(Reprinted from issue 58 of UHF Magazine. To purchase the issue, click here. Or click here to subscribe to UHF)

Four Headphone Amplifiers

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Headphones are a commodity now, sold in convenience stores. At the same time, headphone jacks have been disappearing from good quality home audio equipment.
     But there's no paradox. Those ubiquitous cheap headphones are destined for portables, and it's hard to find better phones. Once you have them, what do you plug them into?
     There's a reason the headphone jack is vanishing. Most of them were fed from dreadful little single-chip amplifiers. If they were fed from the main amplifier, there had to be some system of switching that would knock the system performance down to portable quality anyhow.
     Hence the popularity of the standalone headphone amplifier. You can feed it from your amp or preamp's record-out jacks, meant for a tape deck, that other vanishing piece of equipment. And you can get astonishing quality, as we shall see.
     For this test, we used a pair of Koss professional headphones, the PRO4AAA. We selected two recordings we consider especially revealing. One is There is a Wideness in God's Mercy from Now the Green Blade Riseth (Proprius PRCD9093). The chorale in that piece is especially well recorded, but the voice and instrumental textures can turn to mush or worse under poor conditions. The other is Limehouse Blues from the HDCD version of Jazz at the Pawnshop (FIM CD014-15). Not only does it have fast transients and a good variety of instruments, but its capture of space and depth are considered a model.
     We selected four headphone amplifiers, which -- in this case -- we fed directly from our reference CD player.
     The full review can of course be found in the print issue of the magazine, but here's a preview of the four amplifiers tested.

Creek OBH-11
     This well-known British amplifier maker has a whole set of little boxes that look just like this one. They include phono preamps (UHF No. 56), passive preamplifiers and a couple of headphone amplifiers.
     Because the OBH-11 comes in such an anonymous accessory box, and because it is fed from an AC "brick" power supply (a better supply is available as an option), you might dismiss it as not worthy of your attention. That would be too bad.

Antique Sound Lab MG-Head
     When we were looking over this Chinese manufacturer's tube amplifiers (we chose the AQ-1003, reviewed elsewhere in this issue), we spotted this cute little headphone amp. A tube phone amp...for just $350 (Canadian, equivalent to about $240 US). We couldn't resist.
     And it's single-ended too, though we suspect that's more for reasons of economy than to keep up with fashion. A 12AU7 dual triode drives a pair of EL84 tubes, one per channel. The two transformers on the chassis are output transformer -- the power transformer is much smaller and tucked inside the chassis. The circuit is simple, and the parts used are inexpensive, but what more can you expect at this price?
     The amplifier comes with a perforated cage hiding the tubes, but of course it photographs better with the tubes showing. A front panel switch (the only control other than volume and power) lets you select low impedance headphones (usually 8 ohms, just like loudspeakers) or the more unusual high impedance phones, of 100 ohms or more.

The NVA AP10
     This British-made amplifier may ring a bell, because we actually reviewed it in UHF No. 56. It is the smallest of the company's inexpensive amplifiers, but it is not actually a headphone-only unit. Indeed, in our previous test we hooked it up to our reference speakers (it has binding posts at the rear). And it astonished us. For a mere $519 Canadian (or $399 for a kit that can be assembled in two or three hours) plus shipping, you get a lot of music.
     But the front panel has a headphone jack, and we were determined to try the AP10 out for the purpose for which it appears to have been designed.
     One reason not to use a regular amplifier for headphones is that the residual noise level in most amps is too high for headphones. The NVA was satisfyingly quiet, with a slight buzz appearing only when one's hand was close to the volume control.

Audio Valve RKV MkII
     This is a serious headphone amp, with a price tag of C$1395 (equivalent to $955 US). You can see its workings through its tempered glass top, and you may notice that, although it is a tube amp, it has no output transformers. That's right...this is an OTL tube amplifier, using four PCL 805 tubes in parallel push-pull. Its styling says Germany.
     There are two headphone jacks on the front panel...nice if a friend drops by. Don't leave your phones plugged in, though. We were disconcerted by the pop heard when we turned the amplifier on, perhaps strong enough to take out some headphones! Our sample also had a small buzz in the left channel. The other channel was as silent as you could want, and we presume that's normal.

PARTIAL TEXT: The Microgroove Laundry, Antique Sound Lab AQ1003, Passion I-11, Rogue 88, Jadis Orchestra Reference, Linar 250, Four Headphone Amps, Filters and Cables
FULL TEXT: Watching DVD on a Computer, Passion Kit I-10 Amplifier, State of the Art