Make a Sweet Portable Guitar Amp

Inspired, by gum!
The inspiration for this project was actually two-fold. First, I needed to make a simple, portable guitar amp that would work with an electric bass guitar as well as an acoustic six-string (with a pickup), and could be played through a speaker or headphones. I knew I had all the components needed for a basic "LM386" audio amplifier, but the second part of the inspiration didn't hit me until I was looking for an enclosure for the project. I had a few empty Eclipse gum "bottles" laying around, and upon quick inspection, I realized that the opening at the top was 2¼" (60mm) - exactly the size used by many standard PCs for the internal speaker. The goal from then on was simple - make the entire 1-chip amp fit into the container underneath the speaker, with the lid threaded on over the speaker to hold it in place.

Skills and tools required:
I'm assuming for this project that the builder has some basic electronics experience such as breadboarding simple circuits, wiring and soldering. Tools needed for the electronics assembly are the usual hand tools - wire cutters, needlenose pliers, a low-wattage soldering iron and some rosin-core solder. A helping-hands vise setup would be great if you have one. Not a lot of mechanical skill needed here, just some drilling and light finishing which we'll get to in Step 5. A power drill and a few smaller bits (up to about  ¼") will be needed, and if you've never used one before, a tapered reamer is the perfect tool for getting all the jacks and switches to fit perfectly through the sides of the plastic container.

The components required are included in Step 1. You may have a few of them lying around, but even if not the whole list could be purchased for under $10.

Step 1: Parts and schematic

Parts list:
B1 - 9 volt battery clip and battery
C1 - 4.7μF 16v capacitor
C2 - 100μF 16v capacitor
C3 - 470μF 16v capacitor
C4 - 10μF 16v capacitor
D1 - green diffused LED (just about any color/type will work)
J1 - ¼" (6.5mm) mono jack (guitar input)
J2 - 1⁄8" (3.5mm) stereo headphone jack (headphone output)
R1 - 1kΩ ¼-watt resistor
R2 -10kΩ ½ watt potentiometer
SP1 - 8Ω  ½ watt speaker (2¼" diameter, from an old PC)
SW1 - DPDT "on-off-on" mini toggle switch
U1 - LM386N-1 low voltage audio power amplifier IC

Misc. - breadboard and breadboarding jumper wires (for optional Step 3), hookup wire (small-gauge stranded), small perfboard, solder, knob for R2

Sources:
The only really critical parts here are SP1 for it's diameter and SW1 because of it's special "on-off-on" function.

For SP1, you just need a simple 8Ω speaker - around ½ watt, and exactly 2¼" (60mm) in diameter. I found several of these in my parts box that were pulled from old PC's so it seemed to be a standard size for that application. If you don't have one handy, here are a few places to find them:
- Jameco #135765 (currently $1.59)
- Jameco #10840 (currently $1.95)
- All Electronics #SK-285 (currently $1.25)

For SW1, this type of switch can be easily found. You want to find a mini or sub-mini sized switch with solder lug (not PC-mount) terminals. Here are a few places I found a suitable model where I typically look for parts:
- Radio Shack #275-664 (currently $4.99 each)
- Jameco #21952 (currently $1.55 each)
- All Electronics #MTS-120PC (currently $1.50 each)
- Futurlec #DPDT101 (currently $0.90 each)

The rest of the components should be pretty easy to find wherever you usually buy parts, but here are some links to Futurlec:
B1 - (battery clip) #9VBATTCLIP (currently $0.10 each)
C1 - #C0047U50E (currently $0.05 each)
C2 - #C100U16E (currently $0.10 each)
C3 - #C470U16E (currently $0.12 each)
C4 - #C010U16E (currently $0.05 each)
D1 - #LED5R (currently $0.08 each)
J1 - #P065SCK02 (currently $0.75 each)
J2 - #P035SCK02 (currently $0.50 each)
R1 - #R001K14W (currently $0.10 each)
R2 - #POT10K (currently $0.55 each)
U1 - #LM386N-1 (currently $0.75 each)

My personal recommendation would be to bundle all the parts you can from Futurlec. It takes a few extra days to ship (from Thailand) but the prices are unbeatable. Their electrolytic capacitor pack (#ELEPACK) includes 100 pieces for $3.95. I can't say for sure what values are included in every pack, but the four values needed for this circuit were included when I bought one. They also have a 300 piece resistor pack too (#RES14WPACK) for $2.95, which makes them less than a penny each. Probably don't need that for this project, but it's a good way to re-stock your parts bins.

The schematic:
At the heart of course is the LM386 amplifier, which delivers decent sound with just a few external components. I used the "N-1" version of the chip. This amp delivers 250-325mW of output power, which is plenty for this application. (You could drop in an N-3 or N-4 version pin-for-pin to get either 700mW or a full watt of output power, but you'll need a speaker rated for the additional power - and I would not recommend using the amp for headphone output).

The circuit is designed to deliver the input signal from the guitar via J1 through capacitor C1 into the LM386's non-inverting input at pin 3. The inverting input at pin 2 is connected to ground along with the chip's ground pin 4. The audio output is delivered through capacitor C3 into DPDT switch SW1 which simply directs the amplified audio signal to either speaker SP1 or the headphones connected to J2. The other pole of SW1 acts as a power switch to the circuit, connecting the positive lead from 9V battery B1 to the chip's voltage source pin 6 as well as to the LED power indicator D1 through current-limiting resistor R1. Filtering capacitor C2 sits across the power supply rails. Finally, the RC series circuit of capacitor C4 and potentiometer R2 provides variable gain control for the amp. I found that with these values, I could get an output range from a really clean output into the headphones all the way up to a nice distorted overdrive into the speaker. If you need to experiment with any component values with your particular instruments, those will be the ones. I found with the guitars I worked with that it took a combination of guitar volume knobs and the amplifier's gain knob (R2) to get the right sound level, especially for comfortable headphone listening.

Comments

[xander779]

Hi, I'm using the rev. 2 diagram and I am getting a lot of distortion even at low values on the potentiometer. Do you have any ideas as to what I may be doing wrong?

[mdgrover]

Hello xander779 - thanks for building, and I apologize for the late reply. Your sound should be nice and clean up to higher end of the gain where some overdrive should kick in.

If you haven't solved this on your own already, I would start by double-checking for good connections throughout the circuit, whether you have breadboarded or soldered it. If you have extra parts, try replacing C1 or C3 to see if that helps.

[colourfulmountain]

Hey MDGrover, i have built this successfully, however i have an urgent question about the use of the pot in this schematic. Is this supposed to be used for volume or gain/overdrive? because when i turn the pot completely down, the volume stays at a certain level and never turns the volume all the way down. When the pot is turned up, at the very last 1/8 turn or so, the volume increases and becomes very distorted. I was just wondering if there was a way to wire this in this setup for volume solely. Also, would that potentially clean up the signal? Thanks for this schematic by the way. I urgently await your reply.

[mdgrover]

Hi - thanks for building and for sending your question. The answer can probably be found in the second paragraph of "The schematic" section above:

"Finally, the RC series circuit of capacitor C4 and potentiometer R2 provides variable gain control for the amp. I found that with these values, I could get an output range from a really clean output into the headphones all the way up to a nice distorted overdrive into the speaker. If you need to experiment with any component values with your particular instruments, those will be the ones. I found with the guitars I worked with that it took a combination of guitar volume knobs and the amplifier's gain knob (R2) to get the right sound level, especially for comfortable headphone listening."

Note that the circuit is designed to give a little overdrive (distortion) at the upper range of the gain pot, but it should cut the signal off completely at the lower end of its range. This amp will behave differently with different instruments at its input.

Try adjusting the volume knob of the guitar, and also different values for C4 and R2. You could swap in another 16v capacitor value anywhere from 1μF to 100μF for C4, and try a different pot for R2 anywhere from 5K to 50K.

[rexsnel]

hi, i'm a noob and i really don't know a lot about electronics but i was just wandering if it is possible to make a uv meter using the amplified signal as input?

[mdgrover]

That's a good question - and I'm assuming you mean a VU (volume unit) meter. You should be able to connect one to the output here as with any other audio amplifier. I spotted a couple other projects here that use the relatively simple LM3914 or LM3915 chip for this:

http://www.instructables.com/id/LED-VU-meter-that-dances-to-computer-music/
http://www.instructables.com/id/LM3915LM3916-VU-Meter/

I've made meters like these before, but I haven't done so recently so I haven't actually connected one to one of these amps.

[Flyboy388]

I'm also new at this whole circuit stuff. I can solder and everything fine, but my electrical knowledge isn't great. I'd like to get a battery amp for college so I don't have to lug my big one to a dorm. Is this one easy to build for a beginner? And what is the battery life for this?

[mdgrover]

There are only about a dozen components, so if you've soldered something together of this size before I think you will be successful. If you really want to build your own but would be more comfortable using a kit with a printed circuit board there's a link to one in one of my comments below.

[Flyboy388]

And the downward facing triangles in the diagram are ground right? I'm not used to that notation and want to be sure.

[mdgrover]

Yes - those triangles are ground connections that should all be connected together.

[vde souza1]

Ah, I can't reply that comment o.O

Then, here's it:
Putz! I haven't realized the LED, I messed up something so easy! xD
Thanks!
I think now it's right, I'm just waiting for the pieces, to start assemble it! ^^
Thank you again!
sorry for the bother. ^^

[vde souza1]

Hello Mdgrover!
I'm preparing the materials to start doing this, but I would like to make an amp more simple based on yours, with just an ON/OFF button and without the headphone output, like the step 6, but with the speaker instead the headphone.
please, can you take a look on my schematics, and see if they're correct?
thank you!

[mdgrover]

Thanks for posting!

The input side of your amp circuit should work fine, but the lower-right area of the schematic needs a couple adjustments. I think you may have been confused by the wiring of the DPDT switch SW1 in the original schematic.

Here's what you need to do:
1. Connect the cathode of your LED connected to ground instead of to the amplifier output.
2. Pin 6 of the IC and the (+) terminal of C2 need to be connected to positive side of the power supply. A connection between S1 and R1 would allow S1 to turn your amp on and off.

[Jimbob60]

The only problem I'm having is the wiring of J2 to SW1, SP1, and the negative grouping on the board. If you could post a picture of the J2 - SW1 - SP1, that would be great. Thanks! (I'm using a 5-pin 1/8" panel mount for J2)

[mdgrover]

Hi Jimbob60 - thanks for the comments. There is a picture of my board at the top of Step 5, but it might not help much, since I brought all the wires through the perfboard and then soldered them together underneath. I do like to create a "ground rail" this way - which is actually better depicted in the perfboard illustration at the top of Step 4. That image shows the ground connection between D1, J1, J2 (and SP1), B1 and pin 4 of U1 (and C2). As you can see there, SW1 is not part of the ground circuit.

I think your best bet would be to first connect SW1 and SP1 only, and make sure you get audio at the speaker when SW1 is switched to that side (and no audio when it's flipped the other way).

Once that's working, solder your leads to J2 and test the headphone output by temporarily connecting those leads to the circuit (touching the wires to the correct points in the circuit by hand). The 5-pin jacks can be tricky, so this "trial and error" before you make the final soldered connections should help.

[usefulinsight]

Hi Mdgrover

I need a little more help....

The 1/4 mono plug I am using has 3 pins. I am using trial and error to connect 2 pins together. Is this correct or should I find a plug with only 2 pins?

I also have a 1/8 stereo input which has 5 pins I have pictures of both. Do I just need to figure out how to short some pins together or am I way far off and do I need to purchase a different part?

The 1 k resistor I am using is a 1/2 watt. Your list calls for a 1/4 watt...Is mine okay?
I am breadboarding this project but am having difficulty. My led wont light up and it almost did for a second but went dim very quickly. Could this be from my 1/2 watt resistor?
I am not asking for detailed list of further instructions but would appreciate a little insight when you have the time.
Thanks

[Jimbob60]

As far as your 1/4" jack, I'm using the same one as you are. If you have a guitar cable on hand, plug it in. Then, using a voltmeter (the resistance buzzer setting works well), touch one of the probes to your cable's tip, and touch the other probe to each pin until you get a connection. To find the other pin you need, just touch one end of the probe to the inside of the jack (where the cable is inserted) and check the other two pins.

Tip: Lable one of the pins with a sharpie - a dot will do.

[usefulinsight]

UPDATE

Ive actually figured some things out with a little help.
For the 1/4 mono jack in the right hand pick, I will use the two right pins only and not short any pins. The pin on the far right is connected to the insert hole which I learned is the ground and the middle pin is connected to the prong. The pin on the left is connected to a tab which is connected to the prong until a "male" plug is inserted. I will leave the left hand pin alone.

As for the 1/8 stereo plug in the left hand picture, I was told to connect the 4 left most pins together and together as a whole use this for positive and to use the far right pin as the negative. I will attempt to do this and hope it is correct.

I still have a major issue going on and can use some direction for help.
For now I have everything wired to spec except for the 1/8 plug. I have the negative from speaker going directly to breadboard instead of through 1/8 jack first. One lead from my switch is also not connected to the 1/8 plug. I assume this should be fine to test the 1/4 plug for sound.

Here is the problem

My led wont light up right away and only really does when I play the guitar. Every time I pluck some strings it lights up a little but then dims down when I stop strumming.

The other problem is that I have a lot of feedback all the time. Just turning the system on gives noise out of the speaker. I turn the pot down and it does go quieter but not really. This all happens with and with out a guitar hooked up. When a guitar is hooked up, the sound of playing is heard through the speaker but barely and is accompanied by the same unwanted noise as when no guitar is hooked up.
Any ideas?
also just to be clear the only differences in parts is:
1k 1/2 watt instead of 1/4 watt
all caps are 16 v except the 10 microFarad is 50v and the 4.7 microFarad is 25v. Could any of these parts be of fault?

[usefulinsight]

Hi again

I have success. I came to the conclusion that the breadboard itself was presenting me with problems. There are many ways for wires to be loose and I believe a lot of my feedback was caused some how as well.
I assembled everything on a circuit board and have no problems at all. My led works all the time. There must have been loose connections on the breadboard.

I get no feedback anymore either. I know a breadboard can cause unwanted capacitance so maybe this was to blame.

Anyhow this was a great instuctable. I learned a lot on this build. Thanks you very much for your time.
I hope I am not breaking any rules by leaving such long posts and hope someone else can learn from them.

[mattglas]

I spend 20$ on this and the first time i turned it on and it started smoking.
Must have done something wrong

[Selmerman]

What does +9v to circuit mean on the diagram. We're does the wire go from the switch?

[mdgrover]

Hey there - sorry for the delay, I've been on vacation and offline for a couple weeks. You may have figured it out already, but the "+9v to circuit" shown in the diagram at the top of Step 2 refers to the power being delivered to the circuit (from the battery) when the switch is turned on. In the schematic at the top of Step 1, that connects to the arrow pointing up shown above the LM386 chip (and connected to pin 6).

[Selmerman]

Yes, thank you. I figured out almost instantly after I posted it. I wasn't thinking when I posted it. Thanks anyway!

[mdog93]

Hi again, I built the rev1.0 amp and it works fine, but I find that distortion starts very early on the pot, to get a clean sound out of the speaker i have to have the gain pot at about a quarter and the guitars volume on about 2-3 out of a maximum 10. This is pretty quiet at those settings, but I can't go higher without getting distortion and Its not very nice distortion to listen to its very crackly and harsh- like when the radio isn't tuned into a radio station.

Is there some components such as capacitors or resistors that I could change out for different valued ones to raise the threshold at which the distortion comes in at?

And is there a way of 'softening' the distortion a bit, maybe smoothing it out a little?

Side note: I get a radio hum/ crackle when running it off a AC/DC wall wart that I do not get from the battery, Is this because the power supply isn't shielded or something? I've read that there are special audio grade DC transformers?

[mdgrover]

Hey there - great to hear that you went ahead with the build. Sorry for the delayed response - I've been on vacation (and offline) for a couple weeks.

You might want to try Rev 2.0 if you're looking for a smoother sound - the overall gain is a lot lower, so you could send a larger signal from your guitar and it should distort less. You could also try substituting C1 and C4 with different capacitor values (a little larger for C1, a little smaller on C4). Try them one at a time to see if you're going in the right direction. Hope that helps.

[mdog93]

Ok thanks!

Also i was wondering why the schematic calls for a linear pot, almost all audio electronics i've seen are log because the human hearing is nearly logarithmic.

Cheers

[forgelearner1234]

Hey Dude,

Looking at this thing, Im thinking that what would be really helpful is a step-by-step instruction guide for this thing, or at least some instructions on what goes where, and to what pin on the IC, etc. I only say this because i would like to try to use this with a Scout troop to teach Electricity merit badge.

Sounds like a really cool thing though dude! Keep it up!

[mdgrover]

I love the idea of the merit badge, and I agree that the step-by-step instructions would be needed. I wonder if an electronic kit might be a better way to go - something with a printed circuit board and all the parts included, so they can just insert components and solder them one at a time.

Here's an example that's under $10 and includes what looks like a good quality PCB - http://www.vakits.com/product_info.php?cPath=43&products_id=1481

[forgelearner1234]

Thanks. I tried your instructions, and i think i might work on creating a PCB layout for your project. I ran into trouble with the Circular Perfboards because there wasnt really enough space, so a PCB would be a good alternative for beginners working on this project

[snowpanda666]

hey man,

i just tried building off your schematic and am having the same problem as a lot of the guys in the thread who you haven't responded to.

I quadruple checked my work and still can't get the pot to effect the volume.

[mdgrover]

Are you using the Rev. 1 or Rev. 2 circuit? Also, have you checked the pot with an ohmmeter to make sure it's functioning correctly?

[xsandmanx]

I can't find the potentiometer you listed anywhere. Could I use a 10kohm 1Watt one?

[mdgrover]

Absolutely. In general, you can always go "up" in wattage or voltage on a part, as long as the value (in ohms, μF, etc.) is the same.

[Moustache_Bash]

Hey,

Thanks for the awesome project! I'm breadboarding it right now, but am having some trouble...

My 1/8" jack has three lugs: two are on one side, and one is on the opposite side.
http://imgur.com/6UHGL
^^^ This is how I wired (sorry, bad picture). I have the black wire from the speaker wired to the solitaire lug on one side, and then I wired the blue wire from the switch (I used the same colored wires as you) to BOTH the lugs on the other side. When I plug the headphones in I only get mono.

Also, I'm getting severe distortion regardless of the volume of the input. Also, my LED only lights up when I have the potentiometer wide open, or if I touch the 1/4" input.

I'll keep messing around, but I figured I'd ask to get some ideas.

Thanks!

[mdgrover]

Hey there - I'm glad you're giving this project a go, and going "breadboard first".

It is hard to see the headphone jack clearly, but for most of these 1/8" jacks there should be a way to get both channels going. It may be useful to plug your headphones into it and then use some trial-and-error (apart from the amp circuit). Use a known audio source as an input - this could be two wires coming from a plug that's plugged into a headphone output, or even speaker wires from one channel of a stereo receiver (keep the volume really low if you use speaker wires!). Try different combinations of the two wires briefly connected to the three terminals until you have audio coming to both sides of the headphones.This way you can make sure the headphone jack is good (or not) before you put it back into the amp circuit.

As far as the distortion - that could be a lot of things but I would start by double-checking all the wiring, especially to make sure that all the ground points are connected to ground.

Good luck!

[Moustache_Bash]

Well, I've messed with it some more and everything works except I still get a weird velcro-y sounding distortion that is varied by the potentiometer. I only get a faint sound of music coming out the speaker. On the bright side, pot no longer seems to control the actual flow of current throughout the whole circuit since the LED stays bright unaffected now.

I'll keep tinkering around.

Thanks!

[ncfak94]

Hey mdgrover,

I have a question about the potentiometer. In your diagram, your potentiometer has three terminals. Mine has five. What should I do if I don't want to purchase another potentiometer?

[mdgrover]

Well, it may work and it may not. Usually potentiometers have multiples of three terminals. You may have a standard 3 terminal pot with 2 extra terminals acting as a switch, but I'm just guessing...

See if this helps. You'll need to use an ohmmeter and some trial-and-error to identify the correct two terminals. Connect the ohmmeter across any two terminals and see if you get a resistance reading between 0 and 10,000 ohms. If you do, rotate the knob to see if the resistance changes. When you find two terminals that work this way you're all set.

Note that two different sets of terminals may work, and the resistance may increase and decrease by turning the knob in opposite directions. You'll want to use the terminals that decrease the resistance (and thus increase the gain) when you turn the pot clockwise.

[genericusername5]

Would it be possible instead of hooking this up to the speaker you could hook it up to an earphone jack output? I play my bass alot but I'm always being told to turn it down. If i did just hook this straight up to an earphone socket would i have to change the resistors and capacitors? If you could please get back to me as soon as possible that would be great.

[mdgrover]

That's what J2 is for. If you don't want a speaker at all, follow the schematic and diagrams in Step 6.