Microphone pre-amplifier
The MIC input of a PC soundcard is designed for an electret microphone,
but it can be used for a dynamic microphone. A dynamic microphone
should be connected between TIP and GND.
Sometimes the signal level of a dynamic microphone isn't high enough.
A simple amplifier is enough to get an usable input level.
Design steps:
- Measure the voltage between RING and GND without a microphone connected.
This gives you the internal voltage Vb.
- Connect a resistor Rt of 2-4 K between RING and GND and measure the voltage
between RING and GND again. This is Vr.
- Use Ohms law to calculate the internal resistor: R = Rt * (Vb - Vr)/Vr.
Expect a value of 2 - 4 Kohm.
- Choose the collector voltage of the transistor (normally Vb/2) and
calculate the base resistor. Trial and error also works, a suitable start value
for the base resistor is 1Mohm.
If your computer has a low Vb (e.g 2.4V) changes are that an electret
microphone doesn't work well. An electret draws approx. 0.2 mA. When the
internal resistor is around 4 Kohm, the voltage across the microphone drops to
a low value of 1.6V.
The simple general purpose pre-amplifier below can be used for dynamic and
electret microphones.
It is based on a design from Tomi Engdahl.
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Prototype
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Without the emitter resistor the gain is too high when used as a microphone pre-amplifier for a PC sound card microphone input.
The output resistor prevents the plopping sound when the preamp is switched on.
Remark: if your soundcard is AC97 compliant, the MIC input has an optional
20dB boost. Enabling this is the easiest way to get some extra
amplification :-)
Archive containing the Xcircuit files
(encapsulated postscript).
References:
Archive containing the Xcircuit file (encapsula
ted postscript).
