For anyone who watched the Mythbusters, the catchphrase “Failure is always an option” is one you will hear often. It is also one that i subscribe to in general, where, no matter how well you research and plan, no matter how great your methodology, failure is always a very real possibility.
I was reminded of this in the last couple of days when an audio stage for a DC receiver did not produce the intended results. The audio output from a mixer in a direct conversion receiver is very low and needs a lot of gain to bring it up to speaker driving levels. With that in mind, i have a preamp prior to the mixer giving 10x gain to the incoming RF, and then lots of audio gain post mixer to bring the AF up to a nice listening level.
With that in mind, i thought, OP AMP preamp would be a good idea followed by a LM-386 at 100x gain. Trouble is, I also set the OP AMP to deliver 100x gain and that is 10,000x gain too much and obviously something is going to distort. Changing the gain of the OP AMP to 10x was tried next, and while the audio was nice, it was much lower than the LM-386 in isolation, and the whole point was to have nice sounding audio that would drive a speaker, this was certainly not that. I am guessing there is some impedance miss match there, that i have not accounted for, but to be honest, i really have no idea.
Now, i could run the LM-386 by itself, the audio will be a little lower than i would like, and lets face it, they sound like arse when it comes to CW, which is the main mode this RX will be used on. So i am back at square one, needing to find an audio stage that both sounds nice and gives 150 to 200x gain and is power efficient. Wish me luck HIHI.
The offending circuit and its implemented circuit board.