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A uBitx CW SideTone Volume Fix

 

We all seem to agree that the CW sidetone volume on the uBitx is way too loud. ?Here is what I did to address the issue.?
?
Looking at the uBitx v6 schematic, the CW sidetone passes through a series of 3 Resistor/Capacitor low pass filters to help smooth out the square wave signal from the synthesizer before injecting it into the audio stream. ?These filters can modified to reduce the level of the sidetone signal. ?
?
I increased the values of resistors R251 and R252 from 1K to 4.7K (because that was the values I had on hand), this lowers the low pass filter frequency below the sidetone frequency which is around 700 Hz. ?With the 1K resistors, these two filters have a 3dB cutoff of about 1600 Hz, meaning that they are not attenuating the sidetone at all, just smoothing it. ?Swapping R251 and R252 out for 4.7K resistors lowers the 3dB cutoff to 340 Hz. ?This attenuates the signal in addition to smoothing it out. ?I estimate that this dropped the sidetone volume by about 6dB (1/4 of previous volume). ?
?
It is still a bit louder than I would like, but it is a huge improvement. ?
?
--
73,
?
Steve - KA0SM
 

I found that placing a 10k trim pot in that area does a good job.




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On Wednesday, February 12, 2025 at 10:16:00 PM EST, Steve M <steven@...> wrote:


We all seem to agree that the CW sidetone volume on the uBitx is way too loud. ?Here is what I did to address the issue.?
?
Looking at the uBitx v6 schematic, the CW sidetone passes through a series of 3 Resistor/Capacitor low pass filters to help smooth out the square wave signal from the synthesizer before injecting it into the audio stream. ?These filters can modified to reduce the level of the sidetone signal. ?
?
I increased the values of resistors R251 and R252 from 1K to 4.7K (because that was the values I had on hand), this lowers the low pass filter frequency below the sidetone frequency which is around 700 Hz. ?With the 1K resistors, these two filters have a 3dB cutoff of about 1600 Hz, meaning that they are not attenuating the sidetone at all, just smoothing it. ?Swapping R251 and R252 out for 4.7K resistors lowers the 3dB cutoff to 340 Hz. ?This attenuates the signal in addition to smoothing it out. ?I estimate that this dropped the sidetone volume by about 6dB (1/4 of previous volume). ?
?
It is still a bit louder than I would like, but it is a huge improvement. ?
?
--
73,
?
Steve - KA0SM
 

I had thought about that, but space is kind of tight in that area (below the display/controller board).? A pot in there would allow for some adjustment.
?
Either way, increasing the value of R251 and R252 is an easy way to lower the sidetone volume.
--
73,
?
Steve - KA0SM
 

My fix was a tiny trimpot (1 megohm) between C253 and the high side of RV4 (volume control). I was able to squeeze it onto the board, by lifting one end of C253 (mounting it vertically) and soldering the variable resistor between the capacitor and the vacated p.c. land. A dab of "shoe goop" (or other glue) provides mechanical stability to the trimpot. A larger trimpot could be located elsewhere, with the same wiring. For me, this provides a good range of adjustment (but once set, I leave it alone).
?
It's great, Steve M, that your fix works for you. Thanks for sharing!
 

Thanks for the responses!? This is a topic that seems to come up periodically and I thought it would be good to have a thread on ways to address the issue.
?
So we have the following solutions.
?
- Increase the values of R251 and R252 to 4.7 K ohm or higher.? Increasing R250 from the current 10K ohm would also help.
- Install a 10K trim pot in place of R251 or R252 (most likely R252 given the tight spacing) to allow you to adjust the volume.
- Install a trim pot (1 megohm suggested) between C253 and the high side of RV4 by lifting the one end of C253 and putting the pot between C253 and the board.?
- Putting the resistor on the other side of C253 (between the "output" of R252, which is also the positive side of C252, and the "input" of C253) should also work.?
?
Anyone else come up with additional solutions?
?
--
73,
?
Steve - KA0SM

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