HeadRoom Supreme headphone amplifier Page 2

The DC supply voltage is smoothed with a 100µF electrolytic, then fed to an encapsulated module that converts it to the ±5V-and-ground supply required by the active circuitry. (This 100mA-capable DC/DC converter is said to be the most expensive single part used in the amplifier.) The front or base of the "U" carries the ¼" headphone jack, the front-panel switches, and the volume control, the last a conductive-plastic component from Clarostat. The other leg of the "U" carries a handful of resistors, 100µF supply capacitors, and the heart of the amplifier: a largish, 24-pin module that contains a smaller epoxy-encapsulated board carrying all surface-mount components (fig.2). (This HeadRoom module is available separately at relatively modest cost.)

The amplifier is available in three versions: Standard, Premium, and Supreme, costing $199, $299, and $399, respectively (footnote 4). Apart from the Supreme's front-panel Filter switch, all three look identical, the differences being internal. The Standard's circuit module uses surface-mount LM833 op-amps, whereas those in the Premium and Supreme use the Burr-Brown 2604 op-amp—"a truly killer audiophile quality device," according to Hertsens. The more expensive Supreme also uses Caddock 132 metal-film resistors on the main board, while the Standard uses ordinary 1% 0.25W metal-film resistors. The Supreme's filter applies a moderate amount of high-frequency boost to compensate for recordings that sound too dull with the crossfeed processing active.

Up a little, left a little...
I started this review last Spring with one HeadRoom Basic amplifier sample. Before I even began my listening, Tyll Hertsens sent me an upgraded unit, followed by what was the first of six different Supreme samples. What was happening was that, in addition to Tyll's discovering better-sounding parts, his experience of different headphone models was being broadened. It had become apparent to him that his "one size fits all" philosophy regarding the amplifier's Process and Filter parameters was not going to work. The Sennheiser HD580s, with their weighty low frequencies and extended highs, required different equalization from the very smooth but rather dark-sounding Grado HP-1s and '2s. And the totally in-the-ear Etymotics had different requirements again. The design-optimization process is best described in the words of the indefatigable Tyll:

"This revision activity was catalyzed by the Sennheiser HD580 headphones...Our first reaction was to significantly change the filter. We basically brought the whole high end of the curve up by about 5dB. This brought the highs up close to where they should be, but slightly overemphasized the information just before the notch. The box imaged [well], but the tonality was still hurt by the notch. If we brightened up the box enough in the notch region, the direct-channel component (which is tonally unaffected by the processor) was too bright, and made the HeadRoom sound hard. If we eased off, we started to get the old problem back. What to do?

"We decided to turn down the crossfeed channel a little. The danger is [that, according to theory], the crossfeed level isn't much lower in amplitude than the direct channel. But...we knew that time differences are significantly more important than amplitude differences in correct localization. Therefore, cutting the crossfeed level in half should only slightly affect imaging. But—insert trumpet fanfare here, 'Ta-da'—cutting down the crossfeed level will have a dramatic reducing effect on the depth of the comb filter notch...It definitely sounds better tonally."

I congratulate Tyll for daring to depart from theory when his ears tell him the theory is not quite right. The amount of bass-shelf boost applied to the sum information and bass-shelf cut information will still depend on the particular headphones used, however. What Hertsens now does, therefore, is to offer a limited range of customization options. When the customer phones HeadRoom, he or she is either supplied an amplifier whose EQ is optimized for the customer's existing 'phones, or is sold a matched amplifier/headphones package.

Powering up
The HeadRoom's case gets hot after a couple of hours, implying that its circuit has a thirst for juice. I found alkaline bunny batteries worked best in the HeadRoom, giving about two and a half CDs' worth of listening pleasure, after which the sound degenerated into a clipped, crackling roar. NiCads were a pain, four AA cells not even lasting the 73 minutes of Beethoven's Symphony 9—for long journeys, a little more juice is required. Enter HeadRoom's optional external battery holder, which holds four D cells and plugs into the power jack. With alkalines, this gives a supply good for up to 20 hours. To my surprise, rechargeable Radio Shack NiCad D cells didn't last any longer than their AA cells. (I've since been told that these are a scam, merely mounting an AA NiCad cell inside a D cell's case.)

I used the various HeadRoom amplifiers Tyll Hertsens sent me with a variety of headphones: Sennheiser HD560 Ovations and the new $349 Sennheiser HD580 Precisions; Etymotic ER4S Ear Canal Phones; Sony MDR-484 ear buds; and Beyerdynamic DT901s and '911s. In the comfort of my listening room, I used a Mark Levinson No.31 and Panasonic 3700 DAT recorder (footnote 5) driving either a Krell Reference 64 or a Counterpoint DA-10 (UltraAnalog DAC version) as source. For music on the move, I used a Sony WD-D3 Walkman Pro cassette recorder, an Aiwa HD-S1 portable DAT recorder, or my own Panasonic SL-NP1A portable CD player—followed by a Denon DCP-150 portable CD player on loan from HeadRoom. (CDs were carried in the excellent Laserline case, which holds 12 discs by the center holes. Unlike the cases which hold the discs in clear plastic wallets, the Laserline keeps both top and bottom disc surfaces away from anything that might scratch them.)

I mainly used the Straight Wire LSI Encore minijack-RCA cable provided by HeadRoom to link source components to the HeadRoom amp. This was sufficiently oversized that it wouldn't plug all the way into the Denon, however, so for that player I used a premium OFC cable from Sony.

I first compared the amplifier with the headphone output of the ancient Advent 300 receiver I use in my office system.

Footnote 4: According to a cryptic note Tyll Hertsens posted last October on Compuserve's Consumer Electronics bulletin board, there will also be a Limited Edition HeadRoom Home Amplifier, packaged in an Anvil Case with a Micromega Microdrive CD transport and a Theta Cobalt D/A processor, to give what Hertsens modestly refers to as "the world's best transportable stereo."

Footnote 6: Readers should be warned that the enhanced loudness capability of the HeadRoom makes it even more important to take care about how loudly they listen to headphones. I'm concerned that an entire generation is reaching adulthood with crippled ears, due to their habitual playing of portable tape and CD players at hearing-damage levels throughout their teen years.

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