Pocket Guitar Amplifier

By jobeau

Difficulty: Intermediate
Assembly Time: 3-6 hours

Pocket Guitar Amplifier One of the biggest pains of an electric guitar is the need to hook it up to an amp to hear the actual electric guitar sound. That's not a problem with the pocket-size milli-AMP kit! This electronics project uses a bridged ML386N-3 amplifier circuit to produce in the neighborhood of two watts output power. It runs off a rechargeable 9 volt battery that provides an hour of full volume play per charge.

At two watts, the milli-AMP is more powerful than any commercially available amp (that I know of) of its size; a very pocketable 2.25" by 3.5" by 1" (roughly). Volume is controlled by the volume pot on the guitar, and that controls distortion as well.

Required components:

Case for the charger circuit: I used a case from an Ipod Nano

(1) IC LM317T Regulator TO-220
(2) IC LM386N-3 Low Voltage Audio Power Amplifier DIP-8
(1) Transister MPF102 JFET TO-92
(1) Tantalum Capacitor .22µF TM.22
(1) Tantalum Capacitor 10µF TM10/16
(1) Capacitor Ceramic Disc 0.05µF CD50000/1000
(1) Battery NiMH 8.4 Volts @ 200mAh 10005
(1) Connector 0.25" Audio Jack
(1) Toggle Switch 1MS3T1B1M1QE SPDT 5A On-Off-On
(1) Capacitor 100µF Radial 16V TAP107K016CCS-VP
(1) DC Power Plug Female 2.1mm x 5.5mm
(1) DC Power Jack Male 2.1mm
(1) ABS Enclosure with Clear Top 4.9"L
(1) Case ABS Speedy 3.125 x 2 x .875
(1) 9V Battery Snap with 6" 26 AWG Wires
(1) Prototype Builder 1.5" x 3.5" PCB
(1) 100 Feet Hook-Up Wire 22 AWG Black Solid Tinned-Copper
(1) 1000µF Radial Capacitor
(10) Resistor Carbon Film 240Ω
(10) Resistor Carbon Film 1.5kΩ
(5) Transistor, BC327, TO-92
(1) Speaker, Sq, ferrite, 4" 8Ω, 500Hz - 10KHz
(5) Potentiometer, 1/2W, 500Ω single turn
(1) Wall transformer, 12VDC @ 500mA
(10) Resistor, 1/4W, 10KΩ
(10) Resistor, 1/4W, 470Ω
(10) Resistor, 1/4W, 10Ω


Step 1 – Schematic Review
Schematic one is for the amp. It has been modified from runoffgroove.com to allow for a switch and the charging jack. Schematic two is of the charger. The transistor in the charger schematic is the BC327 or equivalent.

Pocket Guitar Amplifier

Click to enlarge

Step 2 – Amp Assembling

A good idea is to build each amplifier with its own independent circuit (each amp has its own separate connection to ground, positive battery supply, input, etc.) The purpose is to allow a blown amplifier chip to be removed and replaced without disturbing the other chip.

Pay special attention to the polarity of the charging jack. The center pin should be positive, with the sleeve contact being negative. As for the enclosure, a hole has to be drilled in one side of the enclosure for the input jack, then two holes on the opposite side for the switch and charging jack. If you want to add an optional clip to the back you will want to drill that hole now as well. Two holes, each 1/4" in diameter, should be drilled on the narrow end of the enclosure near the speaker, as well as two other holes opposite each other on the sides adjacent to the previously mentioned side. These act purely as ports for the speaker

Port and Input Jack
1. Port; 2. Input jack

As for the speaker place it face down on the top of the enclosure, and a knife used to score a circle around the edge. I used a 1" spade bit to drill out the middle of the circle then used a dremel with a sanding drum to enlarge the hole so the speaker would fit comfortably. The speaker does not have any mounting hardware so you need to glue into place.

Place the speaker

Place your built circuit into the enclosure. The battery goes at one end, with the two jacks next to the battery opposite each other, and the switch and amplifier circuit mounted underneath the speaker. Since the speaker has a relatively shallow mounting depth, there is enough room for two small heat sinks to be placed on top of the amplifier chips.

Place your built circuit into the enclosure
Part of the circuit built onto speaker itself

The heatsinks are cut up pieces of a regular heatsink filed down to fit. They were then glued to the amplifying chips using a homemade "thermal glue" of JB Weld mixed with aluminum shavings. I know not ideal, but it's better than not having any heatsink at all.

Place the heatsink
1. Port; 2. Power switch; 3. Charging jack

Make sure nothing touches the plug tip or its connections. The screw near the green cap is for the clip. The plug is not touching the screw.

Place the heatsink
1. Closeup of how speaker fits above amplifying circuit; 2. Heatsinks; 3. Port

Optional: Amp – Finishing Touches

I chose to add a clip from an old tape measure to the back of the amp allowing the amp to be clipped to the strap on the guitar. For a more professional appearance, the front of the amp is covered in grill cloth from an old pair of speaker grills. White tubing about 1/4" in external diameter was glued around the speaker, and then the grill cloth stretched around it. Hot glue was then used to adhere the edges of the cloth to the backside of the enclosure cover. Where the cloth was folded over on itself at the corner, the tip of a hot soldering iron was used to poke hole in the backside for the screws.

On the front, no holes were made, since the screws could puncture the fabric. Custom decals were made, and adhered to the back of the enclosure. To make these decals, the image was printed onto normal paper, dipped in polyurethane and the excess scraped off. The decals were then placed on the enclosure, and the polyurethane left to dry overnight. A red dot was added near the switch to signify the "on" position.

Place the front cover on
Front cover resting on top. It is not screwed on yet.

Place the front cover on
1. Hole made for screws 2. Red dot to mark ON position of switch

On - OFF Switch

Step 3 – Assemble the Charger

After following the schematic provided in step 1, hook the charger up to the battery in the amp. This may be easier to do with the battery removed. With an ammeter in series with the positive connection of the charger and positive connection of the battery, and the negative connection of the charger hooked directly onto the battery.

With the charger plugged in, turn the trim pot until 30mA is read on the ammeter. Do not go above 30mA. This is the maximum safe charging current for a NiMH battery of this size. Also, make sure that the center contact of the plug is positive, with the external sleeve being negative. An option you might want to follow is to mount the charger in an enclosure. Since the LM317 should be heatsinked, make sure the enclosure is large enough to accommodate it.

120Ω Resistor

Image Note:
1. 12 volts DC in
2. 9 volts DC out
3. The image above shows (2) 120Ω resistors were used in the place of the 240Ω resistor.


Step 4 – Completed Project

Before you use, there are a few notes about the use of the milli-AMP:
  • Do not turn the amp on without a guitar already plugged in. (A painful screech will occur hurting your ears)
  • Do not play the amp while charging. Charge only with the amp switched off.
  • To charge fully from a dead battery will take 12 hours. Do not overcharge the amp.
  • The volume is controlled by the volume pot on the guitar. Because the amp starts clipping above about half volume (entirely dependent on your guitar) the volume also controls the distortion produced by the amp. With the volume turned down enough for the sound to be clean, the "loudness" of the amp is not significantly lower. This depends on the guitar.