May 2011

Headroom Balanced UltraReviewers' ChoiceFor the serious headphone listener, a dedicated amplifier is a must. The headphone jacks built into most audio components, when there’s one built in at all, are usually afterthoughts. They may be connected to a multipurpose operational amplifier (op-amp) with inadequate resolution and able to drive only the easiest headphone loads. Or they may use the power-amp section of an integrated amplifier, which can provide adequate drive but tends to be noisy. Twenty years ago, dedicated headphone amplifiers were do-it-yourself projects or a garage industry. Then along came companies like HeadRoom, who decided to make it a core business, and have continually pushed the level of performance available from headphone amps. It was almost a decade ago that HeadRoom released the first balanced headphone amplifier, and balanced drive quickly became the rage amongst headphone aficionados. 

Balanced circuit topology is nothing new -- many high-end audio companies have long promoted it. While there is some debate as to whether it sounds better than single-ended, there’s no doubt that, properly implemented, it reduces some types of distortion. Balanced drive also doubles the slew rate and voltage swing available from a circuit. In the context of headphones, it also breaks the ground connection between the left and right channels, eliminating crosstalk. For more information about balanced headphone drive, see the articles in the Learning Center on HeadRoom’s website

The typical 1/4” TRS connector at the end of a headphone cable is incapable of carrying the four conductors required by a balanced signal, so headphones must be reterminated, if not completely recabled, to allow for balanced operation. For some designs, that only means buying an aftermarket cable -- something most high-end audio enthusiasts always seem itching to do anyway. For other headphones, it requires disassembly of the unit and soldering in a new pair of wires. HeadRoom offers these upgrades for a limited number of high-quality headphones, or you can buy the headphones directly from them with the upgrades already done. One of the headphones available in a balanced configuration is the top of the Sennheiser line, the HD 800s, which is what HeadRoom sent me for this review. They also sent along recabled AKG K701s, so that I could make some more generalized conclusions about balanced drive. 

The power behind the system was provided by HeadRoom’s Balanced Ultra Desktop amplifier ($1599), built into the same 6” x 6” x 5.5” all-aluminum chassis as the regular Ultra Desktop amplifier and affectionately known as the BUDA. It consists of four separate amplification modules, one each for the positive and negative sides of the left and right channels. There are two inputs on the back: one balanced pair of XLRs and one pair of single-ended RCAs, which are followed by a phase splitter. The rear panel also sports the power switch, a pair of preamp outputs on RCA jacks, and the inlet for the included switching power supply (a bigger, beefier supply is available for an extra charge). 

On the front are two XLR-and-1/4” combo jacks for connecting either one balanced headphone or two single-ended headphones. The silky volume knob is connected to a four-channel potentiometer, in keeping with the fully balanced design. Also on the front are switches for source selection, crossfeed, rear output on/off, and a three-position switch for gain selection. Reviewers are paid to gripe, but I found only two things to complain about. I wish the BUDA’s rear output switch disabled the headphones when the rear output is on, though that would require a more complicated relay configuration and would probably annoy as many users as it would benefit. It would also be nice to be able to use the BUDA as a fully balanced preamplifier -- all the necessary circuitry is there. I admit, though, that having single-ended outputs makes more sense for the vast majority of users. I’m sure that HeadRoom, a small company that actually makes its own products, could accommodate you if you want a balanced output option. 

I had no operational problems with the BUDA. There was a slight thump when I turned it on or off; that’s not at all uncommon, but you’ll want to keep it in mind if you’re using the BUDA as a preamp, lest you get some annoying sounds through your speakers (I doubt they’d be damaging). My review sample had previously done duty at the Rocky Mountain Audio Fest and so needed no burn-in. As with most high-quality electronics, particularly those biased into class-A, I found that leaving the BUDA on for a couple of hours before listening allowed it to give its best. 

Sennheiser HD 800The headphone part of the system was Sennheiser’s HD 800 ($1499.95 when purchased from HeadRoom), recabled by HeadRoom for fully balanced operation. While you can buy an aftermarket cable to upgrade your existing HD 800, you get a substantial price break by buying them fully balanced from the outset ($1649.95). The HD 800s aren’t merely an evolution of the HD 600s/650s, but a complete redesign. Their 56mm drivers -- the largest in any headphone -- is set significantly forward in the earcup and angled back toward the ear. This is said to produce a more natural listening environment from headphones, and it does. One of the reasons I switched to Ultrasone headphones was the greater sense of space created by decentralized drivers, so it’s nice to see companies like Sennheiser and Beyerdynamic now including this feature in their top models. The HD 800s are hand-assembled at Sennheiser’s headquarters in Wedemark, Germany. The level of workmanship is beyond reproach, as is the quality of materials, but they don’t have quite the luxury look and feel of the Ultrasone Edition 8, which commands a similarly lofty price. Although the HD 800s are a little heavier than average, at 330gm without cord, the very large, wonderfully padded earcups and headband make them the most comfortable headphones I’ve ever worn. 

There seems to be some confusion about whether high-impedance headphones are easier or harder to drive than low-impedance designs. For a given voltage, high-impedance ’phones require less current, and so are easier to drive. The trouble is that not all headphones produce the same output for a given voltage input. The claimed sensitivity of the HD 800s is 102dB for a 1V input, which would make them seem very easy to drive. In my own testing, I found them to be about 6dB lower in level at a given volume setting than the Ultrasone Pro 2900, which has a claimed sensitivity of 96dB -- although Ultrasone doesn’t state whether that’s at 1V or 1mW. The bottom line is that the HD 800s are neither particularly easy nor particularly difficult to drive, especially when you consider that any model of this quality will be connected to a headphone amp, not an iPod. What is of greater concern about the HD 800s’ impedance is that it varies widely, from 300 to 625 ohms. That could greatly affect the frequency response, depending on the output impedance of the amplifier with which it’s used. 


The first thing I do with any amplifier or headphone amplifier is to assess the noise it produces with no signal present. I’ve encountered a lot of headphone amplifiers, even some very expensive ones, that make a significant amount of hiss or hum. At high levels, that can be annoying between tracks or in quiet passages, but it obscures the signal even at relatively low levels. 

The HeadRoom BUDA was silent as the proverbial grave. It didn’t matter what gain setting I used or how high I turned the volume knob -- the only indication that the amplifier was even on was the LED. That’s a tremendous accomplishment, and foreshadowed what I was about to hear in my listening sessions. With any of the headphones I used, I never needed more than the Low gain setting; I’m confident that the BUDA could easily drive any pair of headphones to levels that would cause permanent eardrum damage. 

Driven by the BUDA, the HD 800s were effortlessly dynamic. The nonexistent noise floor let me hear every little detail in even the quietest passages, and when louder passages came along, the sense of scale was incredible. Such macrodynamic ease is not merely a result of having power to spare -- though the BUDA certainly had that -- but also control. Using the BUDA to drive the HD 800s was like hooking up a pair of recalcitrant speakers to a multi-hundred-watt solid-state amplifier. It could not only handle wide voltage swings, it could start and stop on a dime. Just as the wide dynamic range greatly benefited large-scale orchestral recordings, that level of precision was a boon to highly rhythmic rock recordings. 

Enhancing the sense of scale were the huge images thrown up by the HD 800s. Some of this sense of space came from the Sennheisers’ large earcups and angled drivers, but another part was what sounded like a slight dip in the frequency response in the presence region (3-6kHz). Through loudspeakers, such a dip can make everything sound as if it’s coming from far behind the plane of the speakers. Through headphones, it seems to create a much greater sense of space between you and the sound source. It’s a shame that only the most dedicated headphone listeners would consider purchasing the HD 800s -- they’re the most like listening to speakers of any headphones I’ve heard. With recordings that don’t have much ambience, the HD 800s brought the performers out a few feet away from my head. With recordings that do have a good sense of hall ambience, the sense of space could be vast. You may have heard something similar through some type of electronic trickery, but that’s wholly different -- this was space without the frequency and phase compromises of digital signal processing (DSP). To believe it, you must hear it firsthand. 

I experimented with the HD 800s through other headphone amplifiers, and found them a little lacking in the bass -- though not as much so as the HD 600s -- and that the bass lacked articulation. I had no such qualms with the BUDA. While the HD 800s still couldn’t reach into the bottom octave with any authority, they did seem to go deeper than I heard with the other amps. The bass was also significantly tighter and more agile. Moving up the frequency scale, the midbass and upper midbass seemed ever so slightly emphasized, giving the overall sound an amount of warmth that was pleasing but not overdone. The midrange was smooth, even, and uncolored. It served male and female voices equally well, which sounds easy to do but isn’t, in my experience. 

It was in the upper midrange and treble that the HD 800s deviated from neutrality. The dip in the presence region, combined with what I believe was a slight reticence in the top octave, made for a sound that was somewhat on the warm and forgiving side. That’s not entirely a bad thing; it renders overly bright recordings or components very listenable. In fact, in my time with the HD 800s and BUDA I never had an unpleasant moment, no matter what I played. The downside was that it stripped instruments of some of their harmonic texture. Brass instruments were less brassy, reed instruments less reedy, stringed instruments less stringy (i.e., there was more of the wooden body and less of the bite of the strings). I believe this is the price you pay for the smooth sound and sense of space that the HD 800s offer. 

A question of balance 

All of what I’ve said so far about the Sennheiser HD 800s applies almost equally to their performance through the BUDA’s balanced and single-ended outputs. To draw some more general conclusions about the contributions of balanced drive, I used, in addition to the Sennheiser and AKG headphones HeadRoom had sent, some balanced Ultrasone Pro 2900s -- the successor to their Pro 2500s, my reference headphones for the past five years. The differences I heard between single-ended and balanced drive were consistent across all three designs. 

The most obvious change wrought by the balanced connection was an improvement in channel separation. Sounds in the left channel were definitively there, and did not cross over at all into the right channel; and vice versa. The result with the Sennheiser and Ultrasone ’phones, each of which can produce a credible soundstage, was a much broader spread from left to right, but it pulled the image back from being in front of my head to being on either side of it, and less far out in front. Whether I thought that was a good or a bad thing varied with the recording. Of course, with the BUDA you can always switch back and forth, depending on which presentation sounds better to you at a particular time. 

Other characteristics of balanced drive were more subtle. With all of the headphones, balanced operation brought greater definition to the bass frequencies. The difference was most apparent with the HD 800s, as their drivers seemed to need that vise-like control to wring the best performance from them. The AKG K701s benefited a little less, as their bass response is quite a bit weaker. The Ultrasone Pro 2900s are already well damped and so benefited least, but the difference was still quite audible. With all of the headphones, I noticed an increase in clarity when using the balanced connection. That’s not something I was left wanting when I used the BUDA single-ended, but when you’re trying to wring the last possible bit of performance from your headphone system, it’s worthwhile. 

Many paths to enlightenment 

The Ultrasone Pro 2900s ($599 single-ended, $799 balanced) is a natural competitor to the Sennheiser HD 800s. Both are open-back designs made in Germany to exacting specifications by companies that know a lot about engineering headphones. Unlike most other headphones, both designs are capable of reproducing a credible soundstage that seems to exist outside the head. The images produced by the HD 800s were somewhat larger and farther away than those produced by the Pro 2900s. The Pro 2900s’ images, on the other hand, are more precise, and demonstrate greater differences from recording to recording. Studio recordings, which shouldn’t have much of a sense of space, don’t through the Pro 2900s; but they did through the HD 800s. 

These headphones also differ significantly in their frequency-response curves. The HD 800s are balanced to sound like speakers in the far field, while the Pro 2900s sound much more like listening to monitors in the near field. Instruments and voices are much more present with the Pro 2900s than they are with the HD 800s, which emphasizes their individual characters or timbres. The Pro 2900s extend more linearly and with more authority into the bass and further into the treble. Recordings that sound great through other high-fidelity systems sound fabulous through the Pro 2900s, but any defect in recording or equipment is made obvious. The Pro 2900s’ brilliant highs show off the full advantage of high-resolution recordings, but draw attention to the curtailed highs of CDs, and make albums that are somewhat hot almost unlistenable. Truth and beauty aren’t necessarily the same thing -- in quantum physics, they’re opposites. For audio engineers, who need to know exactly what is on a recording, the Pro 2900s are the clear winners. For many audiophiles and music lovers, sacrificing some truth for consistent beauty across the widest variety of recordings and equipment will seem a worthwhile compromise; they may prefer the HD 800s.

As for electronics, the Grace Design m902 ($1695) is a natural competitor to the BUDA. The m902 also includes a quite respectable DAC section, but at this level of headphone performance the benefits of even better DACs are readily apparent. In my own system, I use the m902 purely as a headphone amplifier, with either my Ayre Acoustics C-5xeMP universal disc player or my analog rig as the source. The m902 is a little more open than the BUDA and seems to convey a little greater sense of space. Those are both good things with the Ultrasone Pro 2900s, but with the Sennheiser HD 800s the sense of space was overdone. I kept finding myself leaning forward in my listening chair in order to better hear what was going on deep in the soundstage -- and with headphones, that does no good. It’s not that the m902 is less detailed than the BUDA -- all the details are there -- it’s just a question of presentation, which doesn’t work very well with the already set-back soundstage of the HD 800. 

The Grace m902 reaches deep into the bass, but not with the BUDA’s authority. I suspect that if I measured the response of any given pair of headphones with either amp -- which I don’t have the setup to do properly -- I would get very similar results, but the quality of the bass from the BUDA just made it seem as if there were also more quantity, and that it went deeper. This difference in bass was apparent with either headphone, but manifested itself in different ways. The Pro 2900s extend evenly down to below 30Hz and are satisfying in the low end with the m902, but became much more physical -- as much as headphones can be -- with the BUDA. Already-articulate bass response grew even tighter, though walking bass lines were a little drier and less bouncy. With the HD 800s, the difference was astonishing. What was somewhat light and flabby bass through the m902 took on more depth and body, and significantly better control, with the BUDA, especially through its balanced outputs. In short, with the HD 800s, the BUDA was the better choice. 

The more present sound of the Pro 2900s and K701s vs. the HD 800s highlighted another difference between the amplifiers. The Grace m902 conveyed more realistic instrumental timbres than did the BUDA, and the highest frequencies were a touch more delicately rendered. With these headphones, the choice is less clear. As is so often the case when comparing two very high-performance audio products, one is not better than the other -- they just sound different. Most listeners will be able to appreciate both amplifiers, but will prefer one to the other. 


Choosing a pair of headphones is even more personal a matter than choosing within a different category of audio component, because each of us has a unique head-related transfer function and will experience -- hear -- each design differently. Combine that with different tastes in music and sound, and it’s a good thing that there are so many choices of excellent headphones on the market. The Sennheiser HD 800s are clearly some of the best headphones out there. They’re dynamic, detailed, sound exceedingly smooth, and create an out-of-the-head listening experience that headphone lovers and headphone haters alike need to try. Driving the HD 800s with the HeadRoom Balanced Ultra Desktop Amplifier propels them to still greater heights of resolution and depths of bass power and articulation. If you’re in the market for an absolutely top-quality headphone listening experience, you need to audition this system. Maybe as the HeadRoom folks say, “It will leave you grinning from ear to ear.” 

. . . S. Andrea Sundaram 

Associated Equipment 

  • Headphone amplifiers -- Grace Design m902, Furutech GT40, HeadRoom Total BitHead
  • Headphones -- Ultrasone Pro 2500 and Edition 8, Grado SR60, AKG K701
  • Digital source -- Ayre Acoustics C-5xeMP universal stereo disc player, Apple iPod (fifth generation)
  • Computer -- Laptop running Windows Vista and Realtek HD audio ALC 272 with coaxial digital output running foobar2000
  • Analog source -- Michell Tecnodec turntable, HR power supply, modified Rega RB-300 tonearm, Shure V15-X MM cartridge into Trigon Audio Vanguard II with Volcano power supply
  • Interconnects -- DH Labs Revelation, JPS Superconductor, QED Silver Spiral
  • Power conditioning -- Equi=Tech Son of Q

HeadRoom Balanced Ultra Desktop Amplifier
Price: $1599 USD.
Warranty: Two years parts and labor. 

Sennheiser HD 800 Headphone
Price: $1499.95 USD single ended, $1649.95 USD balanced.
Warranty: Two years parts and labor.

HeadRoom Corporation
2020 Gilkerson Drive
Bozeman, MT 59715
Phone: (800) 828-8184


Sennheiser Electronic Corporation
1 Enterprise Drive
Old Lyme, CT 06371
Phone: (860) 434-9190
Fax: (860) 434-1759