Introduction

So, I stumbled upon a schematic of a headphone amplifier on the internet the other day. I've always been interested in buying a headphone amp from HeadRoom, but theirs are pricey. All portable CD players and iPod's do not put out enough clean power. Sure the CD player or iPod is "loud" enough, but after mid volume levels the audio begins to distort.

The parts laid out (click to enlarge)
Distortion is distinguishable on higher-end headphones. I own two pairs of hi-fi German headphones - a $170 pair of open-enclosure Sennheiser HD525's and a $150 pair of closed-ear Sennheiser PXC-250's with active noise-cancelling technology. I primarily use the PXC-250's when traveling on an airliner because it cancels out the hum of the turbojet engines. Active noise-cancelling technology works by a tiny microphone placed on each earpiece to detect ambient noise before it gets to your ears. Then the noise-cancelling module literally inverts the frequencies, turning the noise's sound wave up side down. The result is an ambient noise reduction of up to 25 decibals. I was so impressed with the sound of the Sennheiser noise-cancelling headphones that I bought a pair for my mother. Anyway, bottom line is if you have high-quality headphones and an acute ear for good sound, you will dislike the low power output in the audio of portable electronics.

The headphone amplifier I made is really cool because the circuit is designed to allow the use of a NiCad rechargable 9-volt battery and 12 volt AC Power Adapter. You can actually leave the battery in the case and hook the AC Power Adapter to the back of the amp to recharge it. You don't even have to replace the batteries and it'll provide you with 6-hours of continued music enjoyment!

Specifications

Size: 1 x 2 x 3 inches (a little larger)
Weight: 5 ounces (includes NiCd battery)
Distortion: 0.2% Freq Response: 5Hz to >110kHz (-1dB)
Max Output level: 1.7Vrms (loaded with RS Pro 25 phones)
Noise: Inaudible in quiet room
Output Power: 375mW into 8-ohms
Input Impedance: 33Kohms, minimum
Running Time: 6-hours, est, high-cap NiCd, depending on audio level
Circuit Desc: Uses two LM386 power amp chips, 10-caps, 5-resistors.
Controls: Volume, On/Off switch
I/O's: Input pigtail, mini-phone out. DC power in.
*Source: Stephen Lafferty

Trip to Radio Shack

My first step was to gather all the parts needed for the project. Believe it or not, Radio Shack carried all the parts listed in the schematic. The total came to about $42. I bought a 3"x2"x1" ABS plastic project box originally, but even after enduring over an hour of Dremel tool plastic modification (along with burning plastic smell and plastic pieces flying into my eye), the box would not be large enough for all the components to fit. I ended up buying a box slightly larger.

Also, the guy at Radio Shack tried to sell me their version of a headphone amplifier, but I wanted to make my own. After showing him a copy of the schematic, he thought I was crazy.

The Schematic (click to enlarge)

Building the Amp

I was very skeptical in persuing this project because I have never read nor built a project out of a schematic in my life. I figured I'd give it a try anyhow. Through tutorials found on Google and a book I checked out at University of North Dakota's library, I was able to figure out what the hell all those symbols of resistors, transistors, and connections meant.

Click to enlarge:

Building the amp board

Arranging the capacitors

The completed underside

Completed amp board
Building the amp was probably the most difficult part. I had a very hard time determining which wires and ends to solder together. I had to learn how to visualize the layout of the entire schematic with relation to how it was supposed to look in front of me.

The amplifier circuit uses two LM386 power amps (one for the left channel, the other for the right channel). I was surprised with how small they were - only about a 10mm in length and 7mm wide. Other than the two amps, only a few capacitors and resistors make up the circuit. I had to cut the orange wafer board very precisely. This is where the Dremel tool comes in handy. After about 8 hours spent soldering tiny resistors, capacitors, hook-up wire, and open wires, my amp board was complete.

Preparing the Project Box

Preparing the project box in itself was a pain in the ass. I had to mount a large volume control, on/off switch, audio out jack, DC power jack, and audio-in cord into the box, while making sure there was barely enough room for the amp circuit and 9-volt battery. I used the wire from cheap $1 headphones I bought from Wal-Mart to make the audio-in wire. I had to strip the wires and separate the ground wire from the audio wire, on both channels (also a big pain in the ass).
This box was too small

I had originally purchased a 3x2x1 box but it proved to be too small. I had to extensively modify the plastic support pieces of the box and still everything wouldn't fit. I went back to Radio Shack today and got a slightly larger box so that everything could fit. It definitely helps to have a large assortment of drill bits as the hole diameters for mounting the jacks ranged from 1/8" to a large 7/16" hole for the DC jack. Drilling into plastic leaves large pieces of plastic all over the kitchen, which I have yet to clean up!

The volume control knob had to be extensively modified in order for it to fit. I had to grind metal, cut metal, and endure a few minutes of smelly burnt plastic and flying hot sparks just to make the stupid thing fit! The grinding disc on the Dremel tool worked really well for this issue.

Putting It All Together

At this point I was pretty much anticipating the stupid thing to either A, not work or B, blow up my headphones. I put everything together, soldered all the ground wire to the ground bus on the bottom of the circuit board, and positioned it into the project box. I found it very difficult to solder wires to the IC circuit prongs. They are very small and barely stick out of the perf board. I had to resolder about 4 times for the wire to stay and even after I positioned the amp board into the box it still seemed as if the solder was breaking off.

Before closing the case
After positioning everything in nicely and soldering the remaining ground wires to their appropriate places, I noticed that I made a mistake. I soldered the audio-in pigtail wires to the wrong places on the volume control tabs! I had to re-heat the solder on the tabs, yank the wire off, and resolder.

Testing the Completed Headphone Amplifier

After closing the amp box I quickly grabbed a pair of headphones, connected it to my old portable CD player, and turned on the music. Amazingly, the amp worked! I was so shocked that I didn't miss one solder connection nor break something while mounting the amp.

The music is crystal clear on all audio levels. I cannot believe how much power this little amp puts out. The bass is tight and pronounced while the high frequencies are crisp and clean. The amps inside the typical low-power portable audio players lack the ability to produce clean treble frequencies because all the power is going to the bass frequencies.

Conclusion

Even after about 12 hours worth of work on this thing, I am glad I did it. I learned something new about electronics and learned how to read an electrical schematic.

For a final touch on my amp, I printed out volume and on/off switch labels and applied them to the face of the box. This will probably help save on batteries knowing if the switch is on or off.

Thanks to Stephen Lafferty at MiniDisc.org for his information on how to build this amplifier. Click here for the original website with the amp building information.

Photos

Click to enlarge:


The whole deal

Metal underside

AC Power/Charging Jack

Sennheiser HD525's and iPOD Sennheiser HD525's and iPOD Sennheiser PXC-250's and iPOD
(the ultimate travel companions)

Final Note

Now that you've read my write up, go out and buy yourself a pair of Sennheiser headphones.

For more info on Sennheiser, visit their site at SennheiserUSA.com: