Microphone Preamplifier

One-IC based High Quality unit

Very low current drawing - 9V battery supply

Circuit diagram:

Microphone Preamplifier


P1_____________470K  Linear Potentiometer

R1_______________2K2 1/4W Resistor
R2______________56K  1/4W Resistor
R3_______________1K  1/4W Resistor
R4,R6,R10,R11___22K  1/4W Resistors
R5______________10K  1/4W Resistor
R7_____________560K  1/4W Resistor
R8_____________560R  1/4W Resistor
R9_____________100K  1/4W Resistor

C1_____________470nF  63V Polyester Capacitor
C2_______________1nF  63V Polyester or Ceramic Capacitor
C3_______________1µF  63V Polyester Capacitor
C4_____________100nF  63V Polyester Capacitor
C5,C6____________4µ7  25V Electrolytic Capacitors
C7______________47µF  25V Electrolytic Capacitor

IC1__________LS4558  Dual High Performance Op-Amp (or MC4558)

SW1____________SPST  Toggle or Slide Switch

B1_______________9V  PP3 Battery

Clip for PP3 Battery


This preamplifier is based on a rather unusual circuitry: two low-noise high performance op-amps amplify the very low amplitude signal coming from a microphone. The first op-amp of a dual-IC is wired in the non-inverting configuration and the second op-amp that follows is, on the contrary wired as an inverting amplifier.
This allows to connect the level control into the feedback path of IC1A and at the same time also to ICB.
In fact, when the cursor of P1 is rotated towards R5 the gain of both op-amps is reduced and vice versa. The result is an excellent circuit headroom and the ability to use a linear potentiometer for P1.
Due to the use of a High Performance audio IC, other specifications (signal to noise ratio, frequency response, Total Harmonic Distortion, low voltage supply and very low current consumption) are also first class.


Technical data:

Input Impedance: 2KOhm


1.17mV RMS for 316mV RMS output [-10dB(V)]
2.8mV RMS for 0.775V RMS output [0dB(V)]

Maximum undistorted output: 2.7V RMS [+10dB(V)]

Frequency response: flat from 20Hz to 20KHz

Total harmonic distortion @ 1KHz and 10KHz:

about 0.01% @ 0dB(V)
<0.1% @ +10dB(V)

Current drawing: <2mA @ 9V supply