Tech Forum

I could give you a lot of technobabble, but the truth is there's no practical difference between 1N34A and 1N60 in this application. Most people get a bag of diodes, preferably from different batches, and check them for the loudest signal with best sound quality.

Both diodes — 1N34 and 1N60 —­ are germanium diodes, and both are of similar physical size. The important thing is that this diode family has the smallest forward voltage characteristic, which is important for rectification of small voltages. They both will operate at radio frequencies. The forward current rating and the reverse voltage characteristics are unimportant in this application.

Bottom line: Either one will work in this application. My personal choice would be the 1N60 because the documentation available for this device is superior, with V-I curves to show the typical forward voltage characteristic.

Either should work well. Both have a conduction knee starting around 0.15 or 0.20 volts. The IN60, being slightly newer, probably has more tightly-controlled specs. One source for diodes and their specifications: http://store.americanmicrosemiconductor.com/1n60.html and http://store.americanmicrosemiconductor.com/1n34a.html. BTW, for better Q of the tuner, connect the diode to a tap on the coil.

You can use either type and expect the same results. Both diodes are germanium and both have a forward voltage drop, (often called “turn-on” voltage — where a diode begins conducting), of about 0.3V. A diode with even lower forward voltage is the 1N5817, a Schottky diode, which has a forward voltage of about 0.16 volts. The forward voltage determines how weak a signal can be heard.

The author at the following URL presents a comprehensive table of 1N34 and 1N60 subtypes and a few Schottky diodes used as detectors in crystal radios. If you view it, look in the column labelled Measured Vr. http://wiki.waggy.org/dokuwiki/crystal_radio/detector. However, long before the sensitivity of the diode becomes the limiting factor, four other factors will limit the performance of the radio you propose. Those are:
   Selectivity — only one tuned circuit is used and it is not impedance matched at input or output.
   Antenna length — definitely use more than 10 feet — a goal would be 50 feet and as high as possible.
   Ground losses — connect a wire to Earth or to a large expanse of metal.
   Frequency of operation (also related to selectivity) — with this type of circuit, as you increase frequency, the bandwidth increases. This means it lets through more and more stations at the same time.

If you haven’t had the opportunity to read them, you’ll probably find the insights in the wiki entry on crystal radios time saving. Especially note the sections on tuned circuits, impedance matching, and the problem of selectivity. It can be found at: http://en.wikipedia.org/wiki/Crystal_radio The following URL shows how to connect your tuning circuit and diode directly to your LM386 without the LM741 you have in the middle of the circuit. http://makerf.com/posts/an_lm386_powered_crystal_radio_in_an_altoids_smalls_tin

You specified a number of turns for your coil but I didn’t see any diameter for the it. Starting coil designs would be 56 turns for a 5 inch diameter oatmeal box or 75 turns on a 2 1/8 inch diameter coil, each of which could be used with your 365 pf variable capacitor. 22 to 24 AWG bare enameled wire would work for the 5" diameter. 28 to 30 AWG would work for the 2 1/8" diameter.

To help optimize selectivity, you want what's called a "square coil." This means the coil length is about equal to the diameter. Not critical but helpful. Small diameter wire increases resistance which degrades selectivity. It likely to be less frustrating to first get your design working at the lower AM broadcast band frequencies before pushing up into the shortwave frequencies.

Last, it looks like you might put taps on your coil. The following URL has photos that might give you helpful ideas. Once on the website, click on the “Oatmeal box crystal set.” www.midnightscience.com/download%20files/XSOB1-manual-050108.pdf Please accept my apologies if I've included to much information. Best wishes for your success.