Modification of a WR-3 VLF receiver
WR-3 is an excellent receiver created by Stephen P. McGreevy. Small, compact and very portable which makes it ideal to take it with you, anywhere you go. Here I will describe two modifications that you can add.
First one is a direct output. In case you are perfectionist you would probably want to pick up the signal straight after it passes filter and before it enters preamp. Most preamps will add noise, and if you have obsession with noise (like i do), you would probably like to do this mod. However this is not the main point of installing this mod. You need to keep in mind that LM386 is a low noise op amp. You can record data without worrying about the noise. As a matter of fact, all recording made by Stephen P. McGreevy were done via this op amp. So why doing this mod then?
This is a modification that will provide you a low level signal which will be in a domain of signal generated on typical magnetic loop antenna (4ft diameter, 700ft wire). So now you can connect both the WR-3 and magnetic loop antenna into the same recorder, as their levels will be almost the same. If you have a typical USB recorder, you can record WR-3 to the left and magnetic loop antenna to the right channel. Using this recording method you can do a lot of interesting research and probably some noise reduction. You will record both electric and magnetic fields (which are perpendicular to each other) at the same time.
The trick is to pickup the signal in front of PIN 3 of IC and lead it to a 100 uF capacitor and then to 3.5 mm audio output. Very simple. Looking at the image below you can see capacitor connected to aditional 3.5mm output (2). You can also see there is enough room inside WR-3 to put a switch (1), filter (3), and wires. Just measure everything carefully and it must fit. ( --> ''I've cut this wire twice, and it is still too short !'':).
Second mod is a ''town mod'' as I call it. In case you live in a city, you would probably like to add high pass filter to the output of WR-3 to remove that annoying 50/60 Hz noise. It's primary mission is to save your ears, as listening to that hum can go drive you mad. I designed it as a brick-wall 5 pole passive PI shaped high pass filter. It is a PI shape because there is a difference in impedance from its input and output point as I don't know which earphones you will use. It is designed for about 15 ohm earphones. As a passive filter it will not add any noise, plus you wont need to mess with noisy IC's like you need when building active filter.
From image above, filter's cutoff point is at about 300 Hz and it will go down, at 140 Hz already reaching -60dB. At 50Hz it will already be -100dB. Sounds like a dream filter so far, but it is not just 50/60Hz hum that makes problems, there are also it's harmonics which go up to 1kHz. Most of them will survive, but in general, a filter will do it's job.
I've chosen components for the filter that you can find in almost every decent electronics store. For capacitors you can use (like I did) non polarized ones, as they do exist in 33uF value and I recommend you to use them as you don't need to worry where to connect which side of filter. In case you can only find polarized capacitors of 33uF value, put positive side to input side of filter, and negative to output side (general rule is to put positive side of electrolytic capacitor to higher voltage, and negative to lower voltage side, which is what you will do).
Do not expect miracles from this filter. It will just save your ears, and increase dynamic range of recording, since filter is in front of recorder, so you can put more gain on recorder and pickup more high frequencies if you are interested in them. (I personally like to hear Alpha signals and never filter them. They are a good source for calibrating various things.) Keep in mind that this filter will work only on default output of WR-3. It won't work on direct output, as it is nor connected, nor designed to do so.
You also need to buy a 2 pole switch. It is a switch that breaks two circuits at the same time. This is important in our case as we need to isolate filter 100% from the circuit when it is not engaged. Look at the picture above. You can see that first two contacts are short circutetd (1-1 on schematic below). In the middle we have input and output, and on the right side we have filter connected (2-2 on schematic below).
I am also recommending you to put some isolation on the bottom side of WR-3 box since by moving main PCB you might short circuit it as it might touch the metal case. For example when i bought mine, probably during battery insertion (and without carefully looking) i shortcircuted the PCB of WR-3. Then i started to wonder, why the stuff doesn't work. I even went to store to buy new FET, but the real problem was so simple and it was solved by putting two sheets of paper on bottom side of the box.
So, why is the above schematic blurred? Well the WR-3 is a product owned by S. P. McGreevy Productions. And if you happen to own one WR-3, it wont be a problem to find out what is what on this schematic. And if you don't have a WR-3, then you are probably a wrong guy, in the wrong place, at the wrong time. I won't give you the scheme of the WR-3, go buy the product!
Modified WR-3 ready to fire