DIY Portable Amplifier

As an avid climber, a full day, or weekend, break from my cellphone and e-mail is one of the best perks. Still, like nearly any activity, good music makes it more enjoyable. This desire to merge two of my passions, climbing and music, resulted in this small, yet robust, portable amplifier.

Here's a video of the final system in action.












Materials

•  LM386 audio amplifier

•  3 Capacitors:

C1 = C3 = 220 microF
C2 = 10 microF

• 4 Resistors: 

R1 = 10 Ohm
R2 = 1 kOhm
R3 = 10 Ohm 
R4 = variable 10 kOhm (w/ an SPST or SPDT switch)

• 9 V battery + battery clip

• Speaker (4 - 8 Ohm)*

*Have a broken sound system? Harvest the speaker! Look for one that can fit in the palm of your hand.

•   1/4" (or 1/8") mono panel-mount audio jack

•  PCB Board

• 22 Gauge stranded wire ( ~ 1 ft)

•  Case**

**There are many options! My case is a mini-lunchbox type thing found at a thrift store for $1. Look around the house, or peruse a thrift store for vintage tins, cigar boxes or metal piggy banks. Look for containers made from metal, wood, or rigid plastics.

• Recommended: Piece of wood for speaker mount

This improves audio output by providing a solid resonant surface. This step was somewhat involved (tool heavy), can be accomplished in many different ways, and is entirely optional, so I will just briefly cover this part of the process.


Tools

• Soldering iron, solder + solder sucker. 
• Epoxy or other bonding agent. 
• Wire strippers 
• Highly recommended: Multimeter. 
• For the case, you also need a drill.
• To mount the speaker in wood: a dremel or other wood cutting tool (+ clamps + eye protection)

Build it! Pt. 1

1. Solder wire leads to the audio input.


2. Solder wire leads to the speaker (recommended to use at least 6" leads).











3. Solder wire leads onto the variable resistor (potentiometer) pins.
Check the resistance between the difference potentiometer pins w/ a multimeter before soldering to determine the functionality of each pin.
Aside: Wire color, while entirely optional, helps distinguish the different pin functions. To wire the potentiometer so that the volume increases from the "off" position: Pin 1 goes to ground, pin 2 goes to LM386 pin 3, and pin 3 goes to the positive audio input.

4. Solder wire leads to the switch pins on the bottom of the potentiometer.
Check resistance across switch pins w/ multimeter. This potentiometer switch has a front pin that goes to ground and the left front pin to positive input.






Build it! Pt. 2

If schematic makes sense, skip next two steps (still suggested to build on breadboard first).

1. Build circuit on a breadboard. Recommended to draw out how it will transfer to the PCB board pads.

Connect a battery and check audio input w/ either a multimeter or an actual source (will need a 1/4" or 1/8" headphone cable).

2. Solder LM386 amplifier to PCB board.











3. Solder the battery clip leads and potentiometer switch leads to the PCB board.
The potentiometer leads are inserted between the positive battery lead and the 10 ohm resistor going to the LM386 voltage source pin.
Note: If your variable resistor does not have a switch, it is highly recommended to include a power switch for the battery.


Build it! Pt. 3

1. Solder the remaining components to the PCB board using the PCB pads and/or wires (breadboard pictured for clarity).

A. Connect LM386 pins 2 and 4 to ground.

B. Connect 1kOhm resistor and 10 microF capacitor from LM386 pin 1 to pin 8 (shorter leg (negative pin) of the capacitor connects to LM386 pin 8).











 C. Connect the long leg (positive pin) of the 220 microF capacitor to LM386 pin 5. Connect one side of the 10 ohm resistor to the short leg of the capacitor, and the other side of the resistor to ground. Connect the positive speaker lead to the short leg of the capacitor, and the negative lead to ground.




D. Connect one side of 10 ohm resistor to LM386 pin 6 and other side to positive battery (w/ switch). Connect long leg of 220 microF capacitor to LM386 pin 6 and short leg to ground.












E. Connect the potentiometer pin 1 to ground, pin 2 to LM386 pin 3, and pin 3 to positive audio input.

F. Connect negative audio input to ground.

Case it! Pt. 1

1. Plan + mark location of each of the components.
The potentiometer, or volume knob, power switch, audio input, and speaker are installed on the exterior. The PCB board and battery are on the interior.










2. Drill two holes in case exterior for the potentiometer knob and for the audio input.
If case is not metal, recommended to include a sheet of metal to provide a better ground for the audio input.

3. Remove screw heads from potentiometer knob and audio input. Push potentiometer knob and audio input through the holes and reattach screw heads.

4. Check connections w/ a multimeter and/or an actual audio input (will need a 1/4" or 1/8" audio cable).







5. Dab epoxy onto bottom of case to adhere the PCB board.
To secure the 9V battery: use velcro strips, or make a quick fabric pouch and epoxy the pouch to the case.

Case it! Pt. 2
1. Mount the speaker in wood or in case exterior.
There are different options depending on your chosen case and available tools. It sounds great w/ wood, otherwise you can also mount it directly to the exterior of the case.

One option is to follow the design featured in this tutorial:

A. Cut a piece of wood to fit inside the case (bottom slab). Cut a hole in the middle large enough to pull the speaker through.
 



B. Take a thin piece of wood and cut a notch out. The notch should be sized so that the screw holes of the speaker frame sit on top of the wood.










 C. Optional: Screw speaker into top wood ring. Screw top ring into bottom slab.

Test & Go Adventuring!

Test the electrical connections by plugging in an audio source (e.g. electric guitar or mp3 player). Check that the potentiometer effectively controls the volume and that the switch turns the system on/off. Once everything is working, coat electrical connections in epoxy.

Take your new portable amplifier to bring music to your next outdoor adventure!