Balance Input for Microphone Line

Figure 1 

The circuit was designed for providing a microphone line with a balance input signal to improve the quality of sound with sufficient gain and filter.

• NE5532 – an internally compensated low noise dual operational amplifier with features such as full power bandwidth up to 140 KHz, input noise voltage of 8 nV, common mode rejection ratio, 9 V/us slew rate, high DC voltage gain, 32 V peak to peak voltage swing, wide supply voltage range from 3 V to 12 V, unity gain bandwidth at 10 MHz, and internal frequency compensation
• TL072 – a low noise JFET input operational amplifier with features such as common-mode input voltage range, high slew rate, operation without latch up, compensated internal frequency, high input impedance at the JFET input stage, low noise, low total harmonic distortion, protected from output short circuit, low input bias and offset currents, wide common-mode and differential voltage ranges, and low power consumption
• BC560 – a PNP epitaxial planar Silicon enclosed in a 24-pin dual in-line package or 3-pin TO-92 package typically used for switching and AF amplification due to its low noise and high voltage features

The circuit is made up of the audio from the part of input mixing console from the microphone or from any other source made of high level. The input can be used on the circuit itself or be multiplied to a number of channels that can be attached to the circuit. There are several functions that this circuit may illustrate such as reverse face, phantom power, and signal attenuation, to avoid distortions from signals with high level. The switches are made to function as phantom power for S1, mic-line selector for S2, 20 dB attenuation for S3, and phase inversion for S4.

In extreme conditions, the microphone can deliver a signal of 1 mV up to approximately 1 V. signals of around 100 mV to 1 V can be accepted by the line input and provides amplification. To amplify the signal as necessary up to 1 V on peaks, a microphone preamplifier may be used to deliver a low noise signal to the next circuitry. The phantom power supplied to the microphone by the microphone input is around 48 V.

Using the gain control, the amount of gain can be set although there are some consoles where there is no gain control on the line input. For full mixing console, the gain channels are regulating the signal while the rest of the stages will be added to the next stages. An inversion of the signal is possible through the phase button if present. The low frequencies below approximately 100 Hz are cut using the filter while the signal is being attenuated usually at 20 dB.

The mic/line balance input are normally applied to the mixers which are being used in conference rooms, rental sound systems, churches, hotels, and schools. The direct outputs of the mixer can be utilized with other mixers for usages which require flexible zone distribution, room combining recording, post-production, sub-mixing, and other audio applications.

Source:users.otenet.gr/~athsam/mic_line_balance.htm