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

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.

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