Karaoke Type Amplifier - Essay Example

? To produce a karaoke type amplifier Introduction Karaoke type amplifier is used mainly in voice and music applications where singer sings on mic and out put of an electric music instrument like Guitar, playback music or keyboard is given in the amplifier. Karaoke amplifier mixes and amplifies both inputs and give out put to a load or speaker. The circuit under discussion consists of an amplifier with 6 band Graphic equalizer. It consists of power supply, mixer / pre amplifier, graphic equalizer and power amplifier stages. The circuit will include following essential elements 1. Two inputs, 1 for microphone and a second for Line input, which could be mixed and boosted with a common emitter amplifier stage. 2. A six-band graphic equalizers stage. 3. A common emitter voltage boost amplifier stage. 4. A common collector power amplifier. 5. Circuit works from mains power supply. Circuit Operation Detailed circuit operation is given as under:- Power Supply The power supply is connected to pre amplifier and power amplifier blocks. The Transformer T1 is a simple step-down transformer which takes 240 V Ac current and gives out 9.5 V Ac at current rating of 1.3 amps approximately. The circuit requires steady DC current therefore output of transformer is applied to rectifiers. Transformer works on the principle of mutual induction of two coils. When current in the primary coil is changed, the flux linked to the secondary coil also changes. Consequently, an EMF is induced in the secondary coil. The transformer T1 consists of a rectangular core of soft iron in the form of sheets insulated from one another. Two separate coils of insulated wires, a primary coil and a secondary coil are wound on the core. These coils are well insulated from one another and from the core. The coil on the input side is called Primary coil and the coil on the output side is called Secondary coil. According to Faradays law EMF induced in a coil depends upon the rate of change of magnetic flux in the coil. If resistance of the coil is small then the induced EMF will be equal to voltage applied. A transformer in which Number of turns in primary coil are greater than Number of turns in secondary coil (Np>Ns) is called a step down transformer which is T1 in this case. In this way the Step down transformer T1 converts high alternate voltage of 240V to low alternate voltage of 9.5 V with current rating of 1.3 amps approximately. This voltage is next passed on to a rectifier to obtain a steady DC voltage. Rectifier D1 consists of four diodes which are meant to convert Ac into DC. During each half of current cycle the diodes block reverse current and allow forward current. Rectifier D1 is a full wave rectifier which works during both positive and negative cycle of AC. Rectification is the conversion of alternating current (AC) to direct current (DC). This involves a device that only allows one-way flow of electrons and it is built around a four-diode bridge configuration therefore called a full-wave bridge or rectifier .Regardless of the polarity of the input, the current flows in the same direction through the load. That is, the negative half-cycle of source is a positive half-cycle at the load. The current flow is through two diodes in series for both polarities. Rectifier bridge works half time during positive sinosidal wave and half time during negative sinosidal wave. The result is a steady DC current measuring 9.5 V. This current is applied to Capacitor C18 with value 2500 uf, which charges the in put to around 13.5-14V peak voltage. This voltage is passed to a standard 12 V regulator IC1 that gives steady DC output of 12 V to run the complete circuit. In the power supply, heat is dissipated during transformer and voltage regulation stages. The capability of a capacitor to store electricity is known as capacitance of that capacitor. It is denoted by C. The measuring unit of capacitance is Farad, but Farad is very large unit. Its smaller units are Kilo Micro Farad (KMFD), Micro Farad (MFD), Kilo Pico Farad (KPF) or Nano Farad (NF) and Pico Farad (PF).When AC supply is given to the capacitor the polarity of both the plates changes alternatively according to the input AC. As a result of this capacitor charges in the first half cycle and discharges in the next half cycle. There is 90° phase difference between AC at input and AC at the output of the capacitor. Function of the capacitor C18 is to store the electrical energy and give this energy again to the circuit when necessary. In other words, it charges and discharges the electric charge stored in it. Besides this, the functions of a capacitors being used in the circuit are as follows: 1. It blocks the flow of DC and permits the flow of AC. 2. It is used for coupling of the two sections. 3. It bypasses (grounds) the unwanted frequencies. 4. It feeds the desired signal to any section. 5. It is used for phase shifting. 6. It is also used for creating a delay in time. 7. It is also used for filtration, especially in removing ripples from rectified waveforms. 8. It is used to get tuned frequency. The steady 12 V regulated voltage is next passed on to Mixer / pre amplifier stage. Mixer/Pre Amp Mixing console represents the most demanding area of audio design. The steady advance of digital media demands that every part of the chain that takes music from performer to listener must be near-perfect. A large number of signals flow in a small space and they must be kept strictly apart until mixed. Distortion and noise must not be allowed to disturb the circuit and kept to minimum. A mixer is a non-linear device. If a signal is applied to a linear device, stage, or system, the output will be linearly related to, i.e. directly proportional to, the input. If the voltage across the resistor is increased, or decreased, then the current through it will also increase or decrease by the same factor therefore the current is linearly related to the voltage. It is a characteristic of a non-linear device such as mixer that if the input is a single frequency, the output will consist not only of that frequency but harmonics of it too which is called harmonic distortion. In addition to the original frequency and its harmonics, the output will contain components which are the sums of the various frequencies and the differences between them. They are known collectively as inter-modulation products, or i.m.ps. A diode is the most basic mixer. The mixer design in the given circuit consists of two inputs .J1 is a mic input and J2 is a line input. The input level of J1 and J2 is controlled by potentiometers R1 & R4, which is then fed to the base of Q1 by the coupling capacitors C1 and C20. R1 & R4 adjust the amount of signal passing from their connected inputs, from a maximum of the entire signal to none. The amount of power controlled by a potentiometer R1&R4 is very small. The voltage output and input is no more than a single watt. However, this is enough power to allow simple manipulation of circuit. Potentiometer provides enough power control to trigger the on/off function and increase / decrease the flow of current .The inputs at J1 and 2 have an AC voltage difference of about +/- 40mV to +/- 100mVpp. R5, Q1 and R6 make up a common emitter amplifier. Transistor amplifier operate using AC signal inputs which alternate between a positive value and a negative value so some way of "presetting" the amplifier circuit to operate between these two maximum or peak values is required. This is achieved through biasing. Biasing is very important in amplifier design as it establishes the correct operating point of the transistor amplifier ready to receive signals, thereby reducing any distortion to the output signal.R5 biases Q1, which with R6 produces an AC voltage boost of around 200 times, but inverted. In Common Emitter Amplifier circuit, capacitors C1 and C20 have been used as Coupling Capacitors to separate the AC signals from the DC biasing voltage. This ensures that the bias condition set up for the circuit to operate correctly is not affected by any additional amplifier stages, as the capacitors will only pass AC signals and block any DC component. The output AC signal is then superimposed on the biasing of the following stages. The Voltage Gain of the common emitter amplifier is equal to the ratio of the change in the input voltage to the change in the amplifiers output voltage. Depending on the positions of R1 and R4, the signal is boosted to around 6Vpp. The aim of small signal amplifier is to amplify all of the input signal with the minimum amount of distortion possible to the output signal, in other words, the output signal must be an exact reproduction of the input signal but only bigger (amplified). To obtain low distortion when used as an amplifier the operating quiescent point needs to be correctly selected. This is in fact the DC operating point of the amplifier and its position may be established at any point along the load line by a suitable biasing arrangement. The best possible position for this Q-point is as close to the centre position of the load line as reasonably possible, thereby producing a Class A type amplifier operation. Graphic Equalizer Equalizer used in the circuit is a passive electronic element used for the reason of altering or flattening the frequency response characteristics of a system. Graphic equalizer originates its name from the rough graph of an audio’s altered frequency response which is formed by the glider settings on the front faceplate of an equalizer. The circuit pertains to a classic design by which it can regulate the tone of the acoustic signals into six different ways which includes the low, mid, and high frequencies. Graphic equalizers are used as a standard practice in live sound reinforcement systems, professional recording studios, and some high fidelity systems where a spectrum analyzer is being used by a graphic equalizer to compensate for the room frequency response and room acoustics. Although graphic equalizers can have different designs, all function by dividing the frequency response range into separate bands. The circuit was designed to illustrate the use of tone control circuit by which the audio signals are adjusted before being carried out to output devices such as headphones, speakers, or recording devices through the use of an amplifier. The graphic equalizer used in the circuit is a multiple passive filter configuration. The signal is split and attenuated only.C2 couples the signal from the preamp to the 6 band graphic equalizer, the large section in the middle up to C11, the next coupling capacitor. The signal is split by the six potentiometers R2, R8, R9, R10, R11 & R12, which are the band controls. Each of the six lines has either, a low pass filter, high pass filter or both (band pass filter). Each line only reduces the signal, there is no boost in this filter. In the top line R13 and C10 make a low pass filter with the knee at 80Hz, so frequencies above this are attenuated. In the next line down, the same values appear R15 and C3. This is a high pass filter with the same knee value, and R16 /C4 form a low pass filter at 608 Hz. So frequencies between 80 Hz and 608 Hz pass straight through. High-pass filter allows for easy passage of high-frequency signals from source to load, and difficult passage of low-frequency signals. The cutoff frequency for a high-pass filter is that frequency at which the output (load) voltage equals 70.7% of the input (source) voltage. Above the cutoff frequency, the output voltage is greater than 70.7% of the input, and vice versa. The capacitor's impedance increases with decreasing frequency. This high impedance in series tends to block low-frequency signals from getting to load. This continues on line for line, the other knee points being 1660 Hz, 3770 Hz and 8kHz. C8 and R20 are of course a high pass filter, allowing frequencies over 8kHz to pass. The signal is re-combined by resistors R14, R24, R25, R26, R27 & R38. Voltage Amplifier Voltage amplifier is an eelectronic device that increases an input signal in the form of a voltage or potential difference, delivering an output signal that is larger than the input by a specified ratio. In the current circuit, common emitter amplifier has been used to obtain the required gain. The signal voltage at the transistor stage output is proportional to the input signal current fed into the transistor base multiplied by the output load resistance and the current transfer ratio . The input signal current is equal to the signal voltage at the input divided by the AC base resistance. This base resistance is an inverse function of the current running in the emitter but it is also proportional to the current transfer ratio. In evaluating the ratio of output signal voltage to input signal voltage (i.e voltage gain ) from these factors, the current transfer ratio can be eliminated from the final formula and calculated voltage gain is essentially independent of the value of the current transfer ratio. In the circuit output of graphic equalizer is fed to Capacitor C11.C11 couples to the next section, a voltage amplifier (common emitter).The resistors around Q2 were chosen to supply enough current to the final stage, as well as turn on the transistor enough to keep the collector around 6V.The voltage gain of this section is around 230. A high gain was needed, because the graphic equalizer attenuates the signal greatly. A gain of 230 can be too high, and signal clipping can occur. To prevent this, potentiometer R32 provides variable negative feedback for the voltage amplifier, and is used as a Master Volume control. This will result in more heat dissipation through the transistors, exacerbating the problem of forward voltage drop change. A common solution to this problem is the insertion of temperature-compensation "feedback" resistors as done in the circuit. Power Amplifier Power Amplifier is a type of amplifier built and designed for the purpose of delivering and supplying sufficient and maximum high output power to directly drive loudspeakers or loads, within a given percent of distortion. Amplifier stages divide into voltage gain and final stage is power gain that has both current and voltage gain. Pre-amp is designed to have high voltage gain and final output stage is designed to have high current gain. The last stage is a power amplifier because it must supply power to the loudspeaker or other load. All the previous driver stages are voltage amplifiers, their function being to raise the signal level sufficiently to drive the power amplifier. The signal after Q2 has again been inverted (now the correct way again), and passes through the coupling capacitor C16 to Q4.R33 and R34 should bias Q4 so that it is turned on enough to pull the emitter to around 6V, meaning that in steady state (no input), the transistor is drawing around 270 mA. The gain of this section is 1.C17 then couples the approx. 6V output of Q4 to the speaker that centres on 0V. When the output of Q4 goes below the steady state point (negative part of input signal), C17 drives the speaker into reverse polarity via R35. This is the limiting drive current for the speaker, and therefore the limiting factor in the power output. In this circuit arrangement, the collector node of the transistor is tied to a power rail or a common node, the emitter node is connected to the output load to be driven, and the base node acts as an input. Owing to the physics of the bipolar transistor, the emitter node closely tracks ('follows') the voltage applied to the input node, which is useful in many applications. The common collector circuit is found to have a voltage gain of almost unity, meaning AC signals appearing on the input will be nearly identically replicated on the output, assuming the output load is not too difficult to drive. The circuit has a typical current gain which depends largely on the hFE of the transistor. A small change to the input current can results in much larger change in the output current supplied to the output load. Thus a weakly driven input node can be used to drive a lower resistance at the output node. The common-collector produces an output voltage in direct rather than inverse proportion to the rising input voltage. As the input voltage increases, so does the output voltage. More than that, a close examination reveals that the output voltage is nearly identical to the input voltage, lagging behind only about 0.77 volts. This is the unique quality of the common-collector amplifier: an output voltage that is nearly equal to the input voltage. Examined from the perspective of output voltage change for a given amount of input voltage change, this amplifier has a voltage gain of almost exactly unity (1), or 0 dB. This holds true for transistors of any ? value, and for load resistors of any resistance value. The goal of any amplifier circuit is to reproduce the input wave shape as accurately as possible. Perfect reproduction is impossible, of course, and any differences between the output and input wave shapes is known as distortion. In an audio amplifier, distortion may cause unpleasant tones to be superimposed on the true sound. There are many different configurations of audio amplifier circuitry, each with its own advantages and disadvantages. Read More