Introduction: Miniature Guitar/Audio Amplifier
I wanted a small yet robust amp for practicing while I travel. I wanted something that would fit in my pocket yet still be loud enough to hear.
Presented here is a amplifier based upon the LM386 Audio Amplifier.
There is a standard circuit in the data sheet that is an excellent place to start.
Materials needed (digikey part numbers):
1 - HM359 project box
1 - 668-1237 speaker
1 - LM386N-4 amplifier
1 - BS6I battery conn
1 - CP1-3515 stereo jack
1 - SC1316 stereo jack
2 - 450-1742 knob
1 - 679-1856 switch
1- 3mm LED
1 - 10 ohm 1/4W resistor
1 - 10uF ceramic cap
1 - .05 uF ceramic cap
1 - 420 uF electrolytic cap
1 - 8 ohm resistor
2 - 51AADB24 10K pot
1 - HM1252 circuit board
wire and solder
Step 1: Prep the Enclosure
Careful planning is required the first time you free build a circuit. The circuit board has solder pads but not traces. You will have to use thin wire to make the connections for the circuit to work.
Begin by laying out the components on the circuit board that will need to pass through the enclosure. This enclosure has a removable top panel which will be used for the volume, gain and 1/4 inch stereo jack.
Space is limited to check for fit before drilling.
All drilling of the plastic should be done with a step drill bit. This will make the cleanest holes without breaking the plastic.
Lay out the pots a few spaces back but still in line with the desired position. mark the center of each pot shaft then drill with a step drill tot he tightest fitting hole size. Make a center mark between the pot holes then drill for the stereo jack
On the inside of the top cover position and mark where the speaker will go.
Make a template on grid paper the same size as the speaker.
Tape the template to the inside of the cover as shown then use a step bit to drill holes on the center of every square in the grid. This will form the speaker grille. clean up the holes.
Step 2: Place the Major Components
Solder the pots to the circuit board as shown. then place the stereo jack(note in order to get the final fit I had to trim and modify the stereo jack housing a little)
Next, position and solder the switch on the circuit board and mark a space on the top cover that will need to be cut for the switch opening. Use a small file to cut the opening.
Use a sharp knife to bevel the edges of the switch hole to allow for easier operation.
Drill a hole in the side of the upper case for the headphone jack and fasten it in place. ( I had to recess the hole a bit for the retaining nut to grab)
Step 3: Build the Circuit
The speaker is held in place by using 2 small brackets that come with the serial cable connector hood. ( I had a bunch around that would never be used)
Refer the the circuit shown from the datasheet and the datasheet for the LM386. The basic circuit only has the volume control while the datasheet shows how to add a gain control across pins 1 and 8 of the amplifier.
The speaker is wired in series with the headphone jack. The headphone jack has internal switches that shut the speaker off when the phones are plugged in.
I chose to use a chip socket for the amplifier which make prototyping easier since you do not have to worry about solder heating as much.
Carefully lay the circuit out on the board and begin wiring components together. I added a second pot and cap in series between pins 1 and 8 of the amp to be able to manually set the gain in addition to volume.
Check you connections with a multimeter before adding the amplifier.
I chose to add a LED indicator for power. This was done by using one side of switch contacts from the battery. The LED is in series with a 220 ohm resistor.
Assemble the case and insert the battery.
Step 4: Final Notes
If the speaker is noisy while the headphones work normally, try reversing the speaker connections. If it does not correct the issue, connect a 8 ohm resistor across the speaker contacts.
You may have to place an insulating layer between the speaker and the place where the stereo jack comes through to prevent contact. This will be noted by a loud buzz.
You may have to add some foam in the battery compartment to stop the battery from banging around.
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