USB Microphone Line Level Preamp

Posted on Apr 24, 2012

The computer needs at least 180 millivolts peak-to-peak and the condenser microphone in my headset produces about 20 millivolts peak-to-peak into a 2.2k load with normal speech levels. There are USB ports on the machines, so I can use that for +5 volts. After a quick look in my parts boxes, I decided to put the input jack and output plug on the end of wires so I would not have to tediously mount them on the enclosure. The resulting goal was, therefore, a condenser microphone preamp with 19 db gain that is powered from a USB port that can be closed in a small box to hang off the computer and attach to the microphone lead on the headset.

USB Microphone Line Level Preamp
Click here to download the full size of the above Circuit.

The preamplifier itself is a single NPN transistor, and it gives about 28 db of gain. The electrete microphone element includes an inverting FET (Field Effect Transistor) amplifier, and the drain of this FET is the the output of the microphone element. The NPN transistor in the preamp is connected as an AC transconductance amplifier, meaning that its output is a voltage proportional to the AC input current. The scale factor is set by the 100k feedback resistor. With this value, the amplifier's output is 1 volt peak-to-peak for each 10 microamps peak-to-peak of input current. The drain of the FET in the microphone element needs to be pulled high through a 2.2 k resistor connected to a sufficiently high voltage. That's the purpose of the 2.2k resistor on the microphone input. The other end of the 2.2k resistor connects to the power supply filter, made of the 100 Ohm and the 330 microfarad capacitor. The output of the power supply filter also provides power to the amplifier circuit. Since the collector of the NPN transistor is at about +1 volt, I was concerned that a load resistor inside a computer, which connected to a positive power supply could cause the 4.7 microfarad output signal coupling capacitor to be subjected to revers voltage, possibly resulting in the preamp not working and the eventual destruction of the 4.7 microfarad capacitor. I found that many sound cards put the load resistor on the "Ring"...

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