Classic 2-Transistor Microphone Compressor

Ever wondered how classic radio gear tames loud volume spikes without complex integrated circuits? This vintage-style two-transistor microphone preamplifier with limiter uses a clever pair of Germanium diodes to limit signal peaks automatically. It is a legendary design highly favored by amateur radio homebrewers for single-sideband (SSB) voice communication. Let’s break down the schematic piece by piece to see exactly how it works.

Dynamic and Hi-Z Microphone Matching

The circuit Microphone Compressor provides two separate input ports. You can connect a standard low-impedance microphone directly to the main MIC terminal, where the signal passes through a 0.05 uF coupling capacitor. Do you have a higher impedance source instead, like an old-school crystal or ceramic element? Use the HI-Z input. That port utilizes a smaller 0.001 uF capacitor to properly match the impedance while rolling off unwanted low-end rumble.

Transistor 1: The Buffer Stage

The input signal travels straight into the base of Tr.1. Notice anything unusual about this transistor’s layout? The collector connects directly to the positive 9V power supply rail. Because the collector attaches straight to power, this stage acts as a common-collector amplifier, better known as an emitter follower.

It won’t boost your signal voltage at all. Why use it then? Emitter followers feature exceptionally high input impedance alongside very low output impedance. This setup acts as a perfect buffer, isolating your sensitive microphone from the heavy loading effects of the next amplification stage. A 470K resistor supplies the crucial base bias current from the main power rail.

Transistor 2: The Gain Stage

After the buffer passes the audio along, a beefy 25 uF electrolytic capacitor couples the signal into the base of Tr.2. This is where the real amplification happens. We configured this NPN transistor as a classic common-emitter amplifier.

A 5.6K resistor sits right on the collector to help establish the voltage gain, while a 330 ohm resistor stabilizes the emitter against thermal runaway. Take a close look at the 220K resistor. It runs straight from the collector back to the base. This provides shunt feedback, which stabilizes the DC bias point and lowers distortion. The asterisk on the original schematic means you might need to swap its value to get your specific transistor biased perfectly.

Diode Limiting & SSB Application

How does this track compress the dynamic range? The secret lies at the very end of the line. A 0.1 uF capacitor blocks any DC voltage from escaping the collector of Tr.2, sending pure AC audio through a 10K series resistor. Right after that resistor, two OA81 Germanium diodes sit back-to-back in parallel, tied directly across the output terminals.

When your audio stays quiet, the signal voltage never reaches the forward conduction threshold of the diodes. The audio passes through completely untouched. What happens if someone screams into the microphone? The output voltage spikes, forcing the diodes to turn on.

Because these are vintage Germanium diodes, they begin conducting at a mere 0.2V to 0.3V. They instantly shunt any voltage exceeding that threshold straight to the ground rail. It creates a smooth, analog soft-clipping effect that squashes volume spikes instantly. You get a highly consistent output level that keeps your audio from clipping downstream equipment.

Maximizing Talk Power: Unlike AM or FM, an SSB signal only transmits a carrier when you speak. The power output of your radio depends almost entirely on the peaks of your voice. If your voice has huge volume spikes but a low average level, your signal will sound weak and punch right through the noise floor poorly. This compressor raises the average level of your voice without letting the absolute peaks overload your radio’s modulator.

Preventing Splatter and Overmodulation: In an SSB transmitter, if your audio signal gets too hot, it will overmodulate the balanced modulator or overdrive the linear power amplifiers. When that happens, the radio generates severe distortion. That distortion leaks out into adjacent frequencies as illegal “splatter” interference. The instantaneous soft-clipping action of the back-to-back OA81 diodes acts like a brick wall, guaranteeing your signal stays clean, narrow, and within its bandwidth limits.

Microphone Compressor: Component Considerations

If you plan to build this on a breadboard or custom PCB, keep a few quick tips in mind:

• Power Filtering: The schematic features a decoupling capacitor across the 9V line. Do not skip this! A 10 uF to 100 uF electrolytic capacitor keeps power supply hum out of your audio path.
• Diode Choices: If you cannot source vintage OA81 Germanium diodes, look for 1N34A or 1N270 alternatives. Silicon diodes like the standard 1N4148 will technically work, but their higher 0.7V threshold will let the signal get twice as loud before any compression kicks in.
• Transistors: Standard, low-noise NPN transistors work beautifully here. Try the BC547 or 2N3904 for great, reliable results.

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