Microphone amplifier with balanced input. Scheme, description

Microphone amplifier with balanced input. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

To combat AC hum mains frequency. induced on connecting cables, high-quality microphone amplifiers use a balanced input, which, as a rule, is implemented on balun transformers that are difficult to manufacture and require careful shielding from external magnetic fields.

The figure shows a microphone amplifier circuit that allows you to do without such a low-tech element as a transformer.

Microphone amplifier with balanced input

The basis of the device is a differential amplifier on the op-amp DA1.1 and DA1.2. Its gain is Ku=1+(R8+R9)/Re (Re is the equivalent resistance of resistors R6 connected in series and part of resistor R5 inserted into the circuit) and can be adjusted from 1,5 to 140 by resistor R5. The amplified signal through the coupling capacitors C5 and C6 is fed to the balanced Output 1, and through the second differential amplifier (op-amp DA2) - to the unbalanced Output 2.

Since modern op-amps have almost perfect (more than 70 dB) common-mode suppression, the noise immunity of the amplifier is determined practically only by the consistency of the resistances of the 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%, are not inferior to the noise immunity of the best devices with a transformer input.

The input impedance of the amplifier is 10 kOhm. It is fed from a bipolar stabilized source with a voltage of ± 10 V.

In the amplifier, you can use domestic op-amps K157UD2, KR1407UDZ and (when the voltage is reduced to ± .6 V) KF1407UD4.

Literature

Radio No. 6, 1986 p.64

Publication: N. Bolshakov, rf.atnn.ru

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