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In Reply to: RE: Why can't I receive radio waves using LM386? posted by hifipaul on January 15, 2017 at 20:00:28
He's got a demodulating diode in the circuit. He probably needs a 0.01 μF. cap. to ground, at the diode's O/P.
What I want to know is why this thread is on a TUBE "board".
...since it's shown as a parallel-resonant circuit, the diode should be connected to the junction of the antenna, (variable?) cap and inductor. The bottom of the parallel cap and inductor should be tied to a good earth ground as should the signal (-) input of the 386. A 500-1000pf at diode output to ground would improve sound quality such that it is and probably make the 386 happier too.
Did not examine 386 circuit for correctness but doesn't strike me as optimum load for tuned circuit. High impedance is better.
Why in Tube DIY? Expectation of superior knowledge! ;-)
Right you are about the diode attaching to the antenna. I missed that. Also, the place the diode is currently attached to should be grounded.
From my quick look at the LM386 data sheet, it does use low I/P impedance BJTs. Perhaps a simple JFET source follower between the diode and IC is needed. When I was a youngster, we used tapped tuning coils to connect to the diode and (ultimately) the BJT AF amp.
With a genuine long wire antenna and earth ground, a lightning arrestor is ESSENTIAL. That device can be made from a non-resistor spark plug.
Another tweak for the external antenna/earth ground setup is to connect them to a separate winding on the same form as the tuning coil. That winding has quite a fewer turns. Presto/chango step up transformer.
You can protect this with a GDT - which are about $4 in qty 1 from Mouser.
These guys are mini versions of the OB2 OA2 glow regulators.... They conduct around 90V or so, so your FET needs to have a gate breakdown voltage above that....
Another longwire protection device is a simple neon lamp....
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