This is what a project in progress looks like for me…
I have been hobbled by the low audio in my sBitx since it was purchased back in 2023. This radio is a godsend for FT8 but the SSB microphone level was almost unusable. It was weak and you had to yell at the radio to get it to work at all. I didn’t really want to go into the software and start changing stuff in it, so I decided to bring the level up by adding a pre-amp to the circuit.
So I get out a breadboard and build up a simple audio amplifier circuit on it with a 2N3904 transistor and set about to see what levels I could get with a simple 9VDC battery powering it and feeding the input with the actual microphone I plan to use. I used a couple of potentiometers to adjust the input level and the gain of the circuit so that I could tune it to work with my radio. Below is the circuit I found online (why go to the trouble of designing one from scratch when it has already be invented?) So I built it up and changed 10k resistor and the 100k resistor with the pots I mentioned earlier and it was time to put the scope probe on it and see what I got.
It worked beautifully well. I could drive it to clipping if I wanted but I had PLENTY of gain before that happened. Way more than I needed I would find out later when testing it on the air with my friend on 80 meters. I chose to setup the bench test by simply getting the board out of the radio and powering it without the Raspberry Pi installed, this would power up the audio amplifier circuit without the hassle of it biasing the RF section. I put a scope probe on C119 at the output of the transistor (which you will notice looks very similar to the one I built actually). This circuit powers up when the switch is turned on so you dont need to even have the Pi installed to work with this circuit. For testing I simply connected the output of my amp to the mic jack with jumper wires and then spoke into the microphone while watching the scope. It worked really well to be honest. It worked so well that I looked at the sBitx circuit and figured that full output was 3VDC peak to peak or so as there is a 3.3V zener diode on the input of the IC this circuit is feeding. 3.3V logic protection if what I am figuring and these zeners are clamping the signal to that voltage max for safety.
This was a mistake turns out as it needed me to dial back the input significantly to get good quality audio out of the radio, but I had signal finally! It worked!!!
The yellow trace is what was going into the mic input and the purple trace is what was coming out of the pre amp I built. I was well pleased with this. I then proceeded to move the parts to a perf board and install it into the radio semi permanently to test it.
In the photo below I am pointing at the little “daughter” board I built with my pre-amp circuit on it. I tapped power from the input right at the massive reverse polarity protection diode that I also installed in my radio. The factory diode is really small and if you didn’t consider a small enough fuse in your power cord or worse…no fuse at all, then it will burn open after a short time and will let the reverse voltage wreak havoc on your radio. The reason I know this is that it has happened to others with this radio. This diode, combined with my 10 amp fuse, works like it should. It will blow the fuse every time and not damage the diode. I tested the idea a few times with a few fuses to make sure it would work and it did.
Here you can see how I hard wired the amp into the radio. I lifted the end of the RF choke that was connected to the microphone port and soldered my pre-amp output to the inductor and then my pre-amp input is simply soldered into the through hole the inductor was in. This made for a simple and non-permanent installation should I want to remove my mod at some point. After this, I simply wrapped the board with a little electrical tape and zip tied it in place so it doesn’t wiggle around too much while I test it in the field some. Once I got the pre-amp tied in I realized I was “double biasing” the electrolytic DC blocking capacitor that I was using to couple the pre-amp to the amp in the radio. So I simply removed the SMD resistor RD21 as this resistor is normally used to power the microphone elements and I had my own bias resistor already in the circuit.
Here is what the audio waveform looked like on the radio when transmitting later after I got the radio put back together. Before ( I should have taken a photo…) it was barely visible when you would talk into the microphone. and it was showing power out too!!! Success was had the next day when I made two SSB contacts with it on a POTA activation.
I hope this inspires you to make some improvements to your electronics in some way, this was really fun and rewarding in the end as it enabled a mode on this radio that had been previously disabled by default.