Among the tasks solved with the help of electret microphones, one can distinguish the sound of large rooms (for example, conference rooms, temples, etc.) with a relatively large distance from the sound source, which requires high sensitivity and noise immunity. Industrial microphones for such purposes are quite expensive and, in addition, require an autonomous power source for the preamp.
The purpose of this development was to reduce the cost of manufacturing a highly sensitive and noise-proof microphone, without significant loss of playback quality.
The basis is the scheme [1] of a balanced preamplifier, powered directly from phantom power (+48 V) of a mixing console:
Balanced Preamp Electret Microphone Circuit Diagram
Its main disadvantage is excessive amplification, leading to clipping of the microphone-sensitive microphone inputs of the console. In addition, the electret microphone supply [2] is not rational enough, as well as the temperature-dependent displacement of the transistor bases on six diodes included as stabistors. The presence of these diodes, as well as electrolytic capacitors, increases the size of the board and does not contribute to miniaturization.
An attempt to replace diode stabilization with a reverse-shifted base-emitter junction of a planar transistor (KT315) was unsuccessful due to the increased noise (hiss) in the useful signal.
Therefore, in the subsequent stabilization was applied on the shunt regulator TL431, which demonstrated the practical absence of extraneous noise and high thermal stability of the bias voltage.
The final circuit of the electret microphone preamp is shown below.
Its features were additional collector resistors R7 and R9, about 4.5 times lowering the amplitude of the signal at the connector pins compared to the collectors of transistors VT1 and VT2, as well as setting the bias base VT2 directly from the divider connected to the control electrode of the shunt DA1 (+2.5 V). The electret microphone is powered from the cathode DA1 through the divider R3R6, so that the constant voltage on it is half the power supply (ie, +2.5 V from +5 V) and becomes equal to the voltage on the control electrode DA1. Such a microphone connection provides maximum sensitivity. It was tested in the project [3] and demonstrated its practical applicability.
The diagram is made on surface-mounted components (SMD) on a printed circuit board with dimensions 37 x 15 mm (drawing in * .lay7 format is given in the attachment):
The setting is reduced to equalizing the potentials between the contact points (shown by an arrow), which are displayed on the front side of the board by rotating the trimming resistor slider.
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