Mod: Add Mic Pre-amp Gain and/or Compression


Why increase Mic gain or add compression?

Mic gain is barely adequate on the µBITx and there is no Mic Gain control (only a drive control that impacts on both phone and CW).   Given this is a QRP rig, it would also be helpful to not only provide mic gain control, but to add compression in order to boost the average power output (typically by half to one S point).   Increasing average power output may require changes to the heatsinks attached to the final IRF510 devices.  It is also important to make sure that you do not overdrive your finals, which may cause splatter.

Changing mic gain by adjusting components in the microphone amplifier

There are changes that you can make to components in the µBITx microphone amplifier circuit to increase (or reduce) the gain.

To increase gain you could try making the following component changes:

  • Increase R65 from 1K Ohm to 4.7 K Ohm
  • Reduce R63 from 100 Ohm to 10 Ohm

You could try making adjustment to these resistor values to suit your requirements.

The source for this mod can be found on a blog post of Mr K P S Kang (VU2KR / VU2OWF)  and was intended for the BITX40, but given the circuit is identical it can be translated to the µBITx.   Those who have attempted this mod report good results. suggests you should consider this mod


Adding a Mic amplifier and Compression unit (SSM2167)

Several have now successfully used the SSM2167 to improve mic gain and provide more punch (from 2:1 to 4:1 microphone compression).   Experiments with another unit  (MAX9812) were not quite as positive.

John, VK2ETA, used the small circuit board “SSM2167 Microphone Preamplifier Board Preamp COMP Compression Module DC 3V-5V” found on eBay or Aliexpress as a compression and mic pre-amplifier.

He simply connected the input to the mic, added a 4.7K ohm resistor between the mic input and the 5VDC (taken from the Raduino) for biasing the electret and put a 10K ohms potentiometer in the output to adjust the power level to the mic preamp stage.

He inserted the board prior to the mic input (but did not install on top of the µBITx).  The gain of 20dB is reduced using the output potentiometer. John removed the “R1” resistor and replaced it with a 51K Ohms resistor to get a 4:1 compression factor, up from the 2:1 as delivered.

Simon VK3ELH used a 75k ohm resistor for compression and 1k ohm resistor for the noise gate and a 100k pot on output. At full output, his audio was readable but distorted based on an audio check QSO, so the output has been turned down.

He put a larger heatsink on the IRF510 to cater for the higher average output, as the stock one was getting warm!

The board allows for insertion of two small adjustable multiturn pots.  Mike ZL1AXG removed the resistors and installed pots, to make both compression and the noise gate function adjustable.  The noise gate provides for a threshold below which background noises are not passed through the mic amplifier.  This can be handy for avoiding television audio or family member discussions from “leaking” into your conversations given the compression effect. recommends you consider adding this module