Microphone is added here for the very use it has, and by the ease of making one. It can be used as a good quality microphone. Here is a simple circuit that makes the condenser mikes work, which can also be used as a collar mike. Condenser mikes are rugged, have excellent frequency response, and they develop fairly high signal output. They can certainly be implemented to test later circuits. Make this and I am sure you will make many for many of your friends.

These ubiquitous devices are seen everywhere, in portable tape recorders to cell phones. These are small, sensitive, and rugged and have an excellent frequency response, and high signal to noise ratio. They deliver higher level of signal into preamplifier. It has an internal field effect transistor which needs power at less than half a milliampere. Condenser mike has two terminals; one terminal can be seen as connected to the body of the mike. The other terminal is the HOT terminal. It doubles up as an output terminal and power +ve terminal. A single pen cell can power it.

Circuit is shown in Schematic 35. Condenser mike is biased with a 3.3K resistor to power the internal FET. Output is also taken from this terminal to the Mike (MIC) input of an amplifier. There must be a capacitor at the input of the preamplifier to block the DC voltage from condenser mike. Preamplifiers generally have this capacitor at their input. Negative terminal of the battery is connected to the ground terminal of the mike. It is also connected to the ground of the amplifier through the shield of the connecting mike cable.

Connect 3.3 K resistor to the positive side of single cell battery holder with the other end of the resistor to the HOT terminal through a mike cable. Negative side of the battery holder is connected to the ground terminal of Mike: Solder a shield wire of required length to the

terminals of the mike. Central core wire goes to the hot or output terminal and the external shield goes to the ground terminal. The other end of the shield wire is connected to the appropriate connector suitable to the power amplifier. Use good quality shield wire and appropriate input jack pin suitable for the amplifier. Make two such mikes for stereo applications

Preamplifiers are used to amplify low level signals such as those from mikes, tape heads before they are fed into power amplifiers. Power amplifiers are generally less sensitive. Frequency response also can be suitably trimmed and modified at preamp stage. LA 3161 is one of those widely used in tape decks and amplifiers as a stereo preamplifier.

Block Diagram is shown in Figure 42. LA 3161 has two low noise preamplifiers with good ripple rejection on chip catering to stereo applications. External part count is low and Single In line (SIL))(Figure 43) package makes mounting easy. While the operating voltage is 9V, the IC can tolerate voltages up to 18V. Typical input resistance is 100K and output resistance is 10K with an open loop gain of78dB. Block diagram of the IC is given below. Input is given at Pin 1 and 8, output is taken at Pin 3 and 6, and negative Feedback is given at Pin 2 and 7. Power is at Pin 5 and Pin 4 is the ground terminal. There is an internal voltage regulator.

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