Technical Topics > The RF Workbench
Symptoms of an oscillating amplifier?
Antennae:
makeRF: I'm using the LM386 chip as an amp.
kaliic: The audio sounded fine, just quiet. The LM386 chip would get warm when transmitting. It would get really hot running a speaker without resistors. It wouldn't get warm when running a speaker with resistors.
Its not assembled anymore. It was bugging me why the sound didn't carry very far. I thought maybe because it was oscillating? I'll reply here if I tweak with it.
thanx
makeRF:
Ah, I see... I was thinking you were talking about amplifying the RF.
Yes, the LM386 works as an audio modulation source for the transmitter, but it's not ideal. In my tests the audio was always a bit dampened. It works, but it's not the best. It's really simple to construct, so that's the tradeoff.
As for the LM386 getting hot, I'm not sure if mine did or not... It's been a while since I tinkered with that circuit. What I presented in the schematic was the bare essential parts to get it working. You might have better luck building the full LM386 circuit presented in the datasheet. It has a few more parts that might help with stability. Also, now that I think about it, one reason it might be heating up is the voltage input is too high. You could try to step the voltage down to the chip, which might help. Or, the current might be too high, and in such a case you might need to add in a resistor to limit the current at the input.
ff:
--- Quote from: makeRF on October 16, 2014, 1537 UTC ---the current might be too high, and in such a case you might need to add in a resistor to limit the current at the input.
--- End quote ---
There are several versions of the LM386. Dave Martin's Corsair also used one to series modulate the driver stage of his Corsair. The consensus in our builder's group was that the best modulation came from the LM386-N3 or N4 that was capable of more current output. The typical '386 is the N1, which runs out of gas in these applications. You might have better results with an N3. Worth a try...
Antennae:
Ah, I was using the N1.
makeRF:
--- Quote from: ff on October 16, 2014, 1732 UTC ---There are several versions of the LM386. Dave Martin's Corsair also used one to series modulate the driver stage of his Corsair. The consensus in our builder's group was that the best modulation came from the LM386-N3 or N4 that was capable of more current output. The typical '386 is the N1, which runs out of gas in these applications. You might have better results with an N3. Worth a try...
--- End quote ---
Good point ff! I used an N3 in my build. The audio is still muted a bit, but perhaps not as much as the other variants. I believe I ordered a handful of the LM386 N3's from Jameco at some point. I simply chose the N3 because it puts out more power. IIRC, Radio Shack only sells the N1.
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