Whoa! Who expected for some spare time around for this one! The story behind this begins with my Dad. He tends to fly a lot. And during these long flights he tends to binge the c***p out of movies, series and oldies albums. So one day he comes to me and complains (big surprise there) that he can’t hear anything during the flight, using his big bulky headphones. Don’t get me wrong! I’m a huge fan of big bulky headphones, right? Just recently, after two years of fantasizing about it, I bought myself a pair of Onkyo’s ES-FC300. However, these big headphones have another issue related.
First of all, these headphones have larger characteristic impedance. As any RF engineer knows, different impedance is simply an issue of matching the source to the load, to ensure maximum power transfer (or acceptance? however you like to look at it). But:
1. This is relevant only when you are dealing with PA topology. Many stereo systems use simple, high output impedance, OP-Amps. This is the case in this post.
2. This is only part of the issue. The amplifier has to be able to ouput enough power for the load (the headphone speaker) to be driven efficiently.
I’ll also put in a demand of another nature, here. As this is an in-flight system, it has to be battery powered and to sustain long-enough. Say, at least 3 hours per-charge? So I went around the internet, and found this chip by TI – LM4808. It seemed to suit the purpose, so I ordered some samples (they weren’t free, although I tried). So I quickly assembled the manufacturers suggested scheme, using some parts I had lying around the house and a pack of capacitors I ordered off EBay.

The result was actually better than expected. I connected this bunch of wires between my Sansa MP3 player and my Sony MDR-XB300 headphones and it nearly blew my ears off. The suggested circuit design was a bit too powerful on the trebles, but no meal is free.
Next post, we’ll discuss some circuit design considerations and my feeble attempts at PCB layout. This project turned out a bit more time consuming than I expected, but it sure taught me some things (such as, I’m never going to work full-time as a board designer).
Cheers!