Low distortion 24bit DAC accepts samples from 32 to 96K
Balanced XLR analog inputs
Unbalanced RCA analog inputs
High current output amp drives 50 Ohm phones
24-position level attenuator with 0.05 dB channel matching
Internal linear power supply
High current, low noise toroidal power transformer
No electrolytic capacitors in signal path
Highest quality metal film resistors
Automatic digital de-emphasis filter
10 dB gain boost switch for -10 dB sources
Fast, musical transimpedance output amps
Precision instrumentation balanced input amps
Sealed gold contact relays for input select and gain boost
Freq. Resp.: @0dBu out +/- .25 dB = 22Hz-120K
@0dBu out +/- .5 dB = 12Hz-260K
Max. Output Level: @ 1K, 50 Ohm load = +23dBu
Input Impedance: 4oK Ohms balanced, 20K Ohms unbalanced
Output Impedance: 1 Ohm
Min. Load Impedance: 25 Ohms
Dynamic Range: @0dB gain, 50 Ohm load = 116dB
Distortion: THD=N - 10dB gain, +10dBu out, 50 Ohm load = 0.0051%
Sample rates: 32, 44.1, 48, 88.2, 96K
THD+N at 44.1K, 1 K, +20dBu out = 0.0016%
Noise Floor: 0dB gain, 22-22K = -95dBu; A-weighted: -102dBu
Power Requirement @ 100-115VAC = .25A
Dimensions: 8.5 X 8.5 X 1.7
Weight: 5.8 lbs.
2434 30th Street
Boulder, CO 80306
Grace Design has specialized in mic preamps for 15 years. The 901 Reference Headphone Amp was designed to give engineers and musicians high quality headphone amplification for tracking, mixing and mastering in the studio. The Model 901 was built to drive difficult low-impedance headphones with greater resolution and accuracy than the typical amps found in consoles, recorders, disc players or sound cards. The built-in DAC allows tapping into a variety of digital outputs in the studio and the precise level control is designed to provide accurate stereo balance at any volume level. Subtle differences and details in various tracks can be identified more easily now with this sonic magnifying lens.
Such qualities sound like they could be also right up an audiophiles alley - especially if that audiophile is a headphone fan. So I requested a unit for review. The appearance is compact and gorgeous, with a brushed aluminum front panel and recessed buttons for high gain, analog/digital input, and power on/off - in green and yellow. There are two heavy duty phone plug sockets on the front and a row of four small LEDs displaying the four different sampling frequencies that can be decoded.
On the rear, from left to right, are the detachable AC socket, the coaxial and optical digital input jacks, the AES digital input socket, the unbalanced analog RCA jacks, and finally the balanced analog AES jacks. There is a small toggle switch to change between the AES digital connection and the other two digital inputs. I used a Monster Cable optical cable from the 901 to my digital optical output of my modded Sony 9000ES player, plus AES XLR -to-phone plug cables from my patch bay analog out to the 901 inputs. I ran the 901 for a day with a repeating CD in the 9000ES player and using the DAC input, and another day with the analog feed. I also hooked up my reference AKG K1000 headphone amp using a similar AES XLR cable for comparisons; that unit lacks a built-in DAC. Both amps powered my Grado phones, which I used exclusively since my reference AKG K1000 phones require more power than normal headphone amps like the Grace can provide, and also require a special XLR-in socket.
Since my main interest in high end headphone use is for the proper playback of genuine binaural recordings, I began my tests with a binaural CD from the specialist in African ethnic music, Tchad Blake. Document: Zimbabwe (RealWorld/Womad DOC1) was recorded in the field and includes a variety of music and realistic sound effects. Track 2, Rega Kusarira, is a wonderful a capella male vocal number sounding something like Ladysmith Black Mombazo. The binaural pickup re-created the small and tuneful vocal ensemble surrounding the listener with uncanny accuracy with both the DAC and the analog inputs to the 901. There was quite a pronounced difference in quality, however. The analog input sounded almost muddy and with the highest frequencies rolled off compared to the much greater clarity using the DAC 44.1 input. So for most of the rest of the auditions I compared the digital input on the 901 to the analog AKG amp.
The BIS CD from Tango Futur - Paris-Buenos Aires has an unusual tango quintet of piano, bandoneon, doublebass, sax and percussion. The collection of 19 short tangos is recorded with great presence and clarity. The vibes and percussion were very strong with both the AKG and the Grace, and sonics were very similar. I was hard put to discern a just noticeable difference here.
Thinking it would be good to test some of the other sampling rates of the Grace DAC, I put a new Decca DVD-video into the 9000ES - Ute Lemper Sings Kurt Weill & Michael Nyman. As with many DVD videos, this one employs the 48K sampling rate for its two-channel PCM soundtrack option. Sure enough, the first little LED on the Grace extinguished and the second one - marked 48K - came on. Lempers voice was strong and realistic, and the piano accompaniment had a very natural sonic to it. The track I selected - Le Train du Ciel - opens with a sound effect of a train passing by from right to left. This was also effectively reproduced. In comparison the AKG amp delivered a slightly stronger extreme low frequency reproduction of the train effect, and Lempers voice took on an even greater presence - nearly matching a good binaural recording.
So having made it to 48K I decided to try for 96K: The easiest source was one of Classic Records 24/96 DAADs. I selected one agreed upon by many users as one of the very best of the erstwhile series - All the Works for Orchestra & Piano & Orchestra (alone) by Gershwin - Leonard Slatkin/Jeffrey Siegel, piano (Turnabout/Classic DAAD). Starting with the opening movement of the Gershwin Concerto in F, the 96K LED lit up and we were launched into this exciting concerto with a rich orchestral palette and good clarity on the piano when it enters. However, switching to the AKG amp provided a more impactful piano tone and increased resolution in the orchestral details. Now in switching back to the Grace the piano actually sounded somewhat constricted.
While some of the headphone gimmicks such as the various pseudo-surround-field processors, the HeadRoom circuit, and/or wireless headphones have their place, for critical listening to music in stereo (or DVD soundtracks for that matter) I feel you cant beat a straight no-nonsense dedicated headphone amp to get the very best out of high end dynamic phones. The best I ever heard was the now-discontinued Krell. Next in line for my money is the also discontinued AKG K1000 amp, designed for that companys K1000 earspeakers. Its Class A, built like a tank, has XLR inputs, and its original price was $1200. It lacked the built-in DAC of the Grace and its hard to find used, so as Rosanna Dana would say, Never mind. (The terrific K1000 earspeakers are still available after 14 years, at $916, and can be powered by a small standard stereo amp with a level control.)
- John Sunier