SSM2167 Mic Compressor: Avoiding feedback

John VK2ETA notes that Simon, VK3ELH, pointed out  an issue that when inserting an SSM2167 mic compressor circuit between the microphone and the uBitx mic-preamp, it can create feedback when the microphone was placed near the speaker while in RX.  This is because the SSM2167 module is always on.

The solution John has applied is to connect the shutdown pin of the SSM2167 (pin 3) to the Raduino T/R digital output (D7) through a 2.7K ohm resistor. This disables the chip while in RX and removed the mic feedback issue.

Pictured above is an indication of where he picked up pin 3 on the SSM2167 on his module. The purple wire is connected to what is the right hand side of resistor R4. The 4.7K resistor on the RHS is for the mic-bias and the 51K resistor on the top-left is for bringing the compression ratio towards 4.

John feeds the Vcc pin on the board from the regulated 5V of his Raduino. Measured consumption at 2mA is a very small extra load on the Raduino regulator.

There is a DC blocking cap on the input and output circuits of the board already, so no external blocking capacitors are needed.  However, a bias resistor does need to be added for the microphone.

The 2.7K resistor is not mounted on the module, so is not shown in the picture.

Also not shown on the picture are an axial choke of 100uH between the “in” connection and the Mic, plus a 1nF capacitor between the “in” connection and ground to block RF feedback when Txing on higher frequencies. For John, RF feedback was noticeable from 15m through 10m. Others may not have this issue.

John also has a 10K adjustable potentiometer between the “out” connection on the module and the original Mic input to the uBitx.  His is turned to about 80% through its range.

John mounted his board on header pins so he can remove it as required.  He extended the header pin on the “out” side (bottom LHS on picture) past the board to provide an extra connection for the shutdown wire.

John finds the compression and noise gate work quite well on the module. When he is silent the background noise does not trigger any movement of the power needle, but it goes up as soon as he speaks into the Mic. Also despite showing quite an increase in average power, he hasn’t had any negative comments on his  audio. I was told that it was noticeable, but not unpleasant, “good for DX”.   And this was with a change in the standard resistor value for compression to give around 4: 1 compression.

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