This is basically a low pass circuit which is used to separate out low frequency sounds from audio signals at audio play back devices. A simple loudspeaker is not capable of reproducing all the frequencies of the audible range. Different kinds of loudspeaker are available which can reproduce the sound at certain range of frequencies. Tweeters are the kind of loudspeakers which are used to reproduce high frequency audible sounds and woofers are the general term for the loudspeakers which are used only for reproducing low frequency sounds. In an audio playback device at least the low frequency signals are required to filter out, amplified and fed to the woofer and such kind of circuitry is called bass separator circuit.
This article discusses how to design a simplest active bass separator circuit with design details. The bass separator circuit alone is realized with the help of commonly available op-amp ICs. For demonstrating the working a bass beat is played in a mobile phone which is captured, amplified and mixed with a high frequency musical signal and is then again separated out using the bass separator circuit and reproduced in a loudspeaker.
This 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 mixed signal is then applied to a Butterworth low pass filter to separate out the low frequency components and they are amplified with another op-amp based circuit before fed them to a loudspeaker.
The microphone coupler is a circuit which helps to couple out the weak audio signals generated at the microphone. 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.
TWO STAGE AMPLIFIER
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 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.
The music generated in this circuit with the help of a versatile musical IC UM66. This IC can works in the voltage range of 1.5V to 4.5V. The IC has three pins and the first pin is where the supply voltage is applied and the second pin is connected to the ground and the third pin produces a musical output signal.
Since the maximum voltage rating of the IC is only 4.5V, A 100 ohm resistor is connected between the first pin and the 5V power supply, which will produces a voltage drop when the current flows through it and hence maintain the voltage at the first pin at less than 4.5V.