The next item to look at is the signal output. The audio is being carried over pin 2 of the XLR connector, but what's is with the stuff connected on pin 3? If we look at the mixer side, we see that whatever signals are carried on pins 2 & 3 will be subtracted from each other via an op amp. When a microphone is used, it will actually present a differential signal to the mixer input. Any signals present on both pins will cancel out, and the difference between the signals becomes the amp output. But I only have one signal, right? Ideally yes. However, the cable carrying the two signal lines will be subject to all sorts of noise. So pin 2 will carry the desired audio signal plus some noise, while pin 3 will only carry the same noise as pin 2. When they get subtracted, all that is left is the audio. C4 & R7 are chose to match the frequency response of the JFET buffer output, which essentially makes a high pass filter (this ensures that the noise on both lines is filtered similarly). C4 can just be made equal to C2, but R7 has to be calculated. I did this by running a 1KHz tone through the buffer with no load and with a dummy load to determine the voltage drop internal to the buffer. I took the measurement prior to C2 so that I would not get any reactive component to my values.
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| Simplified circuit showing only power related parts. |
Lastly, there is the phantom power circuit. It's pretty simple and ingenious. The mixer applies a DC bias to both audio carrying conductors, which the device just has to tap off thru a pair of equal resistors. Capacitors at the inputs and outputs block that DC from the signal drivers and receivers, making the rest of the circuit none the wiser. You do have to be careful though, as the more current you draw, the less voltage you will have to bias your circuitry. That's where the zener diode comes in. It will draw whatever current is needed to get the voltage drop across the 6.8K and 10K resistors to leave us with 12V. As long as the current thru the zener is greater than any current your device needs, we'll have a steady enough voltage rail.
So after the solder smoke cleared I plugged the buffer amp into the mixer and happily watched the LED light up. I hooked up an electric guitar and..... well..... it was punchy enough, but piercingly bright as well. This wasn't a huge surprise, but I had gotten away with the circuit up until now so wasn't going to change it at first. C2 is the problem. It's too small. It was creating a high pass filter with the mixer input that didn't include any of the bass. Or mid tones. But it had worked well for years right? Yes, and I found out why, and I'll tell you all about it in the next post.
