SSM2167 Microphone Preamplifier with compression for ubitx HF Transceiver
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:
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
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.
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.
A 4.7K ohm resistor added between the mic input and the 5VDC (taken from 7805 regulator) for biasing the electret microphone and put a 10K ohms potentiometer in the output to adjust the power level to the mic preamp stage. Electret mic can also be powered from raduino regulator using separate line since module power consumption is 2.5 mA, which is a very small extra load on the Raduino regulator. A choke of 100uH between the input +pin & Mic and a 1nF capacitor between the input+ pin & ground are added to block RF feedback when Txing on the higher frequencies.
SSM2167 mic compressor circuit may create some feedback when the microphone was placed near the speaker while in Rx. Connecting shutdown pin of the SSM2167 (pin3) to the Raduino T/R digital output (D7 in raduino board) through a 47KOhms resistor would solve this issue. This disables the SSM2167chip while in Rx and removes mic feedback. However this is not necessary if you intent to operate this module as outboard unit.
The module have provision for adding adjustable pots to make both compression and the noise gate function adjustable. For this, smd resistors R1 and R2 should be removed. The noise gate adjusts for a threshold below which background noises are not passed through the sensitive mic amplifier. This noise gate avoids television audio or background noises like fan noise leaking into qso conversations.
Overall, the SSM2167 can be a useful addition to the uBitx HF transceiver. It can help to improve the audio quality of the transmitted signal, making it sound louder, clearer, and more powerful. Keep in mind that too much compression will overdrive ubitx finals, which may cause splatter. Increasing average power output may necessitate to change the heatsinks attached to the final IRF510.
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