I've been meaning to find an answer to this question for a while, and I figure here's as good a place as any to ask it.
Suppose you have a surround-capable audio card whose surround minijack holes are plugged with the corresponding leads from a 5.1 PC speaker system. Further suppose that said speaker system also provides the Convenience Jack, a headphone connection interface mounted on some appendage of the speaker system.
What sense does this make? Why do they provide this, and how could it possibly work?
In my desktop setup I'm running a SB X-Fi Extreme Music through a Logitech X-530 setup. The audio card consists of a single PCI card providing speaker connectivity via three 3.5mm jacks, assigned in this case to provide Front\Rear\FrontCenter/Sub channel routing, leaving a free three-in-one jack open for headphone connectivity. The speakers provide their Convenience Jack through the front right speaker. Assuming I tell the audio card via its software that I'm using a headphone setup, it's going to want to provide that initially through the unused FlexiJack. Should I be using a mic, that would be in the FlexiJack, so I'd tell the software to route the headphone input through the Front Left/Right hole, which I'd unplug from the speakers and jack in its place. Makes sense so far that if I were to plug my headphones in through the front right speaker hole everything would still work fine.
The problem lies in how these particular speakers provide unified sound; they all wire in through the sub, which conveniently houses the amplifier and power supply. The amplifier and adjacent little switching station for the speaker signals has no way of telling whether or not there's an extra connected device in the Convenience Jack, and so refuse to provide passthrough support for it. My evidence for this, perhaps ill-conceived, is that with the speakers powered off the headphones don't work. My reasoning is that if the Convenience Jack were worth a ***, it wouldn't require the rest of the system to be powered in in order to work as essentially an extension of the Front Channel speaker jack. The more telling evidence is in the nature of the sound itself; there's a marked reduction in throughput and quality when connected through the Convenience Jack compared to the headphones just jacking into the card itself. I know there's a lot of wiring and impedance and whatnot going on, but the difference is beyond what I'd reasonably infer such routing would do.
In any case, I dislike the Convenience Jack. It's a letdown. And when dealing with audio devices, isn't a system without such wasteful excess as the wiring for that jack a better idea? I know these are only eighty dollar speakers, but otherwise these little buggers provide impressive performance for their size and the build quality you'd expect for such a paltry asking price. It seems so wasteful to impose this feature on them that I figured I should gripe about it on the internet, like everyone else with a niggling beef and a weblog that pretends it cares.