Product pitching aside, this is a pretty neat project for somebody with beginner's electronics skill that can follow a schematic. I will state now that the circuit is not entirely of my own design, but was inspired by the Little Gem Mk II circuit from runoffgroove.com. I would recommend checking out the site for some other pretty cool music-related schematics. Even though the circuit is not entirely original, (especially seeing the LM386 is one of the most used amplifier chips available) the design of the enclosure and the charging circuit is of my own creation.
The milli-AMP uses a bridged ML386N-3 amplifier circuit to produce in the neighborhood of two watts output power. It runs of a rechargable 9 volt battery, which provides about an hour of full volume play per charge. At two watts, the milli-AMP is more powerful than any commercially available amp (that I know of) of its size; its size being a very pocketable 2.25 by 3.5 by 1 inches (roughly). Volume is controlled by the volume pot on the guitar, and this adversly controls distortion as well. For an idea of what it sounds like, see the last step of this instructable.
To make this project more obtainable to the average DIY-er, the mili-AMP is available in kit form through Jameco. It includes all the parts needed to assemble a working amp and charger, minus custom touches like the speaker cloth. Click on the button below to purchase the kit!
Step 1: Parts and Pieces
1/4" Mono Jack
12.5 VDC Wall Transformer
2.1 mm Coax Power Jack
2.1 mm Coax Power Plug
9 Volt Battery Snaps
Black Plastic Case
SPDT Toggle Switch
Miniature Full Range Speaker
9v 250 mah NiMh Battery
500 ohm Trim Pot
LM317 Volt Regulator
BC327 Transistor: I had one of these lying around. An equivalent transistor can be substituted
10 ohm, 10K ohm, 1.5M ohm, 240 ohm, 1.5K ohm, 470 ohm Resistors: Scavenged, or purchased from Radioshack/ All Electronics.
0.22 uf, 100 uf 16v, 10uf 16v (x2), 0.05 uf, 1000 uf 25v capacitors: Scavenged, or purchased from Radioshack/ All Electronics.
Case for the charger circuit: I used a case from an Ipod Nano.
A note about some of the parts: If you decide to find your own parts, stick with the speaker listed, as it is the only one I would recommend for this project. It has a resistance of 8 ohms (Nothing lower should be used) and can handle 3 watts. It is also about 1.5 inches wide. Unless you can find something that meets these specs, stick with the speaker listed.
Also, the kit through Jameco comes with a slightly different enclosure for the amp than the one pictured in this Instructable, but it also includes an enclosure for the charger. It does not contain speaker cloth or a clip either.