The circuit shared here is a Microphone Preamplifier with Audio Peak Limiter, often used in audio recording, amateur radio, or intercom systems. It amplifies weak microphone signals and includes peak clipping diodes to prevent signal distortion from loud inputs.
Looking to boost your microphone signal while keeping audio distortion in check? This two-stage transistor microphone preamplifier with peak limiter is just what you need. Ideal for audio hobbyists, ham radio enthusiasts, or DIY audio gear builders, this circuit ensures clear and balanced audio amplification.
Circuit Overview
This preamp is built around two 2N3904 NPN transistors, working as sequential amplifier stages. The design is compact, operates from a 9V supply, and incorporates germanium diodes (1N34) to limit output peaks, protecting further audio stages from distortion or overload.
Component Breakdown
Microphone Input
- Electret MIC: Captures audio signals.
- C1 (0.5µF): AC coupling capacitor to block DC from mic.
- R7 (1kΩ): Bias resistor for microphone power.
First Amplifier Stage (Q1)
- Q1 (2N3904): Amplifies weak mic signal.
- R1 (470kΩ), R2 (3.3kΩ), R8 (1kΩ): Bias network.
- C5 (100µF): Power decoupling capacitor.
- C6 (25µF): Emitter bypass capacitor to improve gain.
Second Amplifier Stage (Q2)
- Q2 (2N3904): Further boosts signal.
- R3 (220kΩ), R6 (330Ω), R4 (5.6kΩ): Biasing and load resistors.
- C3 (1µF): AC coupling to output stage.
Peak Limiter (Diode Clipping)
- R5 (10kΩ): Limits current to clipping stage.
- D1, D2 (1N34): Germanium diodes clip peaks symmetrically at ~0.3V.
- Prevents loud audio from distorting or overdriving next stage.
Notes
- Replace 1N34 with other low-threshold germanium diodes (e.g., OA91, AA119) if unavailable.
- Ensure good power filtering with C5 to avoid hum.
- Circuit is not suitable for stereo mics without modification.
Applications
- Voice preamp for FM/AM transmitters
- Audio input stage for WebSDRs or HF radios
- Preamp for audio recording or PA systems
- Microphone interface for Arduino/Raspberry Pi projects
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