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Very High Gain Audio Amplifier:
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| Description: | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| A signal of only 700uV RMS will allow this amplifier to drive an 8 ohm speaker or phones to an extremely loud level. Although designed to be used with the parabolic microphone also listed on this site, this amplifier is also very well suited for use with any Crystal Set or other low signal output device. Two switches allow for the elimination of frequencies below 100 HZ and/or above 1,600 HZ. Two 9 volt batteries drive this unit with B1 drawing between 8 and 50 mA, depending on volume settings and B2 drawing a constant 3 mA. The over all dimensions of the aluminum case are 3 in. wide by 2.12 in. high by 5.25 in. long. If tape recordings are desired, the signal should be taken from the RCA jack (Tape Output) and fed into the microphone input of a tape recorder. Recordings should sound at about the same volume level as those heard in the phones. If not, change the value of R8. Increasing it, increases gain. |
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| How it Works: | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| IC1 serves as a frequency shaping, audio preamplifier with a voltage gain of 17 while IC2 is the tape output driver with a gain of 2. Thus the tape output has a total gain of 17x2=34. IC3 is a combination driver/power amp with a combined voltage gain of 162. Therefore signals at the speaker/phone output have been amplified 17x162=2754 times. Combo R3, C4 and combo R9, C8 function as RF low pass filters and block incoming radio station signals from passing through the amplifier causing audio distortion. Input impedance to the first stage is 80,000 ohms at audio frequencies and as such will not load down previous circuits. TL071 (low noise ) integrated circuits may be directly substituted for the 741's. An electret (condenser) microphone is employed in the associated Parabolic Microphone project and as such, this amplifier provides 6 volts DC to the tip of a stereo plug for it. For other input devices, just remember that there is 6 volts at the tip and audio signals should go to the small ring connector... not the tip. Although the output of this amplifier is mono, most ear phones have stereo plugs, therefore, a stereo output jack is provided but jumper wired as a mono output. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Construction: | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Begin by drilling holes in the aluminum case as indicated in the associated picture. Prepare the printed circuit board by first cutting it out to 3 1/4 in. long by 2 1/2 in. wide. Transfer the circuit's drill points with a punch and then after cleaning it with it steel wool, draw the circuit with a Sharpie pen followed by about a 20 minute etching. |
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| Rinse in water and then drill the holes with a #62 drill bit. Mount all components and solder. Most of the #22 connecting wires were cut to a length of 2 1/4 in. soldered into the board and then soldered to respective components such as switches, pots, jacks, etc. before mounting them in the case. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Pre mount the RCA jack and ground strap to the case. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Now install all of the switches etc. into the case. Follow this up by using 1 3/4 in. tapped nylon stand off insulator posts to mount the printed circuit board into the aluminum case. If these are not available, simply let the wiring support the printed circuit board. Note that the letters shown surrounding the PC drawing, relate to the same ones on the schematic. |
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| Parts List for The Very High Gain Amplifier: | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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