Introduction: Pocket Sized Guitar Amp

Picture of 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 

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 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

Picture of Schematics

The first schematic is for the amp itself. This schematic has been modified from what it was on to allow for a switch and the charging jack. The original schematic can be found on the 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

Picture of 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

Picture of 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

Picture of 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!

Picture of 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!


kuroneko01 (author)2015-10-27

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.

Debadeep Chaudhury (author)2015-09-17

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.

joshr123 (author)2013-10-22

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

JoeBeau (author)joshr1232014-02-13

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.

avianoguitarist (author)2014-02-13

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.

JoeBeau (author)avianoguitarist2014-02-13

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.

damonarso (author)2012-08-05

could I use headphones with this amps?

JoeBeau (author)damonarso2012-08-06

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

AudioMaximus (author)2012-01-13

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

JoeBeau (author)AudioMaximus2012-01-14

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.

AudioMaximus (author)JoeBeau2012-01-14

How about using a different speaker? one with more wattage or something?

JoeBeau (author)AudioMaximus2012-01-15

Wattage i not too much of an issue. as long as the speaker is designed to handle the wattage put out by the amp, that would not be a factor. But the speaker being used here is good for about 200 to 20,000 hz. For a bass amp, you would more likely need a speaker rated from 10,000 down (or even 8000 down). A speaker designed for those frequencies would be more rigid and have more cone movement capabilities. this will allow the speaker to produce low frequecies without distortion

AudioMaximus (author)JoeBeau2012-01-15

Aright, gotcha. Any idea of where to find a speaker like that that would fit this project? If you have any links, that would be great.

JoeBeau (author)AudioMaximus2012-01-16

The problem is the size of this amp. You cannot get a woofer that is under two inches in diameter. On top of that, bass tones require a speaker with a larger surface area than high pitched tone, in order to obtain the same volume. So i would recommend stepping up the size of the speaker. This would make the amp larger and less portable, but will give it better bass handling capabilities. i would check for some speakers, but two models that i would recommend are:

The second one is larger, so you eould sacrifice portablility, but is better for bass. The second one is smaller, but still larger than the speaker i used in this project.

AudioMaximus (author)JoeBeau2012-01-16

As long as it isn't massive, I can deal with it. My regular bass amp is a pain to lug around, so anything that can fit in a backpack or suitcase will be perfect.

JoeBeau (author)AudioMaximus2012-01-17

You shouldnt have a problem with those speakers. Another thing that might be a god idea is t make sure that the case or enclosure for the amp is sealed. The speaker might perform better with the sealed enclosure behind it.

AudioMaximus (author)JoeBeau2012-01-17

Agreed, it will help focus the soundwaves. Especially at the low frequencies the bass produces.

thrashermanTotalDIY (author)2011-10-09

Good Job. What did you use for wrapping the speakers with?
This would be a great mini amp for me because I play the electric guitar, 24/7 if I could.
NIce Instructable

thank you. If you are referring to the black cloth, it is just grill cloth from an old speaker. It really isnt neccisary, it just looks better that way. Pantyhose could be used as a subsitute if you couldnt find an old speaker with grill cloth.

OK. Thanks!

diogosilvamonteiro (author)2011-12-07

Nice project, man! I have a doubt about the charger circuit: what stops the current flowing when the battery is fully charged? The BC327 is the responsible? Can you explain me a little bit better about its functioning? Thanks!

The bc327 helps with current regulation, while the lm317 is for voltage regulation. There is nothing to turn off the circuit when the battery is done charging, but due to the very low charging current, it would take 3 or 4 hours (or more) of overcharging to damage the battery- charging allready takes 8 to 10 hours. This is due to the battery's type- its NiMh, which is difficult to sense when fully charged, and when designed for a voltage such as 9 volts, takes a looong time to charge. I spent more time trying to make a charger circuit that turns off than I actually did building the rest of the project. In the end, i settled for the circuit shown, cause it works well and is simple. I just tell people who i make this for to put the battery on charge an hour or two before they go to bed and to unplug it in the morning.

NathanRay (author)2011-11-30

If you want to make this for an electric bass, I suggest a larger speaker 4 inches or more. Anything smaller will just make it rattle... a lot. It will rattle a little on the 4inch one.

JoeBeau (author)NathanRay2011-11-30

My friend did plug his bass into this for the heck of it and the results were quite rattley. So you are correct there.

mrmerino (author)2011-11-14

Anything is portable. You just need something big enough to carry it. ;)

JoeBeau (author)mrmerino2011-11-14

Thats true. Or extension cords long enough

abardhaj (author)2011-11-03

The capacitors are all in uF?

JoeBeau (author)abardhaj2011-11-04

yep. the values are listed in the parts list on the second step

Upwards (author)2011-10-24

I'm new to electronics, and have a question. In a portable device like this, what do I connect where the earth/ground symbols are? Since it cannot be connected more directly to the earth like a device plugged into the wall.

JoeBeau (author)Upwards2011-10-24

In a circuit that runs off of a battery, the negative terminal of the battery is used as the ground. Its not a ground like an earth ground, but using ground symbols is easier than having traces running all the way back to the negative battery terminal.

popoya1 (author)2011-10-19

This instructable is really cool and the stuff it uses are so easily available that an average kid like me in New Delhi doesn't have to roam around the entire city to find it. I've just got one question : What do the ports for the speakers do ? Thanks a lot !

JoeBeau (author)popoya12011-10-19

When the speaker moves in and out, not only does it create pressure waves (sound) in front but it creates pressure waves behind it. Since the speaker is mounted in an enclosure, the ports are needed to allow these pressure waves to escape. In high end speakers, the ports are used to tune the frequency response of the speaker enclosure. But here they are simply added for the affect of acheiving more volume, since pressure waves from the back of the speaker can now escape.

popoya1 (author)JoeBeau2011-10-20

Thank you !

JoeBeau (author)popoya12011-10-21

you're welcome

punkhead58 (author)2011-10-17

Very cool, I can't believe that this Instructable has just barely broke 3,000 views, and the guy who put a seatbelt around his knees and called it a chair has almost 20,000. Oh well, think of it this way: it's the EASY Instructables that are recognized, not the necessarily the best ones.

Good work, and thanks for sharing.

JoeBeau (author)punkhead582011-10-17

Thank you. Im suprised too about the lack of views. My frist instructable well exceeded 20,000 views, but unfortunately, this, being my fourth, did not follow suit. My first was about a battery powered tesla coil, so i think the subject matter must also come into play: The more dangerous or "coolest" instructables are also recognized. Again, thanks for the positive feedback.

blinkyblinky (author)2011-10-02

You should use the JRC NJM 2073... more amperage plus, it has two amps inside

JoeBeau (author)blinkyblinky2011-10-03

I just looked that chip up and i see what you are talking about. It looks like it would also be a good one to use if you are looking to build a stereo audio amplifier. The only problem i see is availability. LM386's are available everywhere, meanwhile the NJM2073 i would have to track down. But I'll look into that. thanks.

blinkyblinky (author)JoeBeau2011-10-03

I happen to own a lot...

You're welcome.

Nice idea, though.

JoeBeau (author)blinkyblinky2011-10-03

I checked and they are available through Digikey, so thats good. One thing i am wondering though; what's required for support circuitry? I want to build a headphone amplifier, and can't help but realize this would be an ideal chip to use.

blinkyblinky (author)JoeBeau2011-10-03

The datasheet has plenty of info. When I build this circuitry, I use the stereo configuration which is found on the PDF page 4, the figure to the right. Where the V+ is I use 470uF so I don't have to worry about ordering different parts.

Make an ible when you build it...I'd like to see one else has the patience to try out this chip because they are so used to 386s.

If you need the NJM2073 chip I can mail a few to you if you want.

JoeBeau (author)blinkyblinky2011-10-04

Okay, i found the schematic you are refferring to. That is a lot simpler than some of the support circuitry i have seen for other amps. Now, the diagram to the left of the one you refer to; that is deffinately a mono configuration, but what is the point of that setup? It doesnt looked bridged for higher power or anything. which leads me to my next question: have you ever tried bridging the two amplifiers that are within the chip? or maybe wiring them in parallel?

blinkyblinky (author)JoeBeau2011-10-04

I have wired them in series before but unfortunately, there is some noise. Printing a circuit board might help. It says that when the supply voltage is less than 2 volts it can still operate... I'm not sure, though I'll take a closer look at it and hopefully will be able to answer your predicament.

JoeBeau (author)blinkyblinky2011-10-04

by saying in series do you mean bridged? i never heard of wiring amps in series, unless you are using one as a preamp.

blinkyblinky (author)JoeBeau2011-10-04

I meant something totally different. I meant parallel. The BTL configuration just means that it has double the power. In fact, I was using one as a preamp.

JoeBeau (author)blinkyblinky2011-10-05

Alright. Thanks for your help. I'll deffinately look into it.

blinkyblinky (author)JoeBeau2011-10-05

You're welcome.

kostya (author)2011-10-04

Nice project! TDA2822 is also a good option for portable applications.

JoeBeau (author)kostya2011-10-04

Thanks. the TDA2822 is a chip that i have seen used before. The previous portable amplifier design i made used a TDA2003 chip. It worked well, with an ipod hooked up to it as the input. The problem with using a chip like that (10w car audio amplifier) is its lack of "sensitivity". I could not make it produce more than 2 watts.

About This Instructable




Bio: Why fix it if it ain't broken? Because it's fun.
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