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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…
Well, used as a simple amplifier, the HeadRoom stomped all over the Advent. The AC-powered receiver's bass sounded lumpy and spongy compared with the little battery-powered amplifier. This was probably due to the Advent not having a true headphone output, instead padding down its speaker-level outputs with series resistors. But it sounded so grainy in the highs compared with the HeadRoom that I ultimately ruled it out of contention.
The Denon CD player, on the other hand, has a surprisingly good headphone output, capable of driving Sennheiser '580s to moderately high levels without…
Sidebar 1: Specifications
Description: Battery or AC-powered portable headphone amplifier available in three versions: Standard, Premium, Supreme. Number of inputs: one pair, on RCA jacks. Number of outputs: one ¼" stereo headphone jack. Controls: On/Off, Volume, Process On/Bypass, Filter On/Bypass (Supreme only). Input impedance: >50k ohms. Rated input: 0dBU (775mV) Standard; 4dBU (1.24V) Premium & Supreme. Rated output: 22mW into 100 ohms, Standard; 60mW into 100 ohms, Premium & Supreme. Maximum output (0.02% THD at 1kHz): 400mW into 100 ohms (equivalent to 6.32V RMS). THD:…
Sidebar 2: Measurements
The HeadRoom amplifier was non-inverting for all control settings, and its input impedance was a little lower than specification at 21k ohms at 1kHz (footnote 1). Channel separation in Bypass mode was better than 70dB at low frequencies, worsening to 37.5dB at 20kHz due to capacitive coupling between the channels, probably between the parallel pcb tracks carrying the signals to the volume control. The HeadRoom's background noise was a little higher than I expected, its S/N ratio measuring about 62dB (unweighted, 22Hz-22kHz, ref. 1V). (An earlier sample of the…
So, how to examine the HeadRoom's response shaping and crossfeed? In its bypass mode, the amplifier was flat in response up to the mid-treble, with then a gentle ultrasonic rolloff. This can be seen in both the rounding of a 10kHz squarewave's leading edges (fig.5) and in the amplitude response (fig.6). The -3dB point lies at 27kHz, meaning that audio-band response is down 2.2dB at 20kHz. This will be audible as a slight softening of the overall sound—not a bad idea with headphones! The effect of the filter is also shown in fig.6; it can be seen to shelve up the mid-treble by a mild 3dB.…
Wes Phillips wrote about the Supreme in July 1996 (Vol.19 No.7):
HeadRoom's $399 HeadRoom Supreme headphone amplifier seemed the logical comparison to the Audio Alchemy HPA v1.0/PS3. The portable HeadRoom can run off either a small wall-wart, or an external four-D-cell power pack. I prefer its sound with the battery pack, so I used that for my comparisons.
They were both very good, but the Audio Alchemy sounded just a shade coarser, with a bit of grit in the upper octaves. On "Third Uncle," from Brian Eno's Taking Tiger Mountain (by Strategy), remastered and Super-Bit-Mapped on…
Someday we may speak wistfully to our grandchildren about the "golden age" of digital audio when consumer formats (CD and DAT) contained a bitstream that was an exact bit-for-bit duplicate of the original studio master recording—not a digitally compressed, filtered, copy-resistant version whose sound is "close enough" to the original. Digitally compressed formats such as DCC and MiniDisc represent, in effect, a return to the pre-CD era when consumer-release formats were always understood to be imperfect copies of the studio original. Even the most ardent audiophile accepted the fact that LPs…
For still more efficient coding, linear PCM can be replaced by a "floating-point" code in which leading zeros are eliminated by repetitive division. (The linear PCM codes in a single CD contain more than a billion leading zeros, in small signals and wherever a large waveform crosses the zero-voltage axis.) A small number such as 0.00000001 in decimal arithmetic becomes "1E-8" in floating-point form; the number after the E tells the decoder how many places the decimal point was moved to the left.
With typical program material, lossless coding can cut the bit-rate by at least a factor of…
As with PASC, most of the complexity (and cost) in the AC-2 system resides in the encoder. An AC-2 decoder can be produced as a single under-$20 IC and can be designed to work with either fixed-filter AC-2 codes or the improved sound provided by dynamic-filter AC-2A coding.
Dolby AC-3
Two of the five proposals for high-definition TV currently being tested by the FCC employ AC-2 coding for stereo sound. At the NAB convention, Tom Holman of Lucasfilm went a step farther. He reviewed the psychoacoustic factors that affect the perception of timbre and imaging, concluding that the…
"Ah, I see what the problem is. Your ear canals are larger in diameter than normal."
Size does matter, I guess. I was talking to Shure staffers at their booth at Home Entertainment 2003 last June. I had always had a problem with such in-ear headphones as the industry-standard Etymotics, and was explaining that I was getting on no better with the review samples of Shure's then-new, $499 E5c in-ear cans. With what I now knew to be my wider-than-normal ear canals, I had not been able to get the supplied sleeves, foam or soft plastic, to seal properly. The result was sound that seriously…