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.

Comments

[damonarso]

could I use headphones with this amps?

[JoeBeau]

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

[AudioMaximus]

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]

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]

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

[JoeBeau]

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]

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]

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 parts-express.com for some speakers, but two models that i would recommend are:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=264-1126
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=264-1062

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]

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]

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]

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

[thrashermanTotalDIY]

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

[JoeBeau]

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.

[thrashermanTotalDIY]

OK. Thanks!

[diogosilvamonteiro]

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!

[JoeBeau]

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]

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]

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]

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

[JoeBeau]

Thats true. Or extension cords long enough

[abardhaj]

The capacitors are all in uF?

[JoeBeau]

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

[Upwards]

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]

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]

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]

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]

Thank you !

[JoeBeau]

you're welcome

[punkhead58]

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]

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]

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

[JoeBeau]

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]

I happen to own a lot...

You're welcome.

Nice idea, though.

[JoeBeau]

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]

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 it...no 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]

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]

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]

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]

Oops...
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]

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