The Sound Sack - a Portable Backpack Amplifier




Introduction: The Sound Sack - a Portable Backpack Amplifier

The Sound Sack (picture above) is a portable, light weight, general purpose amplifier. I designed my amplifier specifically for guitar players, but it is very simple to use this as an amplifier for your phone or anything else. The Sound Sack is an Lm386 amplifier circuit connected to a speaker and put inside a backpack. The cost of this project for me was $32.95, but I scavenged a few parts. This project is easy even if you don't know how to build circuits, and the approximate build time for this is about two hours. If you know how to read schematics then this estimate will be significantly reduced.

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Step 1: Materials

Here is a list of all the materials that you will need:

A 9 volt battery and battery clip

A speaker (I used an 8 inch 4 ohm speaker that I scavenged for a guitar amp that I owned) - 8 ohm recommended

Any lm386 amplifier (I used an lm386n-1)

A breadboard (optional if you plan to solder the circuit)

Hookup Wire- any electrical wire

A switch

Alligator clips (optional)

A 100uf (uf means microfarad) capacitor

A1/8in stereo or mono audio jack if you are planning on using an mp3 player

A 1/4in mono audio jack if you plan to use a guitar

Any capacitor between 220 to 1000uf

A 470pf (0.047 uf) capacitor - pf means picofarad

10uf capacitors (x2)

0.1uf capacitors (x3)

10 ohm resistor

10K (10000 ohms) resistor

Backpack of your choosing


An audio taper (logarithmic taper) potentiometer

Protoboard (perfboard)- if you plan to solder

A project box to house the circuit in

scrap metal

- some of the parts are marked not used in the image because I bought different kinds of potentiometers, different values of resistors, and many different values of capacitors.


Wire Stripper


Tin Snips (optional)

Punches (optional)

Drill with bits that are larger than the shaft of your potentiometers

Soldering Iron

If you build this on a breadboard than all components will need to be through hole components

I got my components from a few different online retailers (websites listed below): - all of my components except for the lm386 and the potentiometer came from here - the lm386n-1 - the potentiometer

Step 2: Building the Circuit

In this step I will guide you through the assembly of this circuit on a breadboard. Even if you do plan to solder the circuit I would make it on a breadboard first unless you are experienced. The schematic is included above, but if you don't know how to read schematics, or aren't that experienced, like me, then check out this tutorial on how to build he circuit. I know it is a long video, but it covers a lot of information and makes the whole building process much easier.

The other image that I included was my first attempt at building this circuit. It is really messy, but it should give you a rough idea of what the circuit should look like when finished.

The final photo attached is my speaker, and the purpose of this was to show that I used alligator clips to attach my wire to the speaker, because I wanted to reuse this speaker for a future project. Make sure that the wires attached to your speaker, your potentiometer, audio jack, and switch are long enough. I used about a foot of wire for each contact.

As mentioned before, it is fun to experiment with different values of capacitors as each different value gives you a different sound. The recommended type of potentiometer is a logarithmic potentiometer, but I tried many different types and they all worked, but the audio taper (logarithmic) worked the best.


I included an on off switch in my design. Two wires were soldered on to two of the tree contacts for the switch and one side of it was connected to positive voltage and the other side was connected to the sixth pin of the lm386.

Additionally wires must be soldered to the potentiometer, one to all three contacts, and wires must be soldered to your audio jack of choice. 1/8in mono jack for mp3 players and a 1/4 in jack for guitar and other instruments.


All resistors have colored bands on them and the order of these colors dictates their value, so for the 10k ohm resistor the colors are Brown, black orange and then gold. The 10 ohm resistor is brown, black, black, gold. A tool that I used to figure this out was digikey's 4 band resistor calculator.


Lastly, How to count the pin numbers on the lm386. Every chip comes with a dimple towards the top left and this dimple denotes pine one. under neath that it pin two and below that is pin three and so on. Pin five is the pin directly across from pin four and the pin above pin five is pin six and so on up to pin eight. - photo above

Step 3: to Solder or Not to Solder?

That is the question. Personally, I did solder the circuit together initially, but it didn't work. Troubleshooting, which is common with electronics, is more easily done on a breadboard. If you are experienced with soldering, or want to get started I would recommend this tutorial (see below), and starting with an easier circuit. The required soldering for this project, attaching wires to potentiometers and switches and audio jacks is a good way to start learning, but if you are inexperienced I would recommend breadboarding the circuit.

Step 4: the Testing Phase

The fun part!

Now that you have built your circuit its time to test it. Hook up the nine volt battery, plug in your audio source of choice and test away. The best part about this phase is that you can experiment with different values of capacitors for different sound. You can also experiment with different speakers. I recommend 8 ohm speakers as they tend to sound nicer. Once you have decided on your final circuit put it in a box. I used a project box from Radioshack and I drilled a large hole in the side of it to run my speaker, potentiometer, switch, and audio jack leads out of.

I would recommend testing with an mp3 player because it is a assured audio source that will work and play a sound every time you use it. If you built the sound sack as a guitar amp you will need an adapter like this one.

Step 5: Issues and Tips

I had a few issues along the way and just thought I'd offer some help so that you don't have to build this circuit six times like I did.
If the sound coming out of the speaker is too quiet then substitute the 220uf capacitor for a larger value, and if it is still too quiet substitute the 10uf capacitor for a larger one until it sounded good. I used a 1000uf capacitor between pins one and 8 and a 10uf capacitor in place of the 220uf one.

If you are using a 4 ohm speaker and it sounds scratchy then increase the 100uf capacitor that goes between the positive and negative leads to a higher value (the same can be said for an 8 ohm speaker). If you are still getting a scratchy sound then you should use an 8 ohm speaker as they will always sound better.

If you hear a horrible buzzing noise then increase the 100uf capacitor and if that doesn't work get a higher quality lm386 chip because it is most likely the chip's internal noise and that is near impossible to filter out (by increasing capacitor values) the noise in cheap chips.

Step 6: Putting the Sound in the Sack

This is the building phase of the sound sack.

The building phase is quite simple, but I recommend testing your amp after you put each component in just to make sure that nothing has broken.

Start by taking your scrap piece of metal and cutting out a rectangle. Mine was one inch by four inches. Draw a line down the center of the piece of metal and mark three dots on that line. Drill holes (for your potentiometer, switch, and audio jack) where the dots are located. Test fit each of the components.

Optionally, add electrical tape to the metal surface of the holes.

tip: my potentiometer had a little nub on it that was preventing it from attaching to the metal mounting plate, so just bend the tab off with some pliers

Next cut holes in the top of your backpack (near the handle) I used a set of punches, but scissors would work just as well.

If your backpack, like mine, has a middle section then zip up the big pocket and cut a hole in the middle section. Run your speaker's wires through this hole.

Lastly cut a plus sign into the front of your backpack, or cut a hole and set your speaker in it. I secured my speaker in pace with wire by putting the wire through the speaker's mounting holes, and securing it to the backpack.

Congratulations! you are now the proud owner of a sound sack.

Step 7: Get Out There!

Now that you have created your own Sound Sack. Get out there and share your music with others. If you have any questions of comments, please put them in the comments section below and I will respond as soon as possible. Thanks!

Once again I must stress the need to use an 8 ohm speaker as that is why the the sound quality is not fantastic. That and the fact that it was recorded on an iphone. Thanks!

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    9 Discussions


    4 years ago

    Great project but I would suggest putting a grill over the speaker to protect it.


    Reply 4 years ago

    I agree whole heartedly I should have put a grill on the speaker. The only reason I didn't was because I was running out of time. I was actually toying around with the idea of a fabric circle with a wire support that sat inside the metal mounting plate of the speaker. The only reason I didn't do this was because I don't know how to sew.


    Reply 4 years ago

    Its actually really easy to do basic sewing, just look up a youtube tutorial, you'll get the just of it in no time.


    Reply 4 years ago

    I agree whole heartedly I should have put a grill on the speaker. The only reason I didn't was because I was running out of time. I was actually toying around with the idea of a fabric circle with a wire support that sat inside the metal mounting plate of the speaker. The only reason I didn't do this was because I don't know how to sew.


    4 years ago

    Could this build be used for a personal voice amplifier system? I am an auctioneer and like the idea of a self built portable megaphone system.
    Any tips would B appreciated. Awesome build!


    Reply 4 years ago

    I believe it could be you would just use a microphone as the audio source instead of a guitar. And you could certainly use a smaller speaker if you wanted.


    4 years ago

    For a grill and more solid connection to the back pack a wooden ring with a hole placed inside the bag and then screw the speaker through the bag material into the wood. I would also use some hot melt glue on both the wood and the speaker.

    For wood a 10 inch square of any 3/8 or 1/2 inch plywood should work and the cut out the circle with a jig saw. Jig saws are cheaper than a sewing machine. Also the wood doughnut shape would leave you something g solid to screw down an external grill.

    Neat project.


    4 years ago

    This is a really cool idea! Nicely done!