Balanced microphone preamp

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HETHER IT IS FOR karaoke, public address or for a band, a

is a basic requirement. This Balanced band equaliser and can be used to drive or provide an additional channel for aBalanced microphones are desirable

This Balanced Microphone Preamp comes with a 3-band equaliser and is suitable for Karaoke, public address or many other applications. It can run from a plugpack, its own internal 9V battery or phantom power.
by JOHN CLARKE

present in both signal lines.

In control
a musical instrument so that it sounds crophone or to remove sibilance (the reducing thetreble level and boosting the mid range. Or the bass control can be reduced to suppress popping noises microphone too close.

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Parts List – Balanced Microphone Preamp
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This provides a gain of 46 by virtue of the 100kfeedback resistor between pins 1 & 2 and the 2.2k resistor to the half supply rail (Vcc/2). IC1b drives the following 3-band equaliser stage via a 2.2 F capacitor.

EQ controls
The equaliser stage is based on op amp IC2a and potentiometers VR2, VR3 and VR4. These potentiometers and their associated resistors and capacitors are in the feedback path between pins 6 & 7. Each of the Bass (VR2),Midrange (VR3) and Treble (VR4) feedback networks are effectively in parallel and act more or less independently (ie, with modest interaction). When the tone pots are all centred, the gain over their respective frequency ranges is unity (–1) and therefore the overall Let’s now look at the Bass control in more detail. When we wind the wiper of VR2 fully clockwise towards the output of IC1b, the inputresistance for IC2a now decreases to 18k while the feedback resistance increases to 118k . At the same time, the 15nF capacitor is completely in the feedback circuit across the 118k resistance. Without this capacitance the gain would be –118k /18k or –6.5 (ie, +16dB boost). The addition of the capacitor forces the circuit to give this gain below 100Hz and this reduces towards –1...