Ranting in the dark

Actually, this is my first finished project. It's been working for a long time, but since things like those are never really "completed", it has waited a while for its presentation. Here is my wine-box-gone-headphone-amplifier:



Why a headphone amplifier? Well there are many reasons - boredom, curiosity, the fact that a lot of the sound cards on the market just suck horribly bad. Not only did I have a problem finding one that doesn't dump a horrible amount of noise on top of my music but even when I did that, it was too weak to drive my average quality cans. When you want to listen to cruel drum&bass on proper headphones, you need a little more of a punch.

So I figured, it'll be small and cheap to take care of that little problem and on I went gathering parts for what I found to be the most interesting design around. It's all IC-based. Opamps drive high-speed, high-current (for this type of circuit anyway) buffers, which deliver the power into the headphones. The circuit features an actively driven ground "channel" - that is, the headphones aren't simply grounded at the power supply ground, but at the output of an identically designed channel. Since the ground is virtual, a simple rail splitter that divides into power supply rails in two, this is not only better for sound reproduction, but actually necessary. The circuit and board are designed by the awesomely helpful Tangent and can be found at his site here - http://tangentsoft.net/audio/pimeta/ . He actually has an upgraded version of the PIMETA amp, which I intend to try when I get bored again.

The power supply is the funniest part of this build. I happened to disassemble an old modem to salvage useful parts of the circuit. I literally cut out the unit out of the board, using a Dremel. It comes with power line filtering, fusing, nice laminated core transformer, the whole lot. This AC is then rectified, filtered and regulated by a miniature board I also got from Tangent, which provides an amazingly stable supply for its cost and complexity.

The rest of the parts I calculated and sourced myself, assembled the thing and oddly enough it played music from the very start. It was sitting in raw and naked form on my desk while I was working on it and it that form it played music for months. I finally decided that I should hide the dangerous part, the actual AC IEC connector, inside a box somewhere. That's when I found the use for a wine box I had in the office. I checked and it turned out I can fit the boards perfectly inside. That box actually has quite a history behind it, it's funny to think about when I got it and how it ended up spending time on the shelves in my office. Now it features as a headphone amplifier. I hope Mr. John Blandy doesn't mind the sudden product range expansion of his brand.

The casework is notoriously bad. At least when you know how I've done all the details. I had no tools at the time I did it, so half of it is improvised. The end result is surprisingly good, all things considered. It's shiny, it looks very DIY, but I didn't set out to win a design competition anyway.

For the spec curious - the transformer provides me with 18Vac, which I rectify and filter with 6800uF capacitance. This is tightly regulated by an adjustable regulator to provide 22Vdc which is fed to the board. A total of 4000uF of Elna capacitors provide the energy storage for the circuit. Burr-Brown BUF634 buffers can deliver up to 500mA of current per channel, which can basically deliver sound levels that go far beyond destroying your hearing once and forever. Those are operated by Burr-Brown OPA627 low-noise, high-precision operational amplifiers which - I'm being told - sound as fantastic as they come. The ground channel is actually still an OPA124, since I haven't had the chance to replace it yet. The benefits of doing that are very questionable though. And with the price tag of good opamps maybe I'll never do that.

There is one additional advantage hidden in the box, that justifies its constant use. There is an acoustic crossfeed filter at the input of the amplifier. It's role is is to eliminate the exaggerated stereo effect of listening to isolated stereo channels trough headphones. Stereo recordings are awesome and everything but they were made to be reproduced on speakers and your ears and brain have to mix them up together. With headphones the situation is a little different, since one ear gets only one version of the signal. This filter takes care to add the proper mixing and delays in left/right channels so your ears get what they would get in a normal open environment. I have to say - this changed the way I think about headphones forever. I can keep them on the entire day without feeling awkward now. The filter usually has two mixing levels and a bypass switch but I liked it enough to just keep it on all the time.

With my bassy, low-impedance Sennheiser HD212Pro headphones this baby sounds splendid. I can't get the sound card to do such a good job on the sound, period. Not only it delivers much better kick but now I can keep the sound card output in its optimal range, where it drives a high load, at low volume, so there is much less distortion to the signal. Some day I'll just wire this to an external DAC, but that's a story for another post