If you notice a lot of high-frequency hiss in the receive audio on your BITX40 there is a simple remedy for this.
(This might be more noticeable if use headphones while operating.)
There is a little know feature described in the LM386 Application notes called "Bass Boost".
By simply adding a capacitor and a resistor in series between Pins 1 and 5 of the LM386 you can taylor the audio response of the amplifier.
Wayne Burdick, N6KR (of Elecraft Fame) has been utilizing this trick for years, starting with many of the QRP Rigs he designed for the
Norcal QRP Club.
I used the capacitor and resistor values specified in the Norcal 40a schematic with good results for both CW and SSB receive.
These are 0.01 uF capacitance in series with 1.8 K ohms between pins 1 and 5.
You can try this by tack soldering the capacitor and resistor together and carefully touching one end of the pair to pin 1 and the other to pin 5
on the LM386. If you like how it sounds you can make it permanent. I was too lazy to remove my BITX40 board to install the parts
on the bottom of the board so I just tack soldered the pair directly to the pins on the chip on the top side of the BITX40 board.
P.S. This is an LM386 trick only so it will not work on the uBITX.
Dexter N Muir
There's another possibility, found early on, at low volume levels: too-low resistance from pot. wiper to Earth. Anything from 1k to 10k wiper-to-IC cures it. If you've replaced the (blown?) audio IC with an LM386 and found this hiss, it's a simple cure.