Company Address: 611 North Wallace Ave., Bozeman, Mont. 59771.
Montana! As I unpacked the HeadRoom Supreme Headphone Amplifier and Audio Image Processor, I was singing "I come from Montana. I wear a bandanna. My spurs they jingle as I ride along." I knew that there was more going on in the Big Sky country than riding, roping, and branding, but I never expected an audiophile quality headphone amplifier from there! But since most receivers, cassette machines, and CD players have stereo earphone jacks, why would anyone want a separate amplifier for earphones?
Tyll Hertsens, president of Head Room Corp., once had a job that required a great deal of travelling. Be cause this led to many hours of listening through earphones, he be came acutely aware of the differences between listening with earphones and loudspeakers and did some re search to find the cause of these differences. He found some answers and decided to design and build a headphone amplifier to resolve the problems. His goals were: Consistency when using earphones with different sources, more power out put, and a cross-feed circuit to improve imaging.
A separate earphone amplifier can be designed to provide a uniform frequency response into the wide range of impedance characteristics of different earphones. It can also be designed to supply more audio power than most sources with built-in phone jacks; many CD players, especially portables running on small batteries, necessarily have low-power amplifiers.
The cross-feed circuit is not a new idea, but it is an eminently practical one. It became prominent in the 1960s through work done by Ben Bauer of CBS Labs, and consequently it is sometimes called the Bauer circuit. After you have listened to music with earphones using a cross-feed circuit, you will wonder how you ever got along without it.
An explanation of the difference between loudspeaker and earphone listening will make it clear why a cross-feed circuit is so worthwhile. Let's consider, for example, a recording that has a clarinet predominantly in the left channel and a trumpet in the right. When you listen to this recording with loud speakers, you will hear the clarinet coming from the left loudspeaker and the trumpet coming from the right loudspeaker. However, you will hear the clarinet and the trumpet with both your left and right ears, albeit with slightly different spectra and at slightly different times. The clarinet's sound will arrive at your right ear slightly after it arrives at your left; it will also have a reduced high-frequency spectrum due to shadowing head. These spectral and by your timing clues are what allow you to determine the direction of the sound. When you listen with earphones, the clarinet will be in your left ear and the trumpet will be in your right, but there will be no clarinet sound in your right ear and no trumpet sound in your left ear. This creates a very unnatural listening condition.
Most recordings also have instruments in the center that will help to direct your attention away from this unnatural effect, but it is still there. The cross-feed circuit alleviates this effect by blending a slightly delayed signal, with a reduced high-frequency spectrum, into the opposite channel; this causes multimiked recordings to sound more natural. Recordings made for ear phone listening, with two microphones placed in a dummy head, usually don't need a cross-feed circuit-but such recordings are the exception rather than the rule.
The HeadRoom Supreme has a very good cross-feed circuit. It also has an equalizer filter that can be switched in or out when the cross-feed circuit is switched on. With the filter on, high frequencies in the cross-feed signal are slightly increased. When you order the $399 Supreme, you will be asked what brand and model ear phones you use, the type of music you prefer, and whether you listen at home or while travelling. A technician then adjusts the filter to one of three equalization set tings by selecting a resistor and soldering it to the main circuit board. (If you try the Supreme and find it too bright or dark, you can return it to HeadRoom within 30 days; the company will make the appropriate change, at no charge.)
The Supreme has three on/off toggle switches on the front panel-for the cross-feed circuit ("Process/Bypass"), the filter, and power. The 1/4-inch earphone jack is on the left side of the front panel, and a volume control is on the right. The rear panel has two recessed line input phono jacks, two battery holders, and a jack for the external 5-V d.c. supply. The HeadRoom Supreme is housed in a solid, extruded aluminum case that is about 5 3/4 inches wide, 1 3/8 inches high, and 6 inches deep; the volume control adds about 3/4 inch to the depth. The p.c. board has a large slot, in the center, to allow clearance for the two battery holders mounted to the rear panel; each battery holder accepts two AA batteries. (You can expect a little more than an hour of operation with nickel-cadmium batteries and over 2 hours with alkalines. You can increase this to over 20 hours if you operate the Supreme from the accessory battery holder, which houses four D cells.) There are 25 parts on the circuit board, including a d.c.-to-d.c. converter (that provides +15 and-15 V from a 5-V input) and the encapsulated, two-channel circuit module which is the heart of the amplifier.
The basic amp and processor module is sold separately for those who would like to build their own earphone amplifier; it costs $89. HeadRoom will even help you by supplying schematics, parts lists, and sources. You could put the module into a receiver, preamp, etc. whose power supply provides 12 to 15 V of positive and negative d.c. In fact, one of Tyll Hertsens' goals is to make an IC version of the HeadRoom circuit that audio manufacturers will be able to put into their products.
Rated output is 60 mW (maximum out put is 400 mW), and the amp will drive earphones with impedances from 10 to 600 ohms. I listened to the Supreme with a pair of earphones that measured 17 ohms, and the combination sounded very good; I heard no problems even when I listened at a very high level. There is plenty of output even for low-sensitivity earphones.
I listened to a wide variety of music with the Supreme while switching the cross-feed and the filter in and out. Most recordings benefited from the cross-feed, and many sounded better with the filter turned on. You can usually tell that the cross-feed will make the sound more natural if an instrument can be heard in one ear and hardly at all in the other. For example, Dave Grusin's Discovered Again (Sheffield CD-5) sounded much better with the cross-feed and filter switched on. The cross-feed and filter also made "Wishing Well" on the Schonherz and Scott album, One Night in Vienna (Windham Hill WD-1060), sound more natural. But "Bourbon Street Parade," an Al Hirt cut on Dixieland's Greatest Hits (First Choice FC 4512), benefited from the cross-feed but sounded better with the filter off. Many rock and jazz recordings are made with very little cross-feed on some instruments, and these will definitely sound better with the cross-feed on.
Recordings that sound great without the cross-feed switched in are rare and are usually made by people who use earphones to check the sound, such as Tom Jung of dmp or Craig Dory and Brian Peters of Dorian. Chuck Loeb's "One Man's View" from Simple Things (dmp CD-504) sounded excellent with the cross-feed turned off, as did Pergolesi's Stabat Mater on Dorian (DOR-90196). "La Bamba," on Lift Off (dmp CD-498), was only slightly enhanced by the cross-feed.
There are some recordings that have ad equate cross-feed between channels but can still be enhanced with the Supreme's cross-feed circuit. I have recorded in the Alice Millar Chapel at Northwestern University, where the Millar Brass Ensemble was recorded (Koss CD-1011). The long and narrow hall is very bright and reverberant, but the brightness of the brass instruments was tamed, and the imaging improved slightly, with the Supreme's cross-feed on and its filter switched off.
Some recordings made with a dummy head in a reverberant space can actually sound less natural when the cross-feed switch is on. An example is Joyce Jones at the Ruffatti Organ in Spivey Hall on Rosenhaus Records (3525 Carondolet, Waco, Tex. 76710). This recording was engineered by Wade Bray of Head Acoustics, using an Aachen Head augmented by a pair of Schoeps MK-5 omnidirectional micro phones. The sound is magnificent. However, with the Supreme's cross-feed, the sound field collapsed slightly and wasn't as natural.
If the last few examples cause you to believe that I think the cross-feed process isn't useful, you are very wrong! The majority of recordings will benefit greatly from it. The HeadRoom Supreme head phone amplifier is portable, has audiophile-quality sound, and doesn't cost a fortune. It is one of the best things that has happened to those of us who enjoy listening to music via earphones.
-Edward M. Long
(adapted from Audio magazine, Jan. 1995)
Also see:
Beyerdynamic DT 770 Pro Earphones (Mar. 1993)
Beyerdynamic DT-911 headphones--review, teardown and analysis (Jan. 1993)
Sennheiser HD-540 Reference II headphones (Jun. 1992)
Beyerdynamic DT990 PRO Earphones (AURICLE, Apr. 1992)
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