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Coils

Hi all,

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?

Cheers,

5 comments

5 Comments

LarryDee

Jan 26

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.

johnzybourne

Jan 27

Replying to

Neat! I had never even heard of an OPA2134 before. I'll have to order some as well. Do you know if the MK484 or the 7642 can run into the shortwave bands or are they fixed to AM?

LarryDee

Jan 25

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.

- Larry

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/

johnzybourne

Jan 26

Replying to

Thanks for this. I have a bag of TA7642s laying around here somewhere. As I understand they're very similar to the MK484. I built an AM set with one and paired it with an LM386 and a speaker. It worked alright but squealed when I tuned too far up or down the band. I could never figure out what the problem was and ended up setting it aside. I think I'll try your method of using one of those chips without an LM386 opamp and see if I get better performance with a crystal earphone. Mike's Electronic Parts is a great website, I'll pick up some litz wire. I've only ever used magnet wire, is litz that much of an improvement?

LarryDee

Jan 31

Replying to

Litz Wire has a High Q, so tuning will be tighter. And it is magnitudes higher in "Output" It's kind of like having a 30 foot outside wire, but simply wound up on the ferrite rod. Huge difference.

It is very important to use the correct metal for a ferrite rod in the AM band

Type 61 Ferrite rod NiZn

(not Magnesium)