Against the background of the AC mains frequency. induced on cables that are in high-quality microphone amplifiers use a balanced input, implemented as a rule, difficult to manufacture and require careful shielding from external magnetic fields balun transformers. The figure shows a diagram of a microphone amplifierthat allows you to dispense with such low-tech item, like a transformer.

Microphone amplifier with balanced input

The basis of the device serves as a differential amplifier at the shelter DA1.1 and DA1.2. His gain Ku=1+(R8+R9)/re (re is the equivalent resistance of resistors connected in series and R6 is introduced into the circuit parts of the resistor R5) and can be adjusted in the range from 1.5 to 140 resistor R5. The amplified signal through the coupling capacitors C5 and C6 is supplied to the symmetric Output 1, and through the second differential amplifier (op-amp DA2) - unbalanced Output 2.

Because modern OS have almost perfect (>70 dB) suppression of common-mode signal noise immunity of the amplifier is determined almost only by the consistency of the resistances of resistors R3 and R4, R8 and R9, R11 and R12. R13 and R14, and if they do not differ from those indicated in the diagram by more than 1 %, yields the best noise immunity of the device with the transformer input.

The input impedance of the amplifier 10 kω. Nourish it from the bipolar stabilized source voltage of ±10 V.

In the amplifier you can use native OS CUD, CRUZ and (at lower voltage litany to ±.6 IN) CPU.

(RADIO N 6, 1986, p. 64)

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