This DIY project idea based on the small board module “SSM2167 Microphone Preamplifier Board Preamp Compression Module DC 3V-5V” widely available on eBay and Aliexpress.
Mic gain is barely adequate on the µBITx transceiver since there is no Mic Gain control. It would also be helpful to not only provide mic gain control, but to add compression in order to boost the average power output. If you are looking to improve the sound quality of uBitx transmissions, then audio compression is a good option to consider. There are a number of different compressors available, so you can experiment with different settings to find the one that works best for you.
The SSM2167 is a low-noise audio amplifier with compression and noise gating features. It can be used in the uBitx HF transceiver to improve the audio quality of the transmitted signal. The compression feature helps to reduce the dynamic range of the audio signal, which can make the signal sound louder and more powerful. The noise gating feature helps to reduce the noise in the audio signal, which can make the signal sound clearer.
Here are some of the specific uses of the SSM2167 in the uBitx HF transceiver:
Compression
The compression feature of the SSM2167 can be used to reduce the dynamic range of the audio signal. This means that the difference between the loudest and quietest parts of the signal will be smaller. This can make the signal sound louder and more powerful. The compressor is controlled by single resistor (R1), which allows the compression ratio to be set from 1:1 to over 10:1
Noise gating
The noise gating feature of the SSM2167 can be used to reduce the noise in the audio signal. This means that the quiet parts of the signal will be removed, which can make the signal sound clearer. Noise gating is controlled by resistor (R2), which allows the compression ratio to be set from -40dBv to -50dBv.
AGC
The SSM2167 has a provision for outputting an AGC voltage. This voltage can be used to drive an S-meter, which is a device that measures the strength of an incoming radio signal.
The SSM2167 is typically connected between the microphone input and the audio amplifier stage of the uBitx.The compression circuit on the SSM2167 can be adjusted to provide a variety of compression levels, depending on the desired sound quality.The AGC voltage from the SSM2167 can be used to automatically adjust the gain of the uBitx.
Power Your Electret Microphone Properly
Connect a 4.7kΩ resistor between your mic input and 5VDC from the 7805 regulator to bias your electret microphone correctly. Add a 10kΩ potentiometer at the output to fine-tune power levels for your preamp stage.
The module draws only 2.5mA, so you can power it from either the main regulator or the Raduino regulator without overloading your system. This low current draw gives you flexibility in your installation setup.
Block RF Interference on Higher Frequencies
Install a 100μH choke between the input+ pin and microphone to stop RF feedback during transmission. Place a 1nF capacitor between the input+ pin and ground for additional RF protection. This combination keeps your audio clean when transmitting on higher frequency bands. These components prevent RF energy from entering your audio stages and causing distortion or unwanted oscillation in your signal path.
Eliminate Microphone Feedback During Receive
Connect the SSM2167’s shutdown pin (pin 3) to your Raduino’s T/R output (D7) through a 47kΩ resistor. This automatically disables the compressor when you’re receiving, preventing acoustic feedback when your microphone sits near your speaker.You can skip this modification if you plan to use the module as a separate outboard unit away from your main transceiver.
Customize Compression and Noise Gate Settings
Remove SMD resistors R1 and R2 to add adjustable potentiometers for both compression and noise gate functions. The compression control adjusts how aggressively the circuit reduces your signal’s dynamic range. The noise gate sets a threshold that blocks background sounds like fan noise, TV audio, or room tone from reaching your transmission. This keeps your conversations clean and professional-sounding.
Optimize Power Output Without Overdrive
The SSM2167 increases your average power output by compressing your signal’s dynamic range. This makes your signal sound louder and more consistent to other operators. However, too much compression can overdrive your uBitx finals and cause splatter on adjacent frequencies.
Monitor your power levels carefully and consider upgrading the heatsinks on your final IRF510 transistors if you notice increased heat buildup. Proper adjustment balances improved audio quality with clean spectral output.
For more info
SSM2167 board – improving compression control
Mic Compression and Noise gate with SSM2167 module
SSM2166 (Mic amplifier and compressor) as Audio AGC
Mic Preamp with Audio Compression – Youtube video
Homebrew: SSM2167-based Mic Preamplifier & Compressor, March 12-18, 2019 – youtube video
ubitx SSM2167 mic compressor speaker feedback issue – resolved
Audio pre-amplifier for microphone?
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