No less important part than the ULF power amplifier is the preamplifier which is not only pre-amplification, but its frequency correction using the tone control.
Figure 1 shows a simple diagram of the ULF pre with tone control for bass and treble and volume control. Transistor VT1 is made not so much pre-amp, how many active tone control.
Figure. 1
The timbre of low frequencies is regulated by a variable resistor R2. The timbre of high frequencies is regulated by a variable resistor R4. Customizability bridge connected between the input and output of the cascade on the VT1, turning it into an adjustable active filter.
The input signal is supplied from circuit adjust the tone without any preliminary stages. If the output impedance of the signal source is small it is quite acceptable. But Vysokomol output, for example, if the signal source should serve the old turntable with the piezoelectric pickup, it is necessary to make a preliminary cascade to increase the input resistance, for example, an emitter follower, as shown in figure 2. In this case, the input signal is supplied to the base of VT2, and the signal at the input of the active tone control is removed from its emitter. The operation of the cascade is set by selection of the resistor R10.
Fig. 2
Mode of operation a DC current of the cascade on the VT1 transistor sets the voltage divider R5-RК6.
Variable resistor R9 is used to adjust the volume. His signal is fed to AF power amplifier.
All capacitors should be at a voltage below the supply voltage.
Author: Poptsov G.
