Circuit diagram:
Parts:
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
Comments:
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.
Note:
- C4 should be mounted as close as possible to the supply pins of IC1.
Technical data:
Input Impedance: 2KOhm
Sensitivity:
- 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
