Stereo Preamplifier with Bass-boost

High Quality, simple design

20 to 30V supply

Circuit diagram:

Bass-boost preamp


P1_________________10K   Log.Potentiometer (Dual-gang for stereo)
P2________________100K   Log.Potentiometer (Dual-gang for stereo) (See Notes)

R1,R2_____________100K   1/4W Resistors
R3,R6______________15K   1/4W Resistors
R4_________________10K   1/4W Resistor
R5_________________22K   1/4W Resistor
R7__________________1K   1/4W Resistor
R8________________560R   1/4W Resistor

C1,C2,C5____________2µ2   63V Electrolytic Capacitors
C3________________470µF   35V Electrolytic Capacitor
C4__________________1µF   63V Polyester Capacitor
C6_________________47nF   63V Polyester Capacitor
C7_________________22µF   25V Electrolytic Capacitor

IC1_______________TL072  Dual BIFET Op-Amp

SW1________________DPST  Switch (Optional, see Notes)


This preamplifier was designed to cope with CD players, tuners, tape recorders etc., providing an ac voltage gain of 4, in order to drive less sensitive power amplifiers. As modern Hi-Fi home equipment is frequently fitted with small loudspeaker cabinets, the bass frequency range is rather sacrificed. This circuit features also a bass-boost, in order to overcome this problem. You can use a variable resistor to set the bass-boost from 0 to a maximum of +16dB @ 30Hz. If a fixed, maximum boost value is needed, the variable resistor can be omitted and substituted by a switch.


Technical data @ (30V supply):

Ac voltage gain @ 1KHz: 4

Max. input voltage @ 50Hz: 500mV RMS (280mV RMS @ 20V supply)

Max. input voltage @ 100Hz: 700mV RMS (460mV RMS @ 20V supply)

Max. output voltage: >8V RMS (>5V RMS @ 20V supply)

Max. bass-boost referred to 1KHz:
400Hz = +2dB; 200Hz = +5dB; 100Hz = +10dB; 50Hz = +14dB; 30Hz = +16dB

Total harmonic distortion @ 100Hz and 1V RMS output: 0.02%

Total harmonic distortion @ 1KHz and 1V RMS output: 0.006%

Total harmonic distortion @10KHz and 1V RMS output: 0.007%

Total harmonic distortion @ 100Hz and 5V RMS output: 0.02%

Total harmonic distortion @ 1KHz and 5V RMS output: 0.0013%

Total harmonic distortion @10KHz and 5V RMS output: 0.005%

Current drawing: 2mA