This preamplifier is designed to fit CD players, equalizers, tape recorders, and more, providing an AC voltage gain of 4 to drive less sensitive power amplifiers. Because modern Hi-Fi equipment is often equipped with small speakers, the low frequency range will be sacrificed quite a bit. This circuit also has a bass amplifier, which is intended to overcome this problem. You can use a variable resistor to adjust the bass gain from 0 to a maximum of +16dB at 30Hz. If a fixed maximum gain is required, the variable resistor can be omitted and replaced with a switch.
Parts:
P1 10K
R4 10K
C1 2.2uF-25V
C6 47nF-63V
P2 100K
R5 22K
C2,C5 2.2uF-25V
C7 22uF-25V
R1,R2 100K
R7 1K
C3 470uF-35V
IC1 TL072 BIFET Opamp
R3,R6 15K
R8 470R
C4 1uF-63V
SW1 DPST Switch
Notes:
The diagram shows only the left channel, but R1, R2, R3 and C1, C2, C3 are common to both channels.
To operate the stereo, P1, P2 (or SW1), R4, R5, R6, R7, R8 and C4, C5, C6, C7 must be mirrored.
The numbers in parentheses indicate the right channel IC1 pin connection. Log type for P2 provides more linear bass gain adjustment.
Requires simple step-in and step-out operation, P2 should be omitted and added SW1 as shown in the diagram. For stereo operation, SW1 must be DPST type.
Please note that when using SW1, the acceleration will turn on when the switch is open and off when the switch is closed.