From the first op amp stage we feed both the pin 2(+) of the XLR via 200 ohm resistor (50 to 330 ohms will work) and also a second op amp stage setup as unity gain buffer. If you put the two capsules back to back, you just created a figure 8 response pattern. One capsule feeds the inverting input, one feeds the non inverting input. You could use a 5532 dual op amp also but don't use one that isn't meant for audio use. I like this one as it is very low noise and has great specs for audio use. Then we couple the signal into a differential amplifier based around one half of an OPA2314 Operational Amplifier (op amp). The FET acts as an impedance converter and allows us to generate a usable signal. That voltage shows up on the 1 gig ohm resistor and the gate of the FET. The movement changes the capacitance of the capsule which causes a small voltage to be developed. Sound waves hitting the capsule cause it move with the sound. The capsule contains an internal charge because it is an Electret capsule. I already had nine more of the TSB2555B capsules.) Here is how the capsule converts sound to a signal. Most inexpensive electret condenser capsules have the FET built in but finding a really good one is difficult. This time we are just using it as an impedance converter. Here is how this all works We start with a J305 FET similar to most condenser microphone circuits. Using two of the circuits, one for left and one for right completes the project. So I designed a circuit that combined two capsules into one signal but inverted one of them thus creating a figure eight microphone. I wanted one enclosure and one stereo output, not four individual outputs. I could have just built four of the single mics with electronics into one enclosure but then you have to use a mixer to combine everything correctly. These should be perfect for this project. They are very flat with a slight 3db bump around 10K hz. I recently found some very good microphone capsules, the TSB2555B, which I used in my “build a cheap LDC Instructable”. All we need to do is create two figure eight mics place them 90 degrees apart, and put them in one enclosure. I figured I would just build one microphone that has everything internally all set up in it to do this. Now that my kid is in the high school orchestra (And it is a good one too!), what better time could there be? To achieve this we need two figure eight microphones, of which I only own one. This works really well for orchestras and live music in a great sounding room. But the beauty comes from also catching the ambiance of the room in the rear capsules also. The front mics pick up a great stereo image of the source and you get a nice spread out stereo image. They face the main sound source pointing left and right in an “X”. This recording method uses two figure eight microphones spaced 90 degrees apart. At the time microphones usable for this method were not in existence. He invented it and patented the idea in1931. If you have never heard of a Blumlein Pair, it is named after Alan Blumlein.
0 kommentar(er)
