Simple One-Chip Regenerative Receiver

Crystal radios may be the simplest kind to make, but regenerative receivers are more practical and only a little more complicated. A recent design by [Selenium] is super simple because it uses a single LM386 audio amplifier IC.

You might be surprised that you can convert an audio amplifier to a receiver using just a handful of components (a variable capacitor, a coil, a handful of capacitors, and a speaker). However, [Selenium] realized he could subvert the gain and bypass pins to cause regeneration and wound up with a very simple receiver.

If you haven’t looked at regenerative receivers before, the principle is simple (and dates back to 1912). An oscillator is an amplifier that gets (theoretically) an infinite amount of gain at one particular frequency. A regenerative receiver is just an amplifier that is almost (but not quite) at the point of oscillation. This gives it very high frequency-specific gain and a measure of selectivity. You can also nudge the receiver just into oscillation to receive CW or SSB signals.

[Selenium] built his prototype on an old receiver chassis because it had the IC and the variable capacitor already in place. However, others have built successful copies on breadboards ([Austin Heller] created several good looking breadboard versions) and on PCB material. [Selenium] also released some other unique LM386-based designs that use more parts (and, probably, have better performance). Looks like a simple way to build a practical receiver.

22 thoughts on “Simple One-Chip Regenerative Receiver

  1. Yes – ZN414 was a TRF receiver chip. I built a ZN414 based radio in a cuff-link box in high-school, and listened to the radio during school without the teacher seeing. Even during exams. Thought of putting a recording of exam notes and a small transmitter somewhere outside and having it replay during the exam, but never got to that (and it would have been dishonest). Studying was a bit simpler :)

    Nice hack though. May have to build one of these too

  2. As far as I remember there is no need for an infinite amplification for an amplifier to be come an oscillator. It is enough to have gain 1 provided that the phase shift is 180 degrees at chosen frequency.

    1. You are correct, for only oscillation to occur (this is called the Barkhausen Criterta). However the regenerative receiver uses frequency selectivity in the feedback path to produce high gain. Relatively high gain and selectivity are key charachteristics of a radio receiver, but are not required for simple oscillation to occur.

      1. The voltage gain approaches infinity. There’s a good QEX article on the subject: http://www.arrl.org/files/file/Technology/tis/info/pdf/9811qex026.pdf

        In short, thought, the gain of an amp with feedback is Avf=Av/(1-Mv*Av)

        The Barkhausen criteria Mv*Av=1 So if that’s true you get Avf=Av/0 (and that is lim x/0 => infinity). So it isn’t the gain that is 1, it is the open loop gain times the positive feedback gain, but that’s not the same as the stage gain. Easy to get mixed up.

        Pretty good text book write up at https://books.google.com/books?id=z_CTJ-vIIKMC&lpg=PA373&ots=39no2FgRWz on page 372.

        1. Thank you Mr. Williams, your feedback (no pun intended) clarifies the subject nicely. And thanks for the links. You are correct to point out the magnitude of the loop gain and phase are not the same as the amplifier magnitude and phase or the feedback transfer function. Easy to forget the difference indeed. The Barkhausen Criterion for oscillation is a loop magnitude of one (|A|=1) and zero phase shift (/_ = 0 deg.) in the feedback path…

  3. Similar to regenerative receivers, look for the “reflex radio”.
    https://en.m.wikipedia.org/wiki/Reflectional_receiver
    If you have an amp with good bandwidth, you tune into a station. The trick is: after the amp is a pair of filters. High pass sends the RF to a detecter diode and smoothing capacitor, then feeds the result back into the amp…
    After the second pass, the audio goes through a LOW pass filter for audio output.

    Strong signals can swamp this, so attenuation is typically added between tuning circuit and amp.

    1. This reminds me of when I used to fix glass tube TV’s.

      There was a Telefunken TV that had what was called (from memory) a Ferguson chassis. The LOPT was both the LOPT and the SMPSU transformer and the horizontal transistor (chopper) was both the horizontal drive and SMPSU drive. Filtering separated the two functions even though they shared common components.

      Absolute nightmare. Just looking at the circuit diagram hurt your head lol.

      Anyway, they made repair kits that had a metric fu(k load of parts in it. I don’t think anyone realy tried to diagnose a fault in that chassis more than once. Even if you succeed the first time you don’t want to do it again!

      1. That’s not unusual. In the halcyon days of vacuum tubes, designers went to maddening lengths to minimize the number of active devices. Just about every stage performed multiple functions. By the end, I think GE got it down to just 5 tubes (plus CRT) for a complete TV receiver with their “compactron” tubes. They even eliminated such “unnecessary” parts as the power transformer!

  4. “You might be surprised that you can convert an audio amplifier to a receiver using just a handful of components (a variable capacitor, a coil, a handful of capacitors, and a speaker).”

    Not really.

    For decades before ICs were commonplace low end radios had only a single active device (or less in the case of crystal). More advanced ones than the regen (such as superhets for example) used only a few active devices. That LM386 has 9 transistors, 3 diodes and a bunch of resistors inside it.

    Nothing against the project… just hardly a surprise that it’s possible. For some really minimalist receivers and transmitters Google AA1TJ’s stuff. I’d leave a URL but he took his page down. I know somebody is hosting his archives though, I just don’t remember the URL.

    1. To me the surprise was that you could put the 386 into regeneration. Sure I built one tube receivers and a few design around 602/612s but I thought this was a great repurpose of a chip.

  5. Several years ago, someone gave me an old kit-built regenerative shortwave receiver. The radio, which appeared to be from the 1940s, had been assembled by someone with dubious soldering skills, and after a careful check of all the point to point wiring, and an hour or so re flowing several cold solder joints, I connected a long piece of wire as a makeshift antenna, plugged it in, turned it on and IT WORKED!!!!!. After giving it several minuted to sort of stabilize, the first station I managed to tune in was in the middle of a 1940’s nostalgia broadcast.

  6. All this talk about Regenerative radios reminded me of the old PBS special “Empire of the Air”
    The 3 main characters in the show were DeForest, Edward Howard Armstrong, and David Sarnoff.
    It told the story through the greed of DeForest and Sarnoff to deny Armstrong from getting royalties on his inventions, including Regenerative Feedback.

    1. Armstrong was a true genius. He not only invented the regenerative and superhetrodyne radio circuits; but also the SUPERregenerative receiver. It goes a step beyond the simpler regenerative receiver. I wonder if the LM386 could be used in a superregen receiver?

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