LM318 microphone preamplifier circuit design
This simple microphone preamplifier circuit is based on the LM318 op amp . The LM318 op amp is operated as a standard non-inverting amplifier. Resistor R1 provides an input path to ground for the bias current of the non-inverting input. The combination of R2 and C2 provides a frequency roll-off below 30 Hz. At 30 Hz and above the gain is relatively flat at about 50 dB, set by the ratio R3/R2. R3 furnishes negative feedback from the output to the inverting input of the op amp. C3 ac couples the preamp to the tone control section.
The top half of the tone control section is the bass control. The bottom half controls the treble frequency response. These tone controls (R5 and R8) require audio taper (logarithmic) potentiometers. The 50 k ohm potentiometer on the output can be used to set the output or gain of the preamp .