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.
Step 2: Schematics
The second schematic is that of the charger. The transistor in the charger schematic is the BC327 or equivalent.
Step 3: Assemble the Amp
As for the enclosure itself, a hole has to be drilled in one side for the enclosure for the input jack, then two holes on the opposite side for the switch and charging jack. If you want to add a clip to the back (optional) drill that hole now as well. Two holes, each 1/4" in diameter, should be drilled on the narrow end of the enclosure near the speaker, as well as two other holes opposite eachother on the sides adjacent to the previously mentioned side. These act purely as ports for the speaker. As for the speaker itself, it was placed face down on the top of the enclosre, and a knife used to score a circle around the edge. A 1 inch spade bit was used to drill out the middle of this circle, then a dremel with a sanding drum used to enlarge the hole so the speaker would fit comfortably. Unfortunately, the speaker does not have any mounting hardware, so it has to be glued in place.
Once the circuit is built, it has to be placed in the enclosure. The battery goes at one end, with the two jacks next to the battery opposite eachother, and the switch and amplifier circuit mounted underneath the speaker. See the pictures of the layout.
Since the speaker has a relatively shallow mounting depth, there is even enough room for two small heatsinks to be placed on top of the amplifier chips. The heatsinks are just cut up pieces of a regular heatsink filed down to fit. They were then glued to the amplifying chips using a homemade "thermal glue" of JB Weld mixed with aluminum shavings. I know its not ideal, but its better than not having any heatsinking at all.
(Note: Ignore the grill cloth shown on the amp. That is covered in the next step)
Step 4: Finishing Touches: Amp
Custom decals were also made, and adhered to the back of the enclosure. To make these decals, the image was printed onto normal paper, but then the paper dipped in polyurathane, and the excess scraped off. The decals were then placed on the enclosre, and the polyurathane left to dry overnight. A red dot was added near the switch to signify the "on" position.
Step 5: Assemble the Charger
An option you might want to follow is to mount the charger in an enclosure. Since the LM317 should be heatsinked, make sure the enclosure is large enough to accomidate it.
Step 6: Finished!
Do not turn the amp on without a guitar allready plugged in. A painful schreech will result.
Do not play the amp while charging. Charge with the amp switched off.
To fully charge from a completely dead battery will take 12 hours. Most likely, you won't play the amp until the battery is completely dead, so charging time should take about 8 hours. Do not overcharge the amp.
The volume is controlled by the volume pot on the guitar. Because the amp starts clipping above about half volume (entirely dependant on your guitar) the volume also controls the distortion poroduced by the amp. With the volume turned down enough for the sound to be clean, the "loudness" of the amp is not significantly lower. Again, this depends on the guitar.
Below is a video of a friend of mine, who is the guitarist of the superb band No Response (check them out on Bandcamp here) demonstrating the milli-AMP. It might be difficult to spot the amp at first, but its clipped to the guitar strap:
Now go live a life free of cords and 30 pound amps!