Various cables have appeared that allow you to connect your electric guitar to your iPhone or iTouch so you can record or use some of the amp and modeling effect applications. The issue is that the guitar's tone and volume controls are affected by the iPhone/iTouch microphone circuitry. Also the guitar circuitry sometimes doesn't trigger the iPhone/iTouch to switch to the external headphone/mic socket.
The following instructable will show you how to build a guitar buffer and cable that will allow you to connect your electric guitar directly to your iPhone/iTouch's headphone/mic socket.
To build this instructable you'll need to know how to make a PCB (I used the toner transfer method) and some understating in basic electronics. You could also build it on strip board instead of making a PCB.
Step 1: Circuit Diagram of the Buffer.
The circuit runs of a single 9v transistor battery. When a 1/4 inch plug is inserted it turns on the power to the buffer. I used a high brightness blue LED for the power indicator so I could get away with only using around 1mA for the LED. This means that the whole circuit uses around 10mA or less when running.
The terminals I/P means input and O/P means output.
I haven't included the power diode in the circuit that prevents damage if you connect the battery around the wrong way. I'll show the diode in the wiring diagram.
Step 2: PCB
Here's the layout of the PCB. I used the toner transfer method to make the PCB. LED1 is a high brightness blue LED. You can use a normal LED if you like but you may have to reduce R6 to 1K.
I've added an Eagle CAD brd file and a PDF file that has the PCB layout.
The circuit has unity gain so it's not a preamp just a buffer. The output of the buffer goes into a trimpot so you can adjust the level. Set this trimpot so as not to overload the iTouch/iPhone when your guitar is setup as if you were using a guitar amplifier.
When you first build the circuit, set this trimpot 1/2 way and adjust for the best sound.
The circuit presents around a 1k load into the iPhone so as to automatically switch the internal microphone off.
Step 3: Wiring It Up.
The diagram shows how to wire up the PCB to the external plug and sockets. I have shown how to connect up the 4 pole 1/4" plug looking at the back of the plug. If you have purchased an A/V cable you'll have to figure out what cable colors go where.
I have detailed the plug pin out wiring in the next step.
Step 4: The Plug.
Here's how the 4 pole plug is wired. I managed to pick up a 4 pole 3.5mm (1/8") plug at Jaycar in Australia. They're more expensive than the regular stereo plugs.
Another source of a 4 pole plug can be found on some A/V camcorder or DVD cables. You need to check if they're wired correctly. Some cables have the coax shield connected to sleeve of the plug. If you use these types of cable, then the shield of the coax cable will be used as the microphone connection which will cause hum. So don't use these A/V cables unless you know how they're wired up.
I used an old Nokia cable that had three separate coax cables. Using this type of cable prevents crosstalk although any multicored shielded cable should work as long as the cables not too long.
Step 5: Putting It Into a Box.
I crammed everything into a small ABS plastic box. You could use a metal box to make it more robust and to also reduce electrical interference.