I'm fooling around with crystal sets again, well amplified crystal sets. I'm slowly working my way up to proper reflex and regenerative receivers. I was wondering if anyone had a favorite coil type they liked to use with crystal hookups. What is a good ratio of turns between an antenna load coil and a tank coil?
The litz wire is a Huge improvement. If you build this, with an MK484, you'll get rid of the oscillation. But I did not use the secondary coil for the antenna, I connected the cap directly to the Ls , which is the ferrite rid in this case. The MK484 is actually a better chip, more selective, but just to start out, the 7642's are OK. The Power V+ voltage is 1.5 volts
You can use a voltage divider to get the right voltage
Instead of an LM386, which really is bad, try this headphone amp. You can buy the chip directly from TI. The OPA2134 is absolutely awesome. It's just a headphone/ preamp- amp. If you are going to feed a prebuilt amp module from this headphone amp, do not buy a digital amp Class D, get an class AB linear amp. The class D outputs too much noise, and the radio will be saturated by it. Hope that helps.
I can save you a lot of time. if you use the following for a crystal set, you may not even need an outside antenna or a ground. If you use the following for an MK484, you'll have a much better performance Tuned Radio frequency radio.
I am planning on putting this in the beginners section.
The Litz wire gets wound on the ferrite rod, You can cut the ferrite rod down to size buy scoring it and breaking it. About 100mm width should do for a start. You will experiment on the windings. You'll wind up to about 70mm or so. My suggestion, don't go with small ferrite rods, go with what I have here. Bigger is better.
It would be good to get an LCR meter to check the microhenry value, I use a cheap Proster BM4070. Your budget will determine what you buy, don't go too cheap, it wont work. You can also use the meter to check the value of the capacitor, so you can learn about the effect, and also help connect it correctly.
The Poly Variable Capacitor is simply awesome, Get it with the knob and extension. The litz wire listed is the only way to go, it is very selective. I've tested all this, and it is just great.
Hope that helps.
Poly Variable Capacitor Dual 335pf and 20pf --Mikes
The Special Litz 180/46 wire came -- Mikes
The MK484 came --Ebay or Mikes
The Type 61 Ferrite rod NiZn R40C1- 200x10mm for High Q Amateur & Crystal Radio Coils, AM SW(151884949745) --- Search Ebay https://www.mikeselectronicparts.com/