AC-DC12V

Low-noise microphone amplifier

With the arrival of matched transistors in a single package, it seems that the quest for the zero noise microphone preamplifier has received a new impetus. The present amplifier is based on PMI’s MAT02 transistor pair. The performance of this amplifier is such that even the most demanding audio enthusiast will be impressed by its very low noise contribution.  The amplifier is simple to match to a wide range of microphone impedances and offers a voltage gain of 20 dB or 23.5 dB (amplification of * 10 or * 15 respectively). selectable with a switch. The preamplifier is a two stage direct coupled type with feedback and an input stage that runs on a very low collector current to minimize the noise contribution. The output impedance is about 70 Ω  at gain of 23.5 dB. The preamplifier is built on a compact printed circuit board as illustrated. Its low current drain  (approx. 2.5 mA) and 9 V battery supply make it ideal for portable use in conjunction with a high-quality dynamic microphone. Since resistor R3 is responsible for the input impedance and some other important characteristics of the preamplifier we will have a look at some measured data with different values of R3.

With R3 = 220 kΩ. the input impedance is about 30 kΩ. A THD+N (total harmonic distortion plus noise ) of 0.045% was measured: the noise level was -65dB. measured in a bandwidth of 22 kHz, at an output voltage of 15 mV. Source impedance and overall gain were found to have a negligible effect on these figures.  Changing the values of R3 to 6.8 kΩ resulted in a THD+N figure of 0.042% at an input voltage of 1 mV. and a noise level of impedance dropped to about 1 kΩ. At a source impedance of about 600 kΩ, the THD is smaller than  -95 dB (noise floor). On the down side, however, the lower input impedance results in a reduction in overall gain of about 4 dB. An even lower source impedance of 25 Ω was found to result in a THD of -95 dB.

Parts list.

Resistors :

(1 % resistors metal film from E96 series )

  • R1 = 56.2 Ω ,1%low noise webb2
  • R2 = 22.1 kΩ, 1%
  • R3 = 220 kΩ
  • R4 = 562 Ω, 1%
  • R5 = 866.2 Ω, 1%
  • R6, R7 = 280 Ω ,1%
  • R8 = 2.2 kΩ ,1%

Capacitors:

  • C1 = 6.8 nF
  • C2-C4 = 47μF, 25V , radial
  • C5 = 100 nF
  • C6 = 220μF , 16V

Semiconductors:

  • D1 = 1N4001
  • T1 = MAT02 (Precision Monolithics Inc.; Analog Devices)

Miscellaneous:

  • S1 = PCB mount change-over switch, Fujisoku Type ASID-5M

Low nise 12w

 

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