Audio Power meter power supply version 1.0.
The power switch is a double pole single throw DPST type. The bottom prong of the power plug goes through the bottom pole of the switch and from there it connects to the bottom end of both primaries of both transformers. The top prong goes through the top pole of the power switch, then through a 1/4 amp fuse, then to the top ends of both primaries. Both transformers are labeled 12.6 VAC at 300 mA. If you substitute for these the minimum allowable current is 100 mA. The top section of the supply will be described first. The two ends of the secondary of the top transformer connect to the two AC inputs of a bridge rectifier. The AC inputs are the two points wher a diode anode and cathode connect together. The positive output of the bridge, the point where two cathodes are connected together, connect to the positive end of a 1000 u f 25 volt capacitor. The negative endof this capacitor connects to the negative bridge rectifier output. This is the point where two diode cathodes are connected together. The positive end of the 1000 u f cap connects to the input of a T O 220 12 volt regulator type 7812. The common terminal connects to the negative side of the cap. The output terminal connects to the plus 12 volt output point. There are two more very important capacitors. A 0.1 u f cap connects from the input terminal of the regulator to its common terminal. Another 0.1 u f cap connects from the output terminal of the regulator to its common terminal. These two capacitors must, repeating, must, be placed physically close to the regulator. I don't care what the data sheets say, without these capacitors the regulator will oscillate at about 1 MHz. The negative side of the 1000 u f cap and both 0.1 u f caps and the common of the regulator all connect to ground. This completes the top section of the power supply. The bottom section is almost identical. The differences are as follows. The regulator is a 78L12 T O 92 type. The output of the regulator connects to ground. The negative of the 1000 u f cap connects to minus 12 volts. In addition there is an incandescent lamp connected in parallel with the 1000 u f capacitor. The lamp is a type ML-1819. This is the power indicator for the instrument. An L E D may be used if desired. A current limiting resistor must be used with an L E D. See the circuit for power supply 2.0 below. The bridge rectifiers may be any general purpose 100 volt 1 amp type.
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