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I have been experimenting with a very simple AM regenerative receiver discussed at circuitbasics.com. The schematic is shown here: Regen schematic

I obtain fairly good performance feeding the output to an old Radio Shack amplified speaker. I'd like to build an audio amplifier based on an LM386 op amp as part of the circuit, such as is shown in the same article, for a crystal receiver (see partial schematic below).

LM386 amplifier for crystal radio

My question is this - would the impedance match (radio output to LM386 amp input), and would an LM386 amplifier be suitable for driving a small speaker or 8 ohm earphones? Can someone suggest a schematic for such an amplifier to use with this circuit?

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    \$\begingroup\$ Do you like to fiddle with the regen control instead of using a normal superhet AM radio that has automatic gain control? Do you like the poor sound quality of AM radio? (No high audio frequencies and lots of interference noises). Try the LM386 without C5 (to reduce its gain). \$\endgroup\$
    – Audioguru
    Sep 30, 2022 at 0:43
  • \$\begingroup\$ Yes, I like both things - I am learning how regens work - this is a hobby... \$\endgroup\$ Sep 30, 2022 at 1:23
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    \$\begingroup\$ Tom, your posted schematic is "standard" and should work - I've added one to a regen too. Some users run into problems when it is powered by a weak 9V battery with the "putt-putt-putt" sound of motorboating when driving a 8-ohm (or lower) speaker. C2 is there to help stop motorboating. A larger value may help if you run into this problem. \$\endgroup\$
    – glen_geek
    Sep 30, 2022 at 13:08
  • \$\begingroup\$ @glen_geek Thank you, I will give that a try. I was concerned because I tried connecting the output to a Velleman 4W kit amplifier I had built (based on a TDA2003 IC), and the radio ceased to function. I assumed there was a loading of the circuit caused by impedance mismatch. Will try your suggestion. \$\endgroup\$ Sep 30, 2022 at 13:51

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Reading the data sheet definitely indicates that an \$8 \Omega \$ is within its specification. Follow the guidelines in the data sheet.

The input resistance for the LM386 is 50K. This in parallel with the 10k potentiometer drops the input to as low as 6.3k which will definitely load the collector of the receiver by about ~50%, maybe less. If that gives adequate volume without distortion then you can use it. The only concern is the excess load may lead to collector clipping.

You can buffer the receiver output with an op-amp to eliminate loading. If you want old school, a capacitively coupled emitter follower stage between the receiver and the LM386 would also eliminate loading.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ thank you, I will try a buffer, I've learned something from your answer. Looks like a good way to match the impedances. \$\endgroup\$ Sep 30, 2022 at 13:53
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Usually a "regen" radio is overloaded by strong local stations but does not have enough gain to hear weak distant stations. A normal superhet radio has automatic gain control to prevent these problems.

I am only 77 years old so I was not old enough to make a 62 years old regen AM radio. Instead I made a superhet AM radio that worked fine but sounded so bad that I made an FM radio from a kit that worked well and sounded perfect.

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  • \$\begingroup\$ Actually the regenerative radio was patented in 1914 and used until the 1930s, about 90 years ago. \$\endgroup\$ Oct 1, 2022 at 19:47

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