General Description:
This circuit will amplify voice audio in the range of 50 to 10,000 cycles with little distortion, and have the ability to drive a low impedance load to 16 ohms. The circuit will run from 6 to 15 vdc and give about 20 dB of gain.
Concept:
To build an inexpensive audio amplifier with little parts count that is very reliable and easy to build and implement. Parts are available from Radio Shack.
Construction:
Build the circuit on a breadboard and use a socket so when the lightning hits, well you know......
If the audio input is biased above ground (audio floats on a voltage) a coupling cap will need installed in the "Audio In" lead to the pot. The pot can be any value from 5 K to 100 K. The 100 pF helps eliminate RF bombardment, making it suitable for higher RF environments. Don't forget the 100 uF cap or the circuit will oscillate.
Many op-amp circuits don't drive low impedance loads well, this circuit will handle a load impedance to about 16 ohms but doesn't need to be loaded down to that impedance. The output cap is a 10 uF non polarized electrolytic for impedance's to 600 ohms. The output capacitance should be raised to 100 uF for impedance's to 100 ohms and to 1000 uf for impedance's below 100 ohms (speaker.)
Operating bandwidth is from 50 cycles to 10 kc. Over 10 kc and the unit suffers from poor slew rate, causing distortion, but for NBFM this bandwidth is acceptable. Actually, for this purpose slew rate limitations work to our advantage as it helps make the amplifier less RF susceptible. Increased audio amplification can also be had with the addition of a feedback loop. Consult the RS parts substitution manual for examples.
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