Part number: 15 Haos Redro Last update: 20/07/2009

Microphone preamplifier module

Resources:

21 Overall schematic FIG | PDF
22 PCB layout FIG | PDF (copper) | PDF (copper+component)

Description:

A simple operational amplifier based single channel microphone preamplifier with balanced input and phantom power injection. This module is suitable for dynamic and capacitive microphones.

Table 1: Component function

R1,2 Phantom power resistors. These allow the small DC phantom power current to flow into the signal lines.
R3 Current limiting resistor to prevent large current spikes from flowing into C1 when the phantom power supply is connected.
R4-7 These form a differential amplifier with IC1A. This stage has a voltage gain of 10.
R8-C9 Interstage DC blocking.
R9,10 These set the gain of IC1B to 10 for a total voltage gain of 100.
C1 Phantom power reservoir capacitor.
C2-4 Common and differential mode noise bypass capacitors.
C5,6 AC coupling capacitors. These block the phantom power supply voltage from the differential amplifier input.
C7,8,10 These reduce the gain of IC1A and IC1B at very high frequencies. These are sometimes required for stability with some models of op-amp.
C11,12 Power supply decoupling capacitors.

PCB layout:

The PCB is 62.5mm long, 30mm wide and uses a single-sided layout. The connectors are aligned along the bottom edge of the board to allow vertical mounting to a motherboard.

Power and cooling:

This module requires a suitable split-rail supply of ±15V and a single-rail +48V supply if phantom power is required. The datasheet of the model of op-amp chosen for IC1 should be consulted for power requirements. This module does not normally require any special cooling measures.

Images:

Completed module
Copyright © 2009 Taiyo Rawle. This document is released under the terms of the TAPR Open Hardware License version 1.0 or at your option any later version as published by TAPR: http://www.tapr.org/ohl.html