XinFeed passive crossfeed filter for headphones

Very nice and quite inexpensive

I've previously mentioned my AirHead headphone amplifier from HeadRoom. I like it very much and I use it routinely when I'm listening to something on my iBook at home. Ironically, at the moment I don't have any headphones that are particularly difficult to drive so I mostly use it for its crossfeed.


Yes: When you listen to stereo speakers, most of the right channel goes to your right ear, but some of it goes to your left ear, and vice-versa. Most music is recorded with the expectation that "air mixing" of that sort will happen when it's listened to. But there isn't normally any mixing when you're listening with headphones. The left channel is delivered only to your left ear and the right channel only to your right ear. A crossfeed circuit fixes that; it mixes some of the right channel into the left channel and vice-versa.

Actually, it can be a little more complicated than that. It seems that, on account of the shape of a person's head and ears, the crossfeed that a person perceives from air mixing varies some depending on a sound's frequency. It's straightforward enough to approximate that variation in a crossfeed circuit. Of course, as with many things in audio, it's possible to take matters to an extreme. (There's more at HeadRoom's admirably informative site.)

I'm no nutty audiophile, but after having listened to various things for some time with my AirHead's crossfeed circuit, I find it substantially more pleasant to listen to headphones with crossfeed than without. The sound seems more natural and it seems to be coming from in front of me rather than from beside me.

But what about portable music? It's perfectly possible to carry an AirHead around, and its four AAA batteries last an admirably long time. But it's about the size of a standard iPod, And so while you can carry it, I'm not likely to. And connecting one to an iPod Shuffle would be plain silly.

Happily, there's a solution to the portability problem in the form of XY Computing & Network's XinFeed (specifically, the low-impedance ampless version). The XinFeed is a crossfeed circuit built into a slightly lumpy and asymmetrical widget that's not a lot bigger than the 1/8" mini jack and plug that it has to have. Here's a photo of it plugged into my iPod Shuffle:

For those unfamiliar with the Shuffle's size, the gray control clicker is about 7/8" in diameter, just about the size of a US quarter.

How well does it work? Very nicely. I find that listening with it is much more pleasant than listening without it. It seems to have just about the same effect as my AirHead's crossfeed circuit. I may not be the most discerning listener in the world, and someone else may be able to find a significant difference. But even if someone can, it would be very hard to beat the thing's portability and, at US$30 delivered to me, its price.

Of course, one of the things that makes the XinFeed so portable is that it doesn't run on batteries (it's "passive" as they say). That means that it's going to consume some of the signal in order to do its job. It doesn't eat much, but I found that when I was comparing the sound with it and without it, I needed about two clicks more volume out of my Shuffle to have the sound seem the same after I plugged it in. That means that if your music source is only barely able to drive your headphones satisfactorily, the XinFeed may not be a useful solution for you. Happily, my iPod Shuffle has plenty of power to drive my Sennheiser PX-100s and my Etymotic ER-4Ps even with the XinFeed plugged in. But if I add the cable-thingy that Etymotic sells to make their ER-4Ps work like ER-4Ss, it sounds like the Shuffle is running out of oomph with the XinFeed.

You could probably build something similar to the XinFeed yourself if you know a little about electronics. Googling "passive crossfeed circuit" yields plenty of circuit diagrams and a few pages with a bit more help than that. I know one end of a soldering iron from the other, but I was perfectly happy to pay Mr Xin Feng (who seems to be more or less all of XY Computing & Network) to build it for me. The result is nicer and smaller than anything I'd be able to build and probably cheaper too. Certainly if I valued my time at anything. And, who knows, his circuit may be better than the ones published on the net.

Mr Feng's site is more interesting than elegant, but given a choice between interesting and elegant, I'll take interesting any day.

In all, I'm thoroughly delighted with my XinFeed. When I take my standard iPod on a plane trip or my Shuffle out for a stroll, I'll no longer be thinking that the sound could be a lot more pleasing.

Posted: Mon - March 21, 2005 at 07:09   Main   Category: