I had to rename the super noisy regen to the Signal Digger Regen. People are very literally minded I guess and the circuit got very low views on pinterest. Anyway the new circuit is a variation at least:
I went with the DrM prompt and changed the RF filtering to C4 as a reduced component choice. It depends on the audio amplifier that follows. For the LM386 that has power gain at 500KHz+ you may need extra filtering at least for the AM broadcast band.
I wrote a little about the circuit here:
https://sites.google.com/view/analogelectronics/home/signal-digger-regen
The circuit is more than a little like the B Kainka circuit just the AM detector is a step up.
@Sean O'Connor After looking over your noisy regen circuits and the related discussion, I stumbled upon an idea about how to implement detection more simply in the "noisy" regen. Instead of taking the audio off of the emitter, and instead of implementing a separate detector stage, how about this idea?
Implement the noisy regen as usual. One transistor's will base be tied to Vcc; the other transistor's collector will be tied to Vcc.
For the collector tied to Vcc, insert a load resistor (maybe 4.7k?) and a 10 nF bypass capacitor from collector to ground.
The 10 nF bypass capacitor serves to ground the collector at RF so that the oscillator still works.
The load resistor in the collector serves as a load for the output AF signal.
Intuitively I guess this would work and might provide louder audio -- since you're taking the AF output from the amplified collector output -- than trying to take the weak AF signal off of the emitters. I plan to try this sometime soon, but I was wondering if you had already tried this or if you have any opinions about it.
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EDIT: I started a new topic about this, titled "Improved AM detection in the "noisy regen"?". The post is awaiting approval, however, so may take some days to appear.
QST 1925 03 kind of suggests to me that if you tap a LC resonant circuit with metal near a sensitive receiver you can tune the radio until you hear a click and find the resonant frequency of the LC circuit that way.
Another way is just to blast the LC tank with huge amounts of noise, then listen for the noise peak on a nearby receiver. 😄 This requires inductively coupling the noise into the tank (from the noise source) and out of the tank (into the receiver).
But I think one of the better low-tech ways of finding the resonant frequency is simply to make the tank just barely oscillate with a regenerative circuit like the noisy regen, then to check the radiated signal's frequency on a nearby receiver. You might have to take stray capacitance (added by the oscillator circuit) into account, but if you use high C values in your LC tank that helps in reducing the relative magnitude of the effect of stray capacitance.
I did some optimisation:
I didn't need C6 at around 15 MHz, you might need it a lower frequencies (eg 1 MHz) to keep RF out of the following audio chain. R3 effects the voltage at R4. If you can make R3 1meg then you get a bit more gain but it depends on Q3, Q4 and Q5.
C4 and C7 filter out RF, which is kind of a main trouble point. I think C4 is too high and C7 a bit low.
I have only tried the circuit at higher frequencies (15 MHz). I don't know if there is enough RF filtering for say the AM band. I didn't want to include an RFC to improve RF removal.
R7 can be omitted but allows for an approximate impedance match to the AM demodulation circuit and gives higher gain. It could be reduced to 22k, it depends on the Hfe of Q5.
Q3 and Q4 are a current mirror at DC. If they are approximately matched (same type and batch) then the biasing of Q4 should be correct, else adjust R5.
Persumably Q1 and Q2 should be approximatly matched too, all I can say is I never had any trouble with that cross coupled differential pair Q multiplier circuit. Possibly it is not quite as frequency stable as a properly biased and set up differential pair would be and you have 2 emitters kind of floating around at the end of a relatively high resistance. I kind of noticed slight, more general hand capacitance effects near the receiver even if shielded. Maybe the 2 things are connected.
I don't hear any AM detector noise with the resistor choices given.
I am able to get such high Q multipiication with the simple cross coupled differenial pair that at 15 MHz I can adjust regeneration to get serious audio bandwidth cutting.
For once I did properly design L1 for low loss which may be a factor. There could be some cancelling out of phase shifts due to using the same inductor tap for input to differential pair and output of the differential pair. Maybe that is helping.
I will experiment more with different AM detector input impedances, from very high where the input impedance is just really the base emitter capacitance to a quite low impedance to where the AM detector is just starting to resistance load the the tuning inductor. I mainly want to see if you can get a little more sensitivity with a lower input impedance.
With very high input impedance the AM detector generates a certain amount of noise which is still below the noise floor of the band, it's not really much of an issue. If I can get a bit more gain with lower noise at a lower input impedance then of course that is the choice to make.
Since Q4 is operating at very low current it is possible to use a microwave transistor with say an Ft of 7 GHz+.
You would imagine that can give detection capabilities up to VHF and UHF.
Anyway you cannot normally use those transistors in hobby circuits at the collector current they are normally used at because they will simply oscillate in the microwave region. What I found though was if you operate them at a collector current of below 100 uA they are actually useable by the average person.
I did try them before in a differential pair regen at low collector current and they worked fine. They produced more noise than more conventional transistors. However if you want to try for a VHF regen it is an option.
I thinkt it's a good practice to include C2 over Q3, otherwise the impedance of Q3 forms together with the base-emitter capacitance a low pass filter which causes a gain roll-off at shortwave frequencies.