This instructable describes how to "make" a simple audio headphone amplifier.
It can be used with different devices - MP3 players, Walkmans, Radios, .etc. It can be used also for your own designs - can be connected to the analog outputs of audio DACs, to the outputs of self made radios (for example using TDA7000, or TA7642) or other gadgets.
In comparison with the other instructables, this will not give you an exact instructions how to do the job, but will give you the idea and show you for example how it can be realized in a particular case. The success of this project will relay on your imagination and capabilities ...

The main idea here is - why to make something from scratch, if it exists...
Where an existing audio amplifier can be taken from?
The answer is - from a defect computer CD-R,W, DVD-R,W reader, writer, ROM-drive..
All they have audio output for headphones, which has almost always a volume control.
When those devices broke, normally the malfunction is always in the mechanics, in the laser system, in the optics, but, I think never in the audio headphone amplifier.
Where to find a defect drive?
You decide - at scrapyard, at the place were you company throws away the broken equipment for recycling, in some garage sale, to ask your friends, eBay...

Let's suppose, we have found our defected drive.
Let's go for the first step.

Step 1: Exctracting the Audio Amplifier Board

First step is to disassemble the drive.
The audio amplifier board is normally placed directly behind the front panel of the drive. The PCB in most cases has a long narrow shape. Between the audio amplifier board and the "main" board of the drive a flat cable connection is done. Unsolder it from the main board. May be will be possible to use it, if needed.
Do not forget to extract also the laser diodes and the electric motors - they can be used for other instructables.
On the pictures can be seen the extracted board, which was placed behind the front panel and contains the audio amplifier.

Step 2: Shaping the Amplifier Board.

The second step is to investigate what you have.
It is good practice to take a picture of the board by a digital camera in macro modus, to plot it, if possible on A3 sheet, and to try to understand the structure of the board. You can see that also some additional electric components are placed on the board - switches, LED's for read/write operations.. etc. You have to decide what should be used - do you need light indication of the supply presence, do you need the volume control..
Normally, for the low noise reasons, the audio amplifier occupies a compact area, which must be identified. In this case it is about 1/3 of the whole PCB area placed at the end of the board where the chip is mounted.

The next action is to mark the part of the board which should be used, in the way that the signal paths and supply for the audio amp tracks should be kept safe. There are always some tracks which connect switches, sensors, LED's placed on the other part of the board and can be cut without any influence on the audio amp performance. For the marking I used a dark marker. Now the board can be cut. For that purpose I use usual scissors. You have to cut the board carefully some distance away from the marker line - because of the cracks, which appear during the cut. After cutting the board, it must be shaped - all sharp edges have to be polished. For that purpose an abrasive paper can be used.

Step 3: The Research Work

Now starts the real research work. We have to identify which chip is used for the audio amplifier, to find the technical data (the datasheet), and to track all connections. In this case it is easy to see that the chip is from the type APA3541 (product of ANPEC - http://www.anpec.com.tw ). Using the "Google" the datasheet can be found very easy. The APA3541/4 is an integrated class AB stereo headphone driver contained in an SO-8 or a DIP-8 plastic package with Mute feature . For us the most interesting information, found in the datasheet is : 1)the block diagram with the function pin description ;2) the typical supply voltage -for this case it is 5V ; 3) the possible driven load (can be 16 Ohm).

The task is now to connect the amplifier in the correct way.
I removed the flat cable. I plotted the picture of the PCB with the metal tracks view on a A3 sheet - to be easy to follow each track and connection. You can use markers with different color for each signal.
Let's start with the ground pin - normally the ground is the "fattest" wire on the PCB. It is chip pin #4.Using Ohmmeter you can check this. A suitable place where the ground cable ( "-" of the battery) shall be soldered must be found. There the green lack must removed from the PCB. I scratch it using a big needle. A hole for the ground cable must be drilled there.

Step 4: Power Supply Connection

Next step is to guarantee the proper power supply of the chip. We found that the chip must be supplied by 5V source. Such kind of batteries are not often seen. It is better to use a voltage regulator , which will produce the needed voltage. The most suitable, I found to be a regulator from the type 78L05 - it has 3 pins and small package. Practically it doesn't need an external components. To mount it on the PCB we need to scratch the green lack again in the proper places and to drill 3 holes for its pins.
After that we can mount the regulator, solder it and to bridge the ground line.

Step 5: Connecting the Mute Signal.

In the datasheet was seen that the audio amp has a mute pin - your decision : You can have a switch to mute the amplifier, or to connect the pin hard to the supply line for continuous operation.
I connected it directly to supply line.

Step 6: Connecting the LED As Power Indicator

Because of the existing LED - I decided to connect it as power indicator. Two connection must be done for that purpose :
- the resistor limiting the current through the LED must be connected to the supply line
- the cathode of the LED must be connected to the ground line

Step 7: Connecting the Inputs

Now remains to connect the inputs of the amplifier. I used a cable of defected stereo phones. Depending on the way of use of the amplifier, different cable connections can be implemented. Following the input tracks using the ohmmeter (the inputs of the audio amp chip are connected to the volume control potentiometer, after that through electrolyte capacitors ) I identified the pads for the flat cable where the input signals come. Two holes for the left and right audio signals and an additional hole for the ground cable wire were done there.The audio and the power supply cables were soldered.In the case, you want to connect the amplifier to a mono audio signal source, it is better to short the both inputs together.

Step 8: The Joy...

It is good to find a suitable box for the amplifier and the battery. It can be plastic or metal - in the second case an insulation must be put between the internal walls of the box and the PCB, to prevent a short. A hole for the volume control must be cut.
I used a plastic box suitable for small radio, where a special place for the 9V ( 6LR61type) battery was reserved. I added an ON/OFF micro switch at the cable from the battery "+". I cut a holes on the box side wall for the volume control, for the audio jack, for the LED and for the micro switch knob using a dremel like tool. Finally I fixed the board using 3 small screws.

I connected the battery, switched the amplifier on.....
The sound was quite good...

Enjoy, you too!