W1FB DC Receiver experiments 3
Today I tried to calm the receiver a bit since the audio gain control has to be turned almost to zero with the original W1FB design.
Using my DMM I noted that the input voltage to the Audio Preamp was about the same as the input voltage to the LM386 for acceptable listening levels! Meaning that in practice the pre-amp is not achieving anything and is increasing the noise level.
The audio pre-amp is designed to swamp the noise figure from any added audio filter stage. These generally being noisy in nature.
I simply connected the NE602 directly into the LM386 and bypassed the preamp. This has resulted in much less gain (of course) and a better behaved receiver. Next I removed the 1K feedback resistor in the LM386 to obtain a little more gain.
I also noticed a big difference between my senheiser earplugs and a pair of cheap earphones. Amazing the difference. The cheap pair are better since they do not hear the high frequency hiss in the audio amp.
I need to next increase the .1uF cap on the audio output and see if I can reduce the bandwidth a bit.