LA3161 Preamplifier circuit
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. 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. Wltile the operating voltage is 9V, the IC can tolerate voltages up to 18V. Typical input resistance is WOK 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.
Ready made PCBs, even populated PCBs are available using this very useful IC. You can still build one, with a Veroboard provided proper care is taken about the ground returns. It simply means that one should not connect ground terminals of output and input at the same place. This will create serious oscillations and normal hobbyist will be left in the woods. Please read general instructions for working with amplifiers in the end.
LA4555 Audio Amplifier circuit
Stereo circuit and mono circuit are given in schematic .LA 4555 is basically a stereo amplifier with 2.3 watts into 4 ohms speakers at a total distortion of 10%. With a bridge circuit, it can be configured as mono amplifier delivering 4.6 watts. It has an input impedance of 30K and gain of 51 dB. It has an excellent voltage range of 3 to 13 volts. Both mono and stereo circuits are shown here.
Pin out is given in Figure
Input is given at Pin 8 in the mono circuit and output is taken at Pin 11 through a capacitor (C7) of 470 uF to a speaker of 4 ohms. In the case of stereo circuit, input is given at 5 and 8 pins and output is taken out at 2 and 11 pins respectively for left and right channels through the blocking capacitors C4 and C7.
A detailed description of the pin out will help now and in the future.
In the stereo circuit,
C5, C2 are feedback capacitors, zohich dictate the lower cut off frequency.
CI, C6 are bootstrap capacitors. If the capacitor value is reduced from the recommended 47uF, output at flow frequencies falls.
Cll, CIO are oscillation blocking capacitors. Polyester film capacitors are preferable.
C7, C4 are output coupling capacitors. Lower cutoff frequency depends on their value and quality.
C3 is the ripple filter or decoupling capacitor.
C8, C9 are the power source capacitors.
R2, R2 are oscillation blocking resistors.
ICs dissipate heat as they dissipate more and more power at more and more voltages. The heat must be removed continuously such that IC operates at rated temperature. Failure to do so will result in thermal runaway. If the IC is well protected, output power will fall to a safe area. If not it will eventually fail and pack up. Copper foil area is made as large as possible in the vicinity ofIC to dissipate more heat.
Then heat sink, thermally conductive material such as copper or aluminum is mounted on IC to remove the heat as it develops. It must be of enough size. In case ofLA4555, the IC has fins which are soldered to the PCB and a small heat sink also can be soldered along with it. Solder copper heat sink as shown in the figure below. Aluminum cannot be easily soldered. Method of mounting heat sink is shown in Figure.
