With Karaoke's continually growing popularity over the years‒in all aspects of the community worldwide (i.e. home entertainment, business, etc.), the researchers came up with their sort of own version of Karaoke as a transistor application.
The project, entitled "Transistor Level Implementation of Karaoke Machine with six-band Graphic Equalizer," aims to develop a transistor-based, Karaoke-type amplifier that is able to run from the mains power supply and consists of the following elements:
1. a 12-V regulated power supply,
2. two inputs: a microphone and a line input,
3. a Common-emitter mixer/preamplifier stage,
4. a six-band graphic equalizer stage,
5. a Common-emitter voltage amplifier stage,
6. a Common-collector power amplifier stage, and,
7. a loudspeaker output;
as indicated in the schematic diagram below.
Circuit Design and Operation
Power supplies, as defined by Howard (1998), are electronic circuits, basically composed of four sections: transformer, rectifier, filter, and regulator, designed such that an input ac signal is converted to dc, at any desired level. Shown below is a block diagram of a basic power supply.
Figure 2. Basic Power Supply Block Diagram The input line voltage is either stepped up or stepped down by the transformer, depending on the application; in this case, a step-down transformer, T1 rated at 9.5Vac (10.5Vac on actual testing), was used, giving a peak voltage of 14.84V (Vp = 10.5 x ?2), and allowing the device to run from the mains power supply. In addition to that, the power supply is being isolated by this section from the power line. The rectifier section, specifically a full-wave bridge rectifier D1, then converts the resulting signal, still ac, to a pulsating dc, which is made purer by a simple capacitor (C18) filter section, giving a dc hold capacitor peak voltage of 13.44V (Vp – 2(0.7) = 14.84 – 1.4). This leaves enough voltage overhead for the final section, the 12V-regulator IC1 (LM7812) that maintains output at a constant level of 12V and about 1.3A continuous, regardless of changes in load current and/or input line voltages. This configuration has minimum power loss, and negates the need for a heat sink on IC1. The capacitor C19 removes any spikes from the regulator for a smoother output. Howard (1998) Mixer/Preamplifier When a combination of two or more audio signals is expected in a single output, simply connecting the inputs will result to the degradation of system efficiency and poor overall performance due to impedance mismatches of different signal sources and the amplifier input. Furthermore, the differing signal amplitudes of the sources, too, presents another problem since direct connection may result to higher-amplitude inputs obliterating the weaker inputs, and even worse, damage the sources. By isolating inputs and providing independently variable gains at each of these inputs, an audio mixer eliminates both dilemmas aforementioned, allowing input signals to be blend in the desired ratio. (Gibilisco, 2002) Shown below is a sample circuit of a simple transistor-based two-channel mixer/preamplifier. Figure 3. Transistor-based Two-channel Mixer/Preamplifier In this project, two signals, one from a microphone (J1) and another from a line input (J2) are to be mixed. Potentiometers (R1 and R4) were utilized as volume controls for each channel, adjusting the amount of signal passing from the inputs, from a maximum of the entire signal (Rmicin = R1||R3||RQ1in = 10k||10k||2.3k = 1.58kohm, Rlinein = R4||RQ1in = 10k||2.3k