Pocket Sized Guitar Amp

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Introduction: Pocket Sized Guitar Amp

Even though I do not know from experience- (as I do not play the guitar)- I have heard from many of my friends that one of the biggest pains of an electric guitar is the need to hook it up to an amp to hear actual electric guitar sound. Thats not a problem with the 
milli-AMP!

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

The milli-AMP uses readily avaliable parts, most avaliable at just about any electronic component retailer. The parts list is given below. To guarantee you purchase the correct parts, it is recommended that you order the kit that is available through Jameco via the "Buy This Kit" button. 

Perf board
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
LM386 Amp
500 ohm Trim Pot
LM317 Volt Regulator
MPF102 Transistor
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.
Hookup wire
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 first schematic is for the amp itself. This schematic has been modified from what it was on runoffgroove.com to allow for a switch and the charging jack. The original schematic can be found on the runoffgroove.com website.
The second schematic is that of the charger. The transistor in the charger schematic is the BC327 or equivalent.

Step 3: Assemble the Amp

Since there is a schematic provided, I won't go into detail about building the circuit. One thing that might be a good idea is to build each amplifier with its own independant circuit, i.e. each amp has its own seperate connection to ground, positive battery supply, input, etc. The purpose of this is to allow a blown amplifier chip (no that that should happen) to be removed and replaced without disturbing the other chip. Also, pay special attention to the polarity of the charging jack. The center pin should be positive, with the sleeve contact being negative.

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

These additions are purely optional. I chose to add a clip from an old tape measure to the back of the amp to allow the amp to be clipped to the strap on the guitar. Also, for a more professional appearance, the front of the amp is covered in grill cloth from an old pair of speaker grills. to prevent the speaker cone pusing out and touching the cloth, whit tubing about 1/4" in external diameter was glued around the speaker, then the grill cloth streched around it. Hot glue was then used to adhere the edges of the cloth to the backside of the enclodure cover. At the corners where the cloth was folded over on itself, the tip of a hot soldering iron was used to poke hole in the backside for the screws. On the front, no holes were made, since the screws could puncture the fabric on their own.
 
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

After following the schematic provided, hook the charger up to the battery in the amp (this might be easier to do with the battery removed) with an ammeter in series with the positive connection of the charger and positive connection of the battery, and the negative connection of the charger hooked directly onto the battery. With the chrarger plugged in, turn the trimpot until 30 ma is read on the ammeter. Do not go above 30 ma. This is the maximum safe charging current for a NiMh battery of this size. Also, make sure that the center contact of the plug is positive, with the external sleeve being negative. 

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!

Congratulations on the completion of your milli-AMP guitar amplifier! Before use, there are a few notes about the use of the milli-AMP:

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!

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49 Comments

if i want to use a microphone instead of a guitar, will it work? or i have modification to do? I'm not really good at electronics so if i have to modify it need to be easy.

I've used a C102 transistor instead of the MPF102 as listed. It worked for a while but the sound was all srceechy and broken. Then it stopped working. What do I tweak or do to make it right ! Please help.

You should incase the electronics in epoxy resin so with time it doesn't wobble around and short out

potting electronics is a very good way of ensuring insulation and continued operation. Unfortunately, air flow is really important to keep the electronics cool. It would be possible to pot the electronics so that the tops of the LM386 were showing for cooling, but that is a little complicated giving the space issues here. But potting is definitely a good idea.

The Little Gem MK II puts out about 1 watt RMS, not 2 watts. Each LM386 provides 1/2 watt, so bridging them gives just under 1 watt--due to the drain caused by other components.
I also suggest either putting this circuit on perboard or etching a proper PCB. Having it wired "raw" like this is asking for trouble. If you get a piece of metal crossing IC pins or hitting the JFET--bad news!!!
Plus, this is just not the proper way to wire anything.
Even heat shrinking the bare metal or wrapping with electrical tape is a crude improvement.

Please visit runoffgroove dot com for the schematics, PCB layouts and GOOD info on soldering.

The LM386n-3 offers about .75 watt output typically, with a max approaching 1. Here it is operating at maximum output power, or in the neighborhood of 1 watt for a single chip. Bridged offers close to 2, as I said 'In the neighborhood of 2 watts'. This was never meant to be exact.

Free-wiring things isn't the BEST way to do things. But for this project it is more than adequate. With the way things are fit into the enclosure, a PCB wouldn't fit anyway. The circuit was originally on a proto-board. With proper wiring and layout skills, as well as insulation, it is just as reliable as a circuit board, albeit not as pretty or easy to do repairs on. This amp has functioned correctly for more than 2 years at the hands of my friend who is not particularly careful without issue. This is also not the only project I have built with this technique. I suggest a brief search of many other DIY projects. Free wiring is extremely common amoungst those DIY-ers without the budget for printed PCB, or don't have the luxury of excess space (like here) and the basic soldering skills to accomplish the task.

And I did encourage people to visit runoffgroove; they offer schematic and wiring diagrams. With my changes, and not having schematic design software at the time, I drew my rendition by hand.

While I will take your comments are constructive criticism, the tone should be watched. There is a 'be nice' bubble at the bottom of this box at the moment, urging me to be positive and constructive. This is a DIY community where there are those who will have different ways about doing things. Just because it is different does not mean it is wrong or prone to failure. I am all for constructive criticism; it is how you grow as a DIYer. For this reason I am not flagging the comment you have made, since it offers a different perspective on the project than I took. But I would encourage those posting on instructables to take a more postive approach to their suggestions.

could I use headphones with this amps?

Sure. But you would only want one lm386. This would blow out regular ear buds or cheap headphones

Awesome job, man! I was wondering if this amp could handle the low frequencies of an electric bass, or if any modifications would need to be made to do this. Im a bass player, and this would be perfect for travel. Thanks

The only way to find out would be to try. I could see the driver needing to be replaced in order to prevent ditortion at low frequencies. Also, the gain might have to be turned down. That is bacheived by adding resistors in series with the 10 uf caps. The data sheet on the lm386 has some information on adjusting the gain.