Rogers High Fidelity EHF-100 integrated amplifier

I can imagine the gaiety and mirth that filled the halls of the electronics industry in the 1950s, as engineer after bespectacled engineer realized that the transistor would soon consign to the outposts of oblivion those ancient technologies that had preceded it. Before long—surely no more than a decade—the hated vacuum tube would vanish from the Earth, along with the tube socket, the tube tester, the tag board, the high-voltage rail, and that lowest rascal of them all, the output transformer. What a jubilant time!

I can't imagine what went wrong.

Description
The EHF-100 integrated amplifier ($6350) is one of two products made by Rogers High Fidelity, a new company in rural Warwick, New York. (When I first saw a Rogers amp, at Stereo Exchange in New York City, I assumed the brand name had something to do with the late, lamented Rogers loudspeaker company of Great Britain, but that turned out not to be the case.) Roger Gibboni, who studied engineering at Drexel University and who has over 20 years' experience designing RF circuitry for the military and NASA, founded Rogers High Fidelity in 2009 with an eye to offering very high-quality, US-made tube amplifiers at less-than-extortionate prices.

In contrast with some recent integrated amplifiers, in which power amplifiers are paired with passive front ends, the Rogers EHF-100 has sufficiently high gain to be considered an integrated of the more traditional sort. Here, the majority of voltage gain is provided in the first stage: an EF86 miniature pentode tube. From there, the signal goes to a 12AX7 dual-triode tube, which is used in common-cathode mode to split signal phase for the push-pull output section that follows.

Power tubes in the Rogers amp are KT88s, operated in auto-bias mode: The cathode of each is held above ground by a chunky Dale resistor, ensuring that the KT88's signal grid maintains a steady negative charge relative to the cathode. (I measured approximately 44V across each cathode resistor; consequently, although there was 514V between plate and ground, each KT88 sees a total B+ of only 470V.) The output section of the EHF-100 runs in Ultralinear mode, with plates and screen grids connected to the tapped primaries of very large Hammond output transformers. (I don't know what percentage of primary windings are given over to the screen grid, but other Hammond Ultralinear transformers of my experience are set for 40%.) In addition to its Ultralinear operation, the Rogers EHF-100 also uses a small amount of global feedback. Gibboni says that the amp is fully class-A.

The EHF-100's build quality is excellent—impressively so, given its relative affordability in the context of other US-made tube amplifiers. Every component is hand-wired, most by means of silver-plated hookup wire and thick, clean tag boards. Threaded parts are held in place with Loctite thread sealant and Glyptal red enamel; long runs of cable are tied in place with something very like dental floss (the knots in which also get the Glyptal treatment); and large capacitors are incapacitated with generous dabs of silicone sealant. Solder joins are as close to perfect as I've seen. Parts choices include the aforementioned Hammond transformers (and power-supply choke), plus metal-film signal resistors, polypropylene and silver-mica signal capacitors, Micalex tube sockets, and rhodium-plated copper binding posts from Furutech.

The Rogers EHF-100 offers four pairs of line-level inputs, with one pair of jacks on the front panel—just to the right of the four-position input-selector switch—and the other three on the back panel. A phono section is not provided, although Gibboni says he's considering making one as a standalone component. As with other notable amplifiers, powering up the EHF-100 is done by flipping, in order, two switches: one, Power, to send the requisite voltage to all of the tube filaments; and the other, Operate, to apply the rail voltages. Apart from those and the selector switch, the only other control is a stereo volume pot. (A balance control is not provided.) At the center of the front panel is a power-output meter, calibrated in peak watts from 0 to 100.

The very substantial aluminum-alloy case is finished in what Rogers refers to as an automotive-grade finish, in glossy black (other colors can be had for an additional charge). I love the EHF-100's simple, utilitarian shape; it calls to mind such disparate products as the beautiful Marantz 8B and Shindo Montrachet amplifiers. My only complaints have to do with what seems the amp's excessive size (one thinks of Mary Wollstonecraft Shelley's line "As the minuteness of the parts formed a great hindrance . . . I resolved, contrary to my first intention, to make the being of a gigantic stature") and its disproportionately large, splashy logo and other graphic touches. But that's just me.

Installation and setup
Because my house is only three hours away from the Rogers High Fidelity factory, Roger Gibboni delivered my review sample by automobile, rather than shipping it via brown truck. That said, there was nothing extraordinary about unpacking, installing, or operating the EHF-100—although I was impressed by the exceptional quality of its carton and packing materials. The operating manual was also quite good, and my review sample came packed with a spiral-bound booklet titled "Amplifier Profile": an illustrated collection of reports on tests to which this particular amp had been subjected, including THD vs level, group delay vs frequency, squarewave performance at 1 and 10kHz, and others. I'm told that every Rogers amplifier sold includes such a report.

Source : stereophile[dot]com