What does transformer do on this condenser mic preamp?

[dusan]

In this schematic there is output transformer:

http://www.sdiy.org/oid/mics/Neumann_Handheld.GIF

What does it do? Can it be omitted?

[rhb]

That is essential.  It steps the tube era 48 V phantom power voltage down to what the transistors and op amp require.

Reg

[dusan]

But why is secondary on the output of the opamp?

[newbrain]

Because the transformer does nothing of the sort - as it cannot pass DC.


The op-amp and associated circuitry directly pick the phantom (common mode) voltage through the two 6.8 kΩ resistor and the 0 V connection.
The transformer is rather used to isolate the output of the op-amp from the phantom voltage, letting only the amplified mic signal pass through and not the DC offset at its output.

EtA: Why a transformer, one may ask, instead of a simple capacitor? To provide both polarities of the differential signal.

The top transistor provides a soft start supply to the op-amp and slightly reduces the voltage (probably the op-amp is a 741, capable of working at ±22 V - be careful if you re-implement this with more modern op-amps), and the bottom one polarizes the + input to about half of the supply, still soft starting.

[wasedadoc]

The transformer converts the single-ended output of the op-amp to balanced (differential).

The op-amp is probably NOT a 741.  The input bias current for the inverting input comes through a 470 MOhm resistor. For a 741 that current can be as high as 800 nA.  470E6 times 800E-9 is 376 Volts.

[newbrain]

376 Volts.

Oops!
I did not notice the feedback resistor value, thanks for the correction!