June 2008

HeadRoom Ultra Micro Headphone Amplifier

by Vade Forrester

With all the great speakers available today, why would anyone want to use headphones? Besides tethering you to a headphone amplifier or digital player, they make your ears sweat!

Well, there's portability. It’s a bit of a pain to carry around speakers and an amplifier to use with your iPod. There are also other people to consider. Maybe you want to listen to music without disturbing your neighbors or family. Think thin apartment walls or late-night listening sessions. Headphones let you rock out without disturbing the people around you. Finally, there is sound quality. Some will disagree, but I believe headphones make it possible to achieve really good reproduction without spending huge sums of money. Taking room acoustics out of the equation makes for very direct sound. Even extremely expensive headphones and headphone amps are pretty reasonable compared to high-end speakers and power amps. Possibly the most accurate sound I’ve ever heard was from some Stax Omega electrostatic headphones driven by a custom SinglePower Audio amplifier.

Although most headphones are quite sensitive, serious headphone listeners generally agree that you need a dedicated headphone amplifier to get the best sound. In other words, don’t plan on plugging most headphones into your iPod and getting the most from them. Price-wise, headphone amps range from $100 portables that fit in a shirt pocket to huge $15,000 desktop models. When choosing the ideal headphone amp, one consideration is how big it is. In a portable system, the need for a small amp is obvious. But even in a desktop system, size matters. My audio rack, for example, is always overflowing with various audio goodies, and it’s a bit challenging to find a place for my headphone amp. For this reason, I appreciate small headphone amplifiers.

Honey, I shrunk the headphone amp

HeadRoom’s Ultra Micro headphone amp certainly qualifies as small. Modestly described as "Possibly the highest quality audio per cubic inch known to humankind" on HeadRoom’s website, the $699 USD Ultra Micro measures only 4 1/2"W x 1 1/2"H x 3 1/2"D, plus the size of its external power supply -- 4 1/2"W x 2 1/2"H x 2"D. That’s comparable in size to many portable headphone amps, but the Ultra Micro is definitely not a portable. You must use a power supply that plugs into the wall; there are no batteries to provide power while you’re on the go.

The Ultra Micro does indeed look like a shrunken version of HeadRoom’s Desktop amps; the case is the same shape (slightly concave), the rubber gaskets around the front and rear panels look the same, and the controls are largely the same. But there are some important differences. The smaller case doesn’t provide space for RCA jacks on the rear and a 1/4" headphone jack up front, so all connections are via 1/8" stereo miniplugs -- two sets of input jacks and an output jack.. If you’re using the Ultra Micro with an iPod, that’s just fine; but if you want to connect it to a standard CD player, you’ll need an adapter, hopefully not one from Radio Shack. For some headphones, that will be just fine, but if your 'phones have a 1/4" plug, another adapter will be needed. Fortunately, AKG included such an adapter with my K701 headphones.

Another casualty of the shrinkage is the print on the front panel. I couldn’t read it without a magnifying glass. For the record, from left to right, the controls are: input select, crossfeed, gain (high, medium, or low), the volume control, and the on/off switch.

There are two versions of the Micro Amp: the standard version and the upscale Ultra Micro reviewed here. From HeadRoom's website: "The electronics of the Micro Amp is a Class-A biased, emitter follower design based on the famous Walt Jung Diamond Buffer. It’s a DC-coupled amp (no caps in the signal path), and the left & right channels are completely separate back to the power supply and separately decoupled too. The already excellent Texas Instruments OPA134 op-amp of the standard Micro Amp is replaced throughout by the simply astronomically expensive (and well worth it!) Burr-Brown OPA627, plus all resistors and caps are of extraordinary reference-grade audio quality as well."

Possibly as a result of its class-A circuit, the Ultra Micro runs somewhat warm. Even with no signal, the case is warm to the touch. The power supply is in a separate enclosure, and has a captive power cord. You can place the power supply somewhere other than your rack (like the floor) to keep space demands at a minimum.

The Ultra Micro has an input impedance of 70k ohms, which should be easy for any source, tube or solid state, to drive. Some amps with input impedances of 10k ohms or lower are not compatible with many tube source components.

For the increasing number of audiophiles who use a computer as their source, HeadRoom offers a matching $699 Ultra Micro DAC, which uses the same chassis as the Ultra Micro amp. It has optical, coaxial, and USB inputs and a single line output on a 1/8" stereo miniplug. The rubber flanges on the case of both units have indentations and protrusions that allow them to be stacked. With the Ultra Micro DAC, you can use your computer to build your own music server, and do so for a lot less money than most commercial units. I didn’t have a Micro DAC to try, but I would expect it to offer far better sound than most computers’ internal DACs.

Preparation

HeadRoom said the Ultra Micro needed about 50 hours break-in time, so to be on the safe side I gave it 200. Then the upgraded Astrodyne power supply arrived (see sidebar). Although HeadRoom said it didn’t need any break-in, I’ve never encountered electronics that didn’t benefit from some playing time, so I gave the Astrodyne (and the amp, of course) 200 more hours of break-in. During that period, the amp got rather warm, but the power supplies ran much cooler. The Astrodyne barely got warm to the touch, even running 24/7.

Although it’s not an included accessory, HeadRoom thoughtfully sent me a Straight Wire adapter cable with RCA plugs on one end and a 1/8" stereo miniplug on the other so I could connect the Ultra Micro to my CD player. I also liked the sound of a similarly configured Zu Cable Pivot. Headphones were AKG K701s, still my favorite reasonably priced cans.

Small size, big sound

Lucky me -- I got to write this review twice. It’s all my fault, however. Here's the story: My reference Meridian 508.24 CD player was being used to burn in a balanced-only preamp, so rather than wait for the Meridian to become available, I started listening critically with the vintage NAD 5440 CD player in my burn-in system. At this point, the Ultra Micro sounded rather bright -- on some material objectionably so. The AKG K701 headphones have a treble balance that is ideal for me -- not bright, not rolled off, and not peaky -- but (so I thought) the Ultra Micro made them sound quite bright.

Boy, was that wrong! As soon as I plugged the Ultra Micro into the Meridian CD player, I realized that the brightness was coming from the NAD player. The Meridian was smooth and extended, and it had no rising treble. The NAD player also produced a slight emphasis in the bass. Through the Meridian, the bass was powerful and deep, but the perceived bass emphasis was almost gone. So my first review went into the trash and I went back to the drawing board. That’s what I get for trying to shortcut the process. What this experience showed me was how transparent the Ultra Micro was, easily showing the sonic flaws of the NAD. It’s hard to believe I ever thought the NAD was listenable.

Unlike some IC-based headphone amplifiers I’ve heard, the Ultra Micro could drive my headphones much louder than I’d ever want to listen. And the sound was clean, with plenty of detail. Although the Ultra Micro was definitely not bright, it didn’t roll off the highs either. Playing Kabalevsky’s "Overture to Colas Breugnon" from Bolero! [Reference Recordings RR-92CD], the Ultra Micro portrayed the assorted percussion instruments, including orchestral chimes, with plentiful detail, which requires extended highs.

If the high-frequency extension was not matched with comparable low-frequency extension, the amp really would have sounded bright. Not to worry, however. On "Overture to Colas Breugnon," the low frequencies went low indeed and displayed copious detail. This piece has low frequencies descending into the mid-20Hz range, which most speakers can’t fully reproduce. Through the Ultra Micro, my K701s reproduced the low notes distinctly and powerfully. Not only could I hear that the double basses were playing very low notes, I could also hear the musicians bowing the instruments.

Having established that the frequency extremes were quite extended, I began to focus on to the midrange, where most of the music lies. There I found abundant detail, but it was not at all analytical, just sweet and energetic. The sound was open and (for headphone listening) spacious. I tried the crossfeed circuit, and, sure enough, the reduced separation may have improved the impression of a soundstage, normally lacking with headphones. However, crossfeed trades off some spaciousness for center fill, and I preferred the sound with it switched off

Across the entire audio spectrum, macrodynamics, the difference in volume between the softest and loudest part of a musical piece, were startling -- about as realistic as I’ve heard any system reproduce. Small changes in volume -- the shifts in microdynamics -- were also easy to follow, though less spectacular.

Throughout the musical spectrum, the Ultra Micro sang in a clear, detailed and refined voice -- the sort of sound that a speaker-based system can equal but at nowhere near the price of the Ultra Micro and a set of good headphones. This is the most enticing part of headphone listening -- hearing sound that in important ways far outdistances speakers and electronics arrayed in free space.

The competition

My $595 Stello HP100 headphone amp is a larger unit, closer in size to HeadRoom’s Desktop series, but because it has an internal power supply, you don’t have to worry about finding a place for an external box. Though larger than the Ultra Micro, the HP100 is still reasonably easy to place on your rack or desktop. Unlike the Ultra Micro, the SP100 can serve as a preamp as well as a headphone amp -- it has two sets of inputs and a single pair of outputs -- and its case is large enough to use RCA input and output jacks and a 1/4" headphone jack, so you don’t need special cables or adapters. Unlike many headphone amps, the HP100 eschews op-amps. Its totally discrete solid-state circuit has easily driven any headphones I’ve tried. Does that make it better than chip amps? The proof is in the listening.

Compared to the HP100, the Ultra Micro sounded more energetic -- more extroverted. The HP100 presented music in a calm, relaxed fashion, but the Ultra Micro really rocked out. I was not surprised to find that the Ultra Micro had somewhat more extended high frequencies, but it also had more extended lows, an area where I had not previously found the HP100 lacking. The HP100’s mids were realistically presented, but they sounded a smidgen closed in.

At $100 less than the Ultra Micro, the HP100 is a decent value. Its frequency balance may be just the thing for slightly bright-sounding headphones. However, with my AKG K701s, and with other similarly neutrally balanced 'phones, the Ultra Micro would be hard to pass up.

Sizing up the Ultra Micro

The HeadRoom Ultra Micro has two very important things going for it: size and sound. I found no real flaws and lots of strengths. Its sound was powerful and energetic, yet at the same time smooth and sweet. I never found myself wishing for better performance in any respect. Although the size entails some compromises, if shelf space is at a premium for you, the Ultra Micro may well be the amp you need. Just don’t confuse it with a portable amp.

Like every good headphone amplifier, the Ultra Micro needs to be matched to the headphones it drives. Its extended frequency response was a glorious match for my AKG K701s, but I suspect it would be equally suitable with Sennheiser HD 650s. In fact, a picture on the HeadRoom website shows HD 650s plugged into the Ultra Micro, probably not a casual choice.

Although I haven’t auditioned many other headphone amps of comparable size, I’m fully prepared to believe that the HeadRoom Ultra Micro Headphone Amp is indeed "the highest quality audio per cubic inch known to humankind."

...Vade Forrester
vade@soundstage.com

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