So we went ahead and plugged an iPhone (admission: we also tried an iPad, via a 30-pin extension cable, and that worked just fine too — it even charged our tablet) and put the VAMP through its paces.
First things first: the VAMP sounds good. Quite good — with our primary pairs of test headphones (the relatively inefficient HiFiMan HE-500 and the high-impedance Sennheiser HD-650), it clearly beat both the iPhone and iPad on sound quality and volume. Neither headphone was in its comfort range with the onboard headphone amps of the iDevices; the VAMP did a fine job or remedying the problem. On the HE-500s, even loud mixes like the Talking Heads Remain in Light were dull and boring straight out of the iDevice; the VAMP gave the music useful life.
With a more efficient pair of headphones (we've been using V-Moda's own Crossfade M-80), the SQ differences were less apparent, though the VAMPs horsepower became more obvious — earsplitting levels were a lot easier to achieve given the VAMPs output power of 150 mW/channel into 32 Ohms (Apple doesn't cite specifics, but the stock iPhone 4 measures around 30 mW/channel into a similar load).
VQ was hit or miss for me. With the difficult-to-drive phones, the effect was not necessarily to my taste, but definitely pleasant, and I'd imagine it will be liked by many listeners. Engaging the mode produces a clear tilt toward the treble; engaging it created something akin to cranking up a brightness control. Mick Jagger's vocal and Keith Richards' rhythm guitar on the Stones "Midnight Rambler" became just more forward in the mix, the bass got a little more oomph and correspondingly less definition.
Sitting in a quiet environment, I preferred the flat setting, though when the office was noisier or I was out and about, the VQ EQ made for a nice presence boost that was more competitive with the sounds of the outside world, especially with open-back phones. It's a bit of a hyped sound, but paired with a reasonably neutral set of headphones the effect it's far more minimal an effect than you might expect.
Switching back to the flat setting you'll definitely feel at first that things are too dark, but given a few minutes of listening you'll adjust. Checking out "Jeep's Blues" from the venerable Jazz at the Pawnshop recording, I felt that the VQ setting gave the cymbals too much sizzle and the sax an ever-so-slight strident edge. Basically, I think that your average audiophile is going to much prefer the flatter setting — but it's definitley worth trying out the VQ mode, especially if your tastes run to vocal music, rock, or electronics. And if you're the kind of person who finds HD-650s a little dark, you might find the effect welcome across the board. It's there, and you can easily disable it, so it's certainly not a negative.
With more efficient phones like the M-80s, I felt that the low end became a little too cloudy and the highs too strident; that said, I'm not sure that an amp is even necesary with those headphones. The VAMP, to my mind, makes quite a nice addition for those who want to use high-end phones, but it's worth keeping in mind that the iPhone 4 is very well suited to driving portable headphones right out of the box.
We did some rough comparisons with other devices, of course. We had the ADL Stride and FiiO E17 portable headphone amplifiers on hand, and while these devices aren't direct competitors — neither provides 30-pin connector interface for its DAC, for one thing, and they're not chargers — they are typical of what folks are using to power high-end headphones on the go.
Via a line-out cable, I felt like both the ADL and FiiO devices were able to drive our test cans to somewhat louder levels than the VAMP, even when the latter was used at the higher gain setting. The VAMP's output was more than adequate to reach levels that were, admittedly, excessive, but with lower efficiency headphones you'll definitely find yourself in the upper portions of the analog volume knob's travel.
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