I think you are on the money. Last year, during the lockdown, I built a direct conversion radio, then I added a discrete filter made from 4 transistors. The audio was so sweet!
On Sun, Mar 20, 2022, 10:45 PM Ford Peterson <
ford@...> wrote:
I'm considering ways of reducing the white noise of the uBitX.? Perhaps it is my nostalgic quest to reproduce the audio of an old ARC5 direct conversion tube radio.? Remember the ARC5?? 10 KHz wide you could hear every CW signal above and below the BFO.? You needed to use the filter between your ears and just listen to the one tone and ignore the rest.? You could do it because the rest of the audio chain was perfectly quiet.? Disconnect the antenna and virtually no noise whatsoever.? With my uBitX, the white noise is annoyingly present even without an antenna.
I think it is the LM386.? I've seen this before.? I think the LM386 will actually produce some gain all the way up to 100KHz.? I think they call it thermal noise.? What can be done to make it gone?
The LM386 data sheet talks about a bass boost using a series 10K resistor and 0.033uF capacitor between pins 1 (gain) and 5 (output).?
Using Elsie software, I wanted to see the frequency response of the series R/C with this simple circuit:
Below 500 Hz it attenuates the feedback quite a bit.
Looking at the chip's internal workings you see that there is some feedback working here to knock down the higher frequencies and leaving the listener with the sensation of accentuated Bass.? Consider the R/C between 1 and 5...
This simple R/C is clearly not very selective.? But what do you think would happen if a proper filter were designed to notch out the desired passband of 0-2.5KHz or so?? The high frequency hiss would be knocked down and the desired passband would be accentuated?
Ford-N0FP
