DIY Headphone Amp Circuit

This amplifier helps you to listen to your headphones in even the noisiest of environments.

In some environments --- such as in the cabin of an airplane during a cross-country flight --- the ambient noise of that environment makes it difficult to hear the audio signal through a pair of non-amplified headphones. However, it is possible to build a small headphone amplifier using a few components that are commonly found at a retail electronics store.

Things You'll Need

• LM358 operational amplifier
• 2 200-Kiloohm resistors
• 2 1-Kiloohm resistors
• Electrical wire (18 AWG, insulated)
• Electrical pliers
• Electronic solder
• Soldering iron
• Battery holder (see "Resources")
• 2 size-AA batteries
• Audio plug
• Audio input jack (see "Resources")

Instructions

1. Cut nine 4-inch segments of wire. Remove 1/2 inch of the insulating material from each wire segment end.

2. 

   Audio plugs contain three electrical connections. Left to right, these are: sleeve, ring, tip.

   Place one end of the first wire against the "tip" terminal on the audio plug. Hold the soldering iron to the wire end and the terminal simultaneously, and touch the tip of the solder spool to the terminal so that it connects the wire to the terminal. Smooth out the molten solder so that the soldered joint is flat and shiny (as opposed to dull and lumpy).

3. 

   Solder joints should be shiny and flat, as this photo demonstrates.

   Using the soldering procedure described in the previous step, solder one end of the second wire segment to the "ring" terminal on the audio plug. Solder one end of the third wire segment to the "sleeve" terminal on the audio plug.

4. Place one end of the fourth wire segment on to the "tip" terminal on the audio jack, and solder the wire to the terminal. Solder one end of the fifth wire segment to the "ring" terminal on the audio jack. Solder one end of the sixth wire segment to the "sleeve" terminal on the audio jack.

5. 

   Each metal pin (silver-colored leg) on a microchip corresponds to an electrical terminal.

   Place one end of the seventh wire segment to the "V+" terminal (pin 8) on the operational amplifier, and solder the wire segment to the pin. Solder one end of the eighth wire to the "GND" terminal (pin 4) on the operational amplifier.

6. Twist together one end of the first 200-Kiloohm resistor with the free end of the fourth wire. Place this wire pair on to the first output terminal (pin 1) on the operational amplifier, and solder the connection.

7. Twist together one end of the second 200-Kiloohm resistor with the free end of the fifth wire segment. Place this wire pair on to the first output terminal (pin 7) on the operational amplifier, and solder the connection.

8. Twist together the free end of the first 200-Kiloohm resistor with one end of the first 1-Kiloohm resistor. Place this twisted wire pair on to the first inverting input terminal (pin 2) on the operational amplifier, and solder the connection.

9. Twist together the free end of the second 200-Kiloohm resistor with one end of the second 1-Kiloohm resistor. Place this twisted wire pair on to the second inverting input terminal (pin 6) on the operational amplifier, and solder the connection.

10. Twist together the free ends of both 2-Kiloohm resistors, and solder this twisted pair to one end of the ninth wire segment.

11. Place the free end of the first wire segment on to the first non-inverting output terminal (pin 3) on the operational amplifier, and solder the connection. Place the free end of the second wire segment on to the second non-inverting output terminal (pin 5) on the operational amplifier, and solder the connection.

12. Solder the free end of the seventh wire to the positive terminal on the battery holder. The positive terminal will be marked with a "+" symbol.

13. Twist together the free ends of the third, sixth, eighth and ninth wires. Solder this twisted wire joint to the negative terminal on the battery holder.

14. Insert the audio plug into the audio device headphone jack. Plug the headphones into the amplifier audio jack. Insert the batteries into the battery holder.

References

• Georgia State University: Op-Amp Varieties
• National Semiconductor: LM158/LM258/LM358/LM2904 Low Power Dual Operational Amplifiers
• "Electronic Devices"; Thomas Floyd; 1996

Resources

• Radio Shack: Enclosed 2 AA Battery Holder
• SparkFun Electronics: Audio Jack