In a 1966 technical documentation we found the diagram of an excellent pre-amplifier tubed equalizer for use with hi-fi amplifiers of the time. The circuit uses two dual tubes and includes the preamp and tone control (equalizer) stage.
The power of this circuit between 300 and 400 V must be obtained from the amplifier itself with which it will operate.
As it is a circuit that operates with very weak and high impedance audio signals and, in addition, the gain is high, great care must be taken with the layout of the chassis and arrangement of components so that the buzzing does not occur.
Signal cables shall be short and / or shielded. Capacitor working voltages must be compatible with those at the point of operation.
In figure 1 we have the complete diagram of the preamplifier and equalizer.
