Stereo Mixer for Microphone with 2 Channels

Figure 1 

The circuit was designed to provide 2 channels on a stereo mixer that will be used for microphones while having a crossfader operation.

• 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
• 7915 – a 3-terminal negative fixed voltage regulator with features such as surface mount package availability, contact factory for other voltage options, availability of voltages -5V, -12V, -15V, fold back current limiting, internal thermal overload protection, compensated output transistor safe area, good load and line regulation, current output to 1.5A, no external components required, internal short circuit, and 1.5% internal output voltage
• BD139 – NPN power transistor used for driver stages in hi-fi amplifiers and television circuits because of its low voltage at 80 V maximum and high current at 1.5 A maximum

The circuit is catering microphone and two similar PHONO or line channels with relative adjustment on the following stages. The two channels can be interchanged as they enter the stereo input channels. One of the inputs is an RIAA filter correction for classic stereo input PHONO. This is responsible for the amplification of signal originates from the classic reproduction heads of LP classic discs. The other line input that comes from tuners, CD players, DVDs, etc., are representing high levels.

Using switch S1 and S2 can provide selection of the two stages before being applied to the following stage consisting of adder IC3. The BALANCE is being adjusted by RV5 while the GAIN by RV1. The next stage is handled by IC4 having a tone circuit from a 3-band regulation which is done by RV2-3-4. The potentiometer RV6 used as fader is being driven by the output of IC4. The microphone input is connected to the BALANCE circuit and operates via IC9A. At this stage, the GAIN is regulated by RV13. the next stage is handled by IC9B having a 2-band tone regulation. RV16 regulates the parametric mid band for the GAIN while RV17 for frequency band. The VOLUME is regulated by RV19 while RV18 for the BALANCE.

The operation of pre-fader-listen of proportional channels is done by switches S3-4-5. The headphone circuit is driven by IC14 while RV20 serves as the CROSSFADER between the two stereo channels. The final master VOLUME is regulated by RV21 which adjusts the output signal for final percentage. The audio mixer part of the circuit is being supplied with 15V while the RL1 and VU meter is supplied with 12V. a suitable heatsink should be provided with IC18-19-20 and transformer T1 should be placed in a separate box.

The 2-channel audio mixer is intended for performing or production applications. Some designs are being used in CD/tape inputs for routing flexibility, with sound quality microphone preamplifiers. They are used widely by remix artists, producers, and DJs, while being connected to a laptop or PC.

Source:users.otenet.gr/~athsam/2ch_mixer_eng.htm