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Stereo Preamplifier with Bass-boost
source: RED Free Circuit Designs


High Quality, simple design

20 to 30V supply

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____________22   63V Electrolytic Capacitors
C3________________470F   35V Electrolytic Capacitor
C4__________________1F   63V Polyester Capacitor
C6_________________47nF   63V Polyester Capacitor
C7_________________22F   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 a 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.


  • Schematic shows left channel only, but R1, R2, R3 and C1, C2, C3 are common to both channels.

  • For stereo operation P1, P2 (or SW1), R4, R5, R6, R7, R8 and C4, C5, C6, C7 must be doubled.

  • Numbers in parentheses show IC1 right channel pin connections.

  • A log type for P2 ensures a more linear regulation of bass-boost.

  • Needing a simple boost-in boost-out operation, P2 must be omitted and SW1 added as shown in the diagram.

  • For stereo operation SW1 must be a DPST type.

  • Please note that, using SW1, the boost is on when the switch is open, and off when the switch is closed.

Technical data (30V supply):

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




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