A simple stereo preamp with tone control
The stereo preamplifier shown here has separate bass and treble controls, a volume and balance adjusters and a contour switch. With this, the frequency response can be adapted to the frequency-dependent sensitivity of our hearing, depending on the volume set. The circuit of the tone control is often referred to as a "shelving equalizer" in the specialist literature.
The amplification of the single-stage arrangement is canceled again by the tone control network. In connection with a power amplifier that has sufficient input sensitivity (e.g. 500mV), such as the 50W power amplifier shown elsewhere on this website, you get an integrated amplifier with the corresponding input sensitivity.
The circuit can also be easily combined with IC power amplifiers (e.g. TDA2030) to form an integrated amplifier. The template for this circuit was provided by the corresponding circuit part of a stereo amplifier with a hybrid power amplifier module from Sanyo from the 1970s. Instead of the specified transistor of type 2SC1571, a BC239 or better still a BC550 can be used with good success.
By using a bootstrap arrangement, the input also allows the connection of very high-impedance signal sources. A selector switch at the input could be used to switch between tape, radio and CD, for example. To connect a magnetic record-player pick-up system, a two-stage preamplifier with cutting characteristic equalization is required, as shown elsewhere.