Simple audio compressor for SSB HF Radio rigs
An audio/speech compressor is a device that reduces the dynamic range of the audio signal. This means that it makes the loud parts of the signal quieter and the quiet parts of the signal louder. This can be useful for SSB HF radio rigs because it can help to improve the readability of your transmissions.
Without compression, your voice may sound weak and quiet when you speak softly, and then suddenly loud when you speak loudly. This can make it difficult for other hams to understand you, especially if they are using weak receivers. Compression helps to level out the volume of your voice so that it is easier to understand regardless of how loud or soft you speak.
In addition to improving readability, compression can also help to improve the quality of your transmissions. This is because it can reduce the amount of noise that is present in your signal. Noise is any unwanted sound that is mixed in with your voice signal. It can be caused by a variety of factors, such as interference from other radio signals, electrical noise from your equipment, or even the sound of your own breathing. Compression can help to reduce the level of noise in your signal, making it sound clearer and more professional.
The audio signal goes through passive components C1, R1, D1, C2, R3. Part of the speech signal charge the D3/D4 detector and creates a control voltage for T1 which is easily available for general-purpose transistor 2N3904. The fading time depends on capacitor C4 and resistor R5. With an audio input variation of 50dB, the output signal gets +-3dB. This simple circuit can be used in SSB transceivers. I used 1N60 diodes that I could more easily find. I have not tested this using more commonly available diodes but I do not see the reason why this should not work using them.
Project files available at https://oshwlab.com/vu3dxr/audio-compressor-two-transistor