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Build an Audio Buffer Amplifier By Kevin K. Custer W3KKC Some HTML cleanup by Mike Morris WA6ILQ

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 or any of the mail order suppliers (Mouser, DigiKey, etc).

Construction:
Build the circuit on a breadboard and use a socket so when the lightning hits, well you know...... You could mount the breadboard inside a mobile speaker case, with the volume control pot mounted in the outer case. The use of a pot with a switch allows turning the amplifier off when you aren't there. (note from Mike WA6ILQ: I've seen one with the pot mounted dead center in the speaker grille.)

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 application will determine which direction the positive side of the cap faces. You may want to use a nonpoliarized cap in a test amplifier. 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 (i.e. a 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 sub manual for examples. Power can be from 8 to 15 volts. Caps should be 25v or greater.

Another take on the same idea is on the Radio Shack page at this web site. Scroll down to the Model 277-1008 Mini Audio Amplifier schematic.

This extra is from Danny Musten KD4RAA:
I am using your audio buffer template w/the LM-386 on all my link radios, with one change: the addition of a .0047 uF capacitor in series with a 15k resistor from pin 1 to pin 5 of the IC gives a -6 dB per octave de-emphasis slope to the circuit. I connect these monitor amps to the discriminator test point. This addition gives uniform sound throughout. See the schematic addition below. The switch is optional and when open has the alplifier in normal mode or with it closed the circuit can handle de-emphasized (also known as "flat") audio.