Introduction: Little Gem Amplifier

Picture of Little Gem Amplifier

I needed to build an inexpensive amp to go with the Cigar Box Guitar I built for my girls for Christmas. The inspiration for this project came from Make magazine's $5 cracker box amp. That project was based on the 1/2W Little Gem amp from RunOffGrove. It is a great sounding little amp with separate volume and gain controls. It is also inexpensive and easy to build.

Materials

  1. Enclosure
  2. Speaker
  3. Speaker grill
  4. LM386N amplifier chip
  5. LED
  6. 9 volt battery or power supply
  7. 25 Ω 25W potentiometer
  8. 5K Ω 25W audio taper potentiometer
  9. 0.047 µF capacitor
  10. 0.01 µF capacitor
  11. 220 µF electrolytic capacitor
  12. 100 µF electrolytic capacitor
  13. 10 Ω resistor
  14. SPST switch
  15. 1/4" mono phono jack
  16. 8 pin DIP IC socket
  17. wire
  18. knobs
  19. PC board

The handle, knobs, feet, and power cord were all from an old PA amplifier I rescued from the trash. The speaker and grill cloth were from an old speaker I found in the same trash pile. The aluminum panel for mounting the controls came from my old clipboard that I accidentally ran over. All of the electronic parts can be ordered online.

Step 1: Make the Enclosure

Picture of Make the Enclosure

There was no way a cardboard amplifier was going to survive in a house with a 2 and a 6 year old. So I dug through my pile of reclaimed junk in the basement and found a wooden index card box. (It's so satisfying to finally use something that you've been holding onto for years for no reason other than you know you are going to need it one day!) I used this great Dremel circle cutter to cut out the speaker opening. To clean up the look, I removed the hinge and clasp and glued some wooden strips in the corners of the interior so I could screw the lid on. I used a reciprocating saw to cut out an oval section in the side of the box so I could recess the controls.

Step 2: Wire the Circuit

Picture of Wire the Circuit

I built the amp just as laid out in the schematics with a few changes. I swapped the battery for a 12v wall wart that I had saved from some discarded electronic device. I also added a power switch and led power indicator. The LM386 power amplifier chip was mounted in an IC socket so I could easily swap it out just in case I blew it up during construction. Other than that, just solder it up as indicated in the illustration.

Step 3: Finishing

Picture of Finishing

I made a speaker grille by cutting a hole in a piece of 1/8" MDF and covering it with the reclaimed grill cloth. Then I screwed it over the speaker using some screw cups to give it a more finished look. I housed the transformer inside the amp to make it easier to transport. A piece of scrap aluminum sheet was screwed inside the box under the oval cutout to mount the controls. A reclaimed handle was fastened to the top. I didn't do any sanding or finishing on the outside of the box. I prefer the worn look of the original wood.

Step 4: Play!

Picture of Play!

The final price was probably closer to $15 rather than the $5 claimed in the original article, still cheap enough. The hardest part of this project was finding the free speaker. The amp makes a nice mellow sound, loud enough for a room. All that's left now is to plug in your favorite guitar and enjoy! If I knew how to play, I'd demo it for you. ; )

Comments

JohnE12 (author)2016-11-07

"(It's so satisfying to finally use something that you've been holding onto for years for no reason other than you know you are going to need it one day!)" Brilliant quote of the year...you get my vote just for that ... however its also a great project. Thank you.

uncle frogy (author)2016-10-30

the look of the finished project in photos does not really indicate size very well.

it looks bigger than it must be as an index card box is probably not 18" high by 18" wide by 8" deep as an amp

good job!

uncle frogy

entomophile (author)uncle frogy2016-10-30

Good point. It's 6.5" x 8.5" x 10".

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