RF noise: lm386 vs lm741

Both devices are ancient and the LM386 was notorious for instability. Suggest you use something more modern.

If you're trying to amplify a crystal detector you need a high input impedance, use a TL071 for example with a high Rin bias resistor.

Graham

Wow! that's a long post, The crystal raido question reminded me of the L, variable C, diode, antenna and ear plug kit that's in my son's room (from the Simthsonian??). With a big enough antenna you can hear AM radio. From there you can add some amplification.

You also seem to be having some problem with the concept of ground. For an isolated circuit you are free to define the ground of the circuit at whatever voltage you want. When you want to connect to other circuits it's good to have a common reference.

George Herold

I don't really see what the difficulty is. Every time I try to build an audio amp using a LM386 and turn the gain high, putting my finger on the input yields an audio output of the local AM station. I'm not even trying to build a crystal radio!

I think you're concentrating too much on the audio amplifier stage and not enough on the radio. Unless you live far far away from any AM broadcaster, just a random wire and any sort of rectification will generally yield a big enough signal for a crystal earphone.

Tim.

Of course in that case, the input of the 386 is being overdriven by the radio signal, and is rectifying it, acting just like the diode in the "crystal radio".

The thing is, no amount of audio amplification will make up for lack of enough radio signal. If the signal isn't big enough to be properly rectified by the diode, then more audio amplification won't compensate. All it will do is make a signal that is strong enough louder in the headphones, or even allow for feeding a speaker.

One needs either a bigger antenna, or to amplify the radio signal before it goes into the diode.

WHich of course is why "crystal radios" were given up relatively early in the days of radio, first with Tuned Radio Frequency receivers that had tuned amplification before the detector, and then the regenerative receiver that used a single stage to amplify the signal over and over again before going to the detector.

Michael