The following page was taken from Ken's Hobby Electronics Page. There's some good stuff there!


It's always handy to have a little amp kicking around to trace audio signals, test mics, CD tape and TV audio outputs. You know, something that doesn't weigh a lot and isn't clumsy. There are tons of uses for this little circuit.

There are a couple of versions of this amplifier chip. Both are 8 pin DIP packages and the difference between the two are apparent by their part numbers. Either are suited for this circuit provided the supply voltage does not exceed the recommended 5 to 12 volt DC range. Power output can range from about 325 mW to about 750 mW within this supply range when using an 8 ohm speaker. Power it with batteries or a small DC supply...why not solar cells or a lttle windmill generator?

The circuit shown has gain of about 200. VR1 is the volume control. The voltage rating of the DC blocking capacitor C1 should exceed the supply voltage of any piece of equipment you want to probe if you're using this as a signal tracer. Tube amp circuitry supply rails can exceed 600VDC, so make sure you choose C1 with this consideration in mind.

You can grab more info on this chip and many other audio ICs at National Semiconductor's web site at If you live in North America, you can get these chips from Digikey at Great customer service happening here! A note to Canadians - Digikey takes care of duty and all the other crap and can have an order to your door IN LESS THAN 24 HOURS!!! Here's the schematic...

Nothin' to it, right?

parts list

C1, C2   10uF 16V electrolytic    
C3   .1uF capacitor    
C4   .05uF capacitor    
C5   220uF 16V electrolytic    
R1   10 ohm 1/4w resistor    
U1   LM386 amplifier    
VR1   100K "A" taper pot    
SPKR1   8 ohm speaker