Microphone pre-amplifier

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This microphone amplifier can be used for amplifying signals from dynamic microphones.
It features:

- Symmetrical input (reduces pick up of hum)
- Low noise
- Gain of 27 dB (22 x voltage gain)
- Output swing: 5V p-p for one output (or 10 V p-p measured between two outputs)
- Output is short circuit proof

Transistor T1 is a current source of about 2 mA.
T2 and T3 are a differential amplifier with 22.7 times voltage gain, the gain can be reduced by increasing the value of the (100 Ω) emitter resistors.
The signal is buffered via IC1, this must be a low noise, rail-to-rail opamp, capable of supplying at least 50 mA output current, I used a TS922 as opamp.
The input and output connector are XLR type.

The output can also be asymmetrical loaded, taking the signal from one output, and leave the other output unconnected, or shorted to ground.
In this case the voltage gain is halve the value, so 11 times ( = 21 dB).

The amplifier is running on a +10 V supply, supplied by this circuit.
The coil L1 reduces ground loops, and hum at the amplifier output.
The used part number for L1 is FPP2-25-1/a made by "Schurter", it has two 10mH coils in it.
The input voltage of this circuit can e.g. be supplied by a non regulated 12V mains adapter, with only a small load these things have about 15V output voltage.

The circuit (without mains adapter) is in my case build into an aluminium box, for shielding it against electrical fields.
The aluminium box gives however no shielding for magnetic fields, for this reason place the mains adapter (= producing magnetic fields) at a distance of at least 10 cm from the amplifier to prevent hum.

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