The different sounds in songs like the sound of the guitar, drums, the voice of the singer etc. are recorded as separate tracks using separate microphones. More than 10 numbers of tracks are very common in normal quality songs. The audio mixer circuits find its applications in both recording and playback side. When recording a song the audio mixer circuits are used to mix two or more different sounds into a single track. The audio mixer devices used by the DJs for playing songs are familiar to most of the people.
This article discusses about the design of a simple audio mixer circuit. An op-amp
based summing amplifier is used here to mix two sounds. The audio
mixing is demonstrated with the help of mixing a high frequency musical
sound with a low frequency bass beat, where the musical sound is
generated by a musical IC and the bass beat played at a mobile phone and
is captured and amplified through a microphone and amplifier circuits.
DESCRIPTION
This audio mixer circuit
uses two stage amplifiers with a microphone to capture and amplify the
bass beats played on an external device so that it should have enough
loudness when mixing with other sounds. A music generator IC is used to
produce high frequency musical sound which will be then mixed with the
audio mixing circuit. The audio mixing circuit alone is a very simple
summing amplifier made with an op-amp.
The
microphone coupler is a circuit which helps to couple out the weak
audio signals generated at the microphone. There are different kinds of
microphones which have different working principle, but all of them have
a diaphragm which vibrates according to the sound signals. As the
diaphragm vibrates the current flowing through the microphone varies
according to the sound signals amplitude which made the diaphragm to
vibrate. Here in this circuit a condenser microphone is used which and
the varying current is made to flow through a resistor across which the
equivalent voltage get generated due to the current flow. This voltage
across the resistor will be having a DC voltage on to which the varying
voltage gets added up. This varying voltage is separated out from the DC
voltage with the help of a coupling capacitor and fed to the following
amplifier circuits.
With a condenser microphone a 10K resistor and a 0.1uF coupling capacitor is used in most of the circuits.
Here
a single transistor based amplifier circuit is used as the first stage
amplifier for the audio signals coupled out from the microphone. This
circuit is designed to have extremely high gain so that the audio
signals are get amplified enough. The transistor is connected in a
common emitter configuration and fixed bias technique is used for
biasing the transistor.
The
volume controller used here in between the transistor stages is a
simple potentiometer, which will attenuates the output of the first
stage amplifier before it is fed to the second stage amplifier. This
volume controller circuit helps to keep the amplitude of the signal
within the input range of the second stage amplifier circuit.
The
second stage amplifier is exactly similar in design with the first
stage amplifier. This amplifier simply amplifies the signal more and at
the output of this stage one can obtain a good enough voltage amplified
signal which is ready to be current amplified by the following current
amplifier circuit.
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