vs frequency curves for various settings of the tone controls appear in Fig. 2. Here the 0 dB level cor- responds to 1 V. The current consumption of this preamplifier is modest at about 5 mA. When the circuit is cor- rectly balanced, the indicated measuring points should all be very nearly at ground potential. The circuit shown here must, of course, be duplicated to obtain a stereo preamplifier. St 2 C5 Up 47p ...