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This is a great shortwave Radio
I am putting together a pre-amp / headphone amp for my radio. The CMOY headphone amp is pretty famous, and it works great. The only modification I made was the addition of precision adjustable resistors for R4. That will allow me to adjust the gain without messing around. I used a prototype board from Amazon, ElectroCookie Snappable PCB, Strip Board with Power Rails, one of the easier boards to use. I have built this exactly as shown in the schematic and it works great, except I use one 9 volt battery. My next iteration should have better placement on the board. Inching my way toward an AM/FM Stereo SW radio. I'm cheating on the FM radio, using a module. Going to use a TDA2822M Audio Amplifier Board 2.0 Channel Stereo module for the speakers. My goal here was to build a great AM, SW. The rest is modules.
I went over my breadboard several times before I posted this. It appears that I short the coil to ground when I connect the C3 to the tank. Poking around , when I connected to the gate of Q1, it worked great, with one battery powering the whole thing, no noise or hum. I know that is also the antenna, but when I remove power, the signal drops off dramatically, power it up, it increases the signal. Also, If I remove Q2, the signal drops .
I'm using the time station at 3330Mhz from Canada, which is usually burred in noise for me. It comes in strong with this preamp. I'm sure I am missing something, but it works. Probably getting the signal back via ground from the emitter.
Short the tank coil to ground at DC? or AC?
The only DC path to ground I can see in your regen would be for a drain to gate short in the jfet oscillator to occur.
Maybe C3 is bad? Is it possible that the whip preamp itself is oscillating? If it is, you could get all sorts of weird stuff happening.
Those are huge value coupling capacitors in that whip preamp. I would think 10nF or even 1nF or less would be fine for the frequencies your regen is operating at. The two circuits may be too tightly coupled together.
Thanks for you having a look, I appreciate it very much. Ooops, I posted the wrong schematic. Here is the one I used. When I connect C3 to the antenna connection below, using the same battery and ground, it just hums like a bad circuit. When I use two batteries, and don't connect the grounds, it works but there is distortion. Using one battery to power everything, when I connect from from the gate to the antenna, it works perfect. I did try substituting the caps, but no luck. I'll try again. BTW, the performance is unbelievable, so I hope I get it to work.
@LarryDee First, use two batteries (the working configuration) and use your multimeter to measure the circuit voltages at strategic points. Then, connect both circuits to the same battery and confirm the circuit voltages are the same. This will confirm that you have not made any basic wiring errors.
If the circuit voltages all look OK, then: It's conceivable that connecting both circuits to the same battery is now causing the entire receiver to motorboat (oscillate at some audio frequency), preventing any reception at all. Probably half of the time I spent debugging my regenerative superhet was (and still is) related to stopping kind of unwanted AF oscillation.
The usual way to fix this is to insert RC decoupling networks at strategic locations in the power supply line. You can read a little about this here: http://www.ke3ij.com/amp.htm .
Also, your schematic does not show any AF amplifier. I assume that you are using one. How is your AF amplifier powered? Is it using the same battery as the antenna amplifier and the regenerative detector? If yes, then this again can cause problems with unwanted AF oscillation, due to the typically high current draw of an AF amp. As the AF amp draws more current, it may slightly change the voltage seen by other, earlier stages. These earlier stages may then end up trying to amplify this voltage change, causing a feedback loop and AF oscillation. This AF oscillation can then prevent reception of any signals as the whole circuit is busy amplifying its own self-generated noise and oscillating.
I use a lm386 amp module, has its own battery. I'll do the tests you suggest.
The issue is that both circuits - receiver and preamp - work fine when each is connected to its own battery power source, but does not work at all when one battery powers both?
What type of battery (ies) are being used?
That's a head scratcher.
2 - 9 volts. Kinda weird. It definitely amplifies the signal. But wiring them together, I'm missing something simple.
These two circuits work great. Does anyone know how to run these both off of the same battery? I tried connecting the grounds and positive to the points on the schematics, but the whole project goes dead. I thought I could do a hack with ultra low current voltage regulators for each, but would like to know the proper way to integrate these circuits. The preamp works so good I'm going to make a stand alone version. The copper plate is a small copper clad board. I use a wire, but the plate does work, but it's meant for mounting outside, really cool either way. As it is, it pulls in signals I could never get before, and it isolates the antenna. I feed the output of the preamp as is directly to the antenna, but connecting the grounds caused it to fail. Thanks.
I finally got my circuit to work the way I wanted. It tunes from below 3 MHz to just over 17 MHz. With qrp-gaijin suggestions, I figured it out. I m only refencing my findings to this circuit, so what I say here may not be useful in other circuits.
I could not find or get to work properly, any RF preamp circuit or a grounded gate circuit with this circuit. They just messed it up, gain was terrible. As configured, it sounds great, so I stopped trying to add the preamp. I use a Fielect Toroid Core Ferrite Choke Iron Powder Inductor Ferrite Ring 19.8x33x11.1mm from Amazon. The wire is Fielect 0.55mm Inner Dia Magnet Wire Enameled Copper Wire Winding Coil from Amazon. I found that the thicker the wire (lower gauge) the better the gain. But as you'll see below, you can only go so far. Through experimentation, I found that the winding really affects performance. You can wind it to any frequency range you want, with a 365 pf capacitor, but there seems to be a point where it works, but not well. You need a good amount of turns to get a high quality signal. That presents a problem for frequency coverage. With 40 turns = around 8uH C5 at 470pf, and a 365pf tuning capacitor, I could cover 3 MHz to almost 10MHz. If you use this calculator.
You'll see that it should go much higher. But in practice, it doesn't. Qrp-gaijin mentioned I should try a 4700pf for both C5 and C6, and changing the values led to success. Qrp-gaijin suggestion had me focus on the correct components, or I never would have gotten this to work in the higher frequency ranges. Since raising the values didn't work, I thought, maybe they should be lower. When I lowered C5 to 100pf, and changed the coil to 16 turns, the circuit tuned around 8 MHz to 17 MHz with nice fidelity and good gain. C6 is left at 470pf. The amount of turns on the core is very critical, less turns gets higher in frequency but no gain, more turns and it doesn't cover the band I want, and with high turns and a low value C5, it oscillates real bad. As it is, the selectivity is really great, but the tuning is tight. A gear down would work well. You should take notice of the 5K pot VR1 instead of 2 K, it just works better. I will have to use a switch to move the coil and cap in and out, but this little thing works as good as my Tecsun PL310 and has great fidelity, using a 2 foot wire. For bigger or powered antennas I will couple to the ferrite core with a few turns to keep it from transmitting. Next step is to get it off the breadboard onto an all copper board, Manhattan style (I think that name sounds cool) Hope you find this interesting. If anyone would like please let me know if you want a post with the parts list and suppliers.