There is no hard and fast rule about whether lo-Z or hi-Z sounds better. (Z is the electronic symbol for impedance.) All it tells you is how much current the headphone wants. Impedance isn't the only thing you need to consider: you also have to talk about sensitivity. This tells you how many milliwatts you will need to achieve a given volume level. It's basically a measure of how efficient the headphone is at turning power into sound.
Lo-Z high-efficiency headphones -- e.g. Grados -- do need a fair amount of instantaneous current, but you end up keeping the volume control down because they get loud with relatively little voltage. If you don't have enough current capability, they'll sound thin.
Hi-Z low-efficiency headphones -- e.g. high-end Sennheisers, AKGs -- don't need much current but the volume control has to get turned up to give enough volume. If the amplifier in your portable isn't designed well, it can run out of voltage swing before the volume knob stops turning. If that happens, the amp will start clipping, which sounds horrible. Most portables are designed so that when the volume knob stops turning, you're at the limit of the amp design so you don't actually get clipping. You just get very little volume with the volume control turned all the way up.
In both of these cases, the solution is to add an amplifier. In the first case it's to add current drive to round out the sound, and in the second case it's to add voltage boosting to get the volume level up.
Lo-Z low-efficiency will be some kind of speaker rather than a headphone, or headphones designed like speakers. These need lots of voltage _and_ current. Which is to say, they need a power amp.
I can't think of an example of high-efficiency hi-Z phones. It would imply lots of volume for very little current or voltage, which means very little power. If it's possible at all, it's very rare, and it probably doesn't sound good -- it would imply materials tradeoffs and such that aren't compatible with Hi-Fi. But time marches on -- who knows what the materials scientists will come up with next?
|the PCDP manufacturers...have limited the maximum current -delivery capability of their op-amps. As a result, using 16-ohm headphones with such PCDPs may produce clipping distortion at just above half-volume
Current starvation won't produce clipping, it'll just give thin sound, robbed of bass. If you're getting clipping, you have also hit the amplifier's voltage limit. The surest way to do that is to plug some unpowered PC speakers into a headphone amp. What Eagle Driver describes will happen -- it'll start clipping horribly well below full volume level, and you'll never get much volume out of the speakers. The solution is more current and
voltage. You need a power amp.