Balancing amplifier for electret microphone. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The proposed amplifier for an electret microphone has a practically symmetrical current output to the subsequent amplifier of the mixing console with a symmetrical input, receiving "phantom" power through the signal wires of the connecting cable.

As you know, in professional sound amplification equipment, microphones are connected to balanced (balanced) inputs of amplifiers, which allows using relatively long lines without any noticeable influence of external noise sources on them. For a condenser microphone, the polarizing voltage (usually 48V) is supplied through the input connector from the console's power supply.

Condenser microphones are relatively expensive, and in many cases in amateur practice it is possible to replace them with electret microphones. Such small-sized electret microphones are used, as a rule, in wireless radio microphone devices from Shure, Sennheizer, AKG, or in consumer sound recording equipment. The microphone is a design in which an amplifying stage is also built in to match the microphone with a relatively low-impedance input of the device following it. A thin and flexible cable is removed from the capsule - two cores in the screen.

If it becomes necessary to use an electret microphone, for example, as a pickup for a string or wind instrument, it is not at all necessary to use a radio channel. This microphone can be connected to the balanced microphone input of a mixing console through a simple device, also powered through the input connector.

(click to enlarge)

The matching amplifier (its circuit is shown in the figure) is inseparably connected to the BM1 microphone, which excludes its arbitrary disconnection during operation, causing a voltage surge in the entire amplification channel and a loud click in the speaker system. The microphone is powered through resistor R1 from the emitters of transistors VT1 and VT2. The signal from the microphone is fed to the input of the differential stage (the base of the transistor VT1) through the capacitor C1. From the collectors of transistors VT1 and VT2, the balanced signal is fed to the microphone input of the mixing console or other device.

It also supplies voltage to power the entire device: directly to the collectors of transistors VT1, VT2 and through resistors R5 and R6 to the base circuits of the differential stage.

To stabilize the base bias voltage, a "stabistor" of six silicon diodes connected in series was used. A stable voltage of about 3,6 V is formed on them. The use of a zener diode is undesirable, since the noise level increases markedly.

The device uses a Sennheizer MKE-2 microphone. You can use a similar microphone from another company. Instead of C536A transistors, you can use other low-noise ones with a maximum voltage Ukb >= 50V, for example, domestic KT3102A or KT3102B with a pair selection according to the specified parameter, as well as according to the parameters h21e and Ube. It is advisable to use capacitors C1 and C2 specially designed for audio equipment - Black Gate or other well-known companies.

For mounting the amplifier, a breadboard with a grid of holes 2,5x2,5 mm was used. The whole device is housed in a battery case similar to the Krone, which is connected by a conductor to a common wire. The microphone cable is connected to the board: the red wire - to the connection point of R1 and C1, and the blue one - together with the braid to the common wire. To the points on the board, to which the collectors of both transistors are connected, and to the common wire, a balanced cable is connected, on the other end of which an XLR connector plug is soldered.

You can check the electrical modes of the device by connecting it directly to the microphone input of the mixing console (phantom power must be turned on) or to a stabilized voltage source of +45 ... 48 V. By connecting pins 2 and 3 of connector X1 to the positive output of the power source through resistors 33 kOhm, and pin 1 to negative. When measuring the current in the power circuit break, you need to make sure that it does not exceed 2 mA. Then you need to measure the voltage across the resistor R2; it should be about 3 V. Next, the device must be tested in action.

Author: M. Sapozhnikov, Ganei Aviv, Israel

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