Grace Design Model 901 

Reference Headphone Amplifier Review

ã 2002 By Eddie Ciletti

"Eliminating the Weakest Link"

There are times when only Headphones can reveal the fine detail and nuance that engineers need when choosing preamps and microphones, listening for background noises or doing quality control. That’s why the Grace Design Model 901 Reference Headphone Amplifier is the answer, the type of product my hardcore customers would buy in a heartbeat and I totally get it. By eliminating an often overlooked weak link in the chain (see the Sidebar "Bench Perspective"), the Model 901 encourages critical listeners to match the quality of this device with better than average headphones. 

This is the first Grace Design product I have ever reviewed. Its 1-RU height and half-rack width is deceptive because at five-plus pounds, at first I suspected that a dummy weight had been added to make the unit "feel" more substantial (and to keep it from sliding around). I couldn’t have been more wrong. The Model 901’s considerable "heft" is due to its thick steel chassis, toroidal transformer (no wall-wart, yeah!!) and quarter-inch thick aluminum front panel construction. Inside, the build quality and components are top notch.

As a converter alone, the Grace 901 can justify its $1495 List Price because the foundation is high quality 24-bit / 96kHz Digital to Analog conversion interfaced to the outside world via rear panel AES, SPDIF and Toslink connectors. Most equipment does not have conversion of this quality both in terms of components and execution, plus th converter sub-assembly can be upgraded to a higher sample-rate module if and when it becomes available. 

A front panel switch chooses between the Digital or Analog inputs (XLR or RCA), the Level Control is a rotary switch with precision resistors — to ensure long trouble-free life and channel-to-channel stability — followed by a high quality, high power output amplifier. A front panel "Range" switch ensures that the Model 901 can drive any impedance headphones via the two front panel jacks. The unit could probably drive a pair of speakers though I didn’t try that! It is recommended that the analog input of the Model 901 be driven from a low-impedance source (typically 100-ohms or less, 1k-ohms is too high).


Over the years, when customers with critical ears discussed headphones, a few names have been repeated over and over. On the short list that also includes Grado, Stax and Sony, I acquired a pair of Sennheiser HD600 headphones to make this marriage complete. The first "test" was a Jazz Trio consisting of Acoustic Bass, Drums and Piano in a live recording space with no control room. (Click this Jazz Link for full session details plus an mp3 sample.) During the recording I worked exclusively in the headphones and it was heavenly as there was no isolated control area. On playbacks we listened on ProAc Studio 100 monitors while the headphones were passed around.

The session was recorded to a Fostex DV-40 4-channel DVD- RAM recorder. Two channels came direct from the board plus another pair from an Audio Technica Stereo Shotgun mic in the balcony — using either an AT815ST or an AT835ST. From the Fostex, the main mix pair was digitally connected to the Grace 901 while the shotgun pair was fed via analog connection. After the session, The Grace 901 was connected to a Z-Systems digital detangler using either an Alesis Masterlink (at 88.2kHz /24-bit) or a Technics CD player as sound source. 


Level Setting
Sony V-6 (7506)
Fostex T20

TABLE-1: List of headphones tested with the Grace Model 901 Reference Headphone Amplifier. (The spaces between the rows group the headphones into similar sonic categories.) H and L refer to the Range Switch (for high- and low gain) which more often than not corresponds with the level required to drive high- and low-impedance headphones, respectively.

Table-1 is the list of headphones tested along with the Model 901 Headphone Amp. At left the list is in descending order of personal preference, the spaces between the rows group the headphones into similar sonic categories. The level settings are "clock-wise" and quite relative although very in the ballpark in terms of Impedance. The "H" and "L" refer to the Model 901’s "Range" switch, High and Low Gain optimizes the level for high and low-impedance cans.

What is so impressive about the Grace Model 901 is the silence; there’s not a hint of hiss, extraneous noise or hum. When partnered with the Sennheiser HD-600 (about $299 "street"), the sound is open, effortless, extended and spacious. The top is delicate and the bottom full. Nearly everyone knows what the Sony cans sound like — not as nice as the HD-600 — but not so far away as some of the other headphones on the list and about one-third the street price. Considering the Impedance difference (300-ohms and 0-ohms), the Model 901 had no difficulty driving either pair. For more info on the headphone test, see the SIDEBAR: Can-Can.

Taking the time to audition various headphones, source material and the Grace Model 901 Reference Headphone amplifier is an exercise I recommend to everyone. Those who rely heavily on headphones have most likely done the research enough to know the value of this product in the Classical and Jazz environments. For everyone else, I highly recommend the Grace 901, especially when sonic comparisons make it difficult to distinguish whether the gear or the source material is at fault.



Auditioning multiple headphones (listed in Table-1) was a revelation if only because time is rarely taken except at the time of purchase. I’ve used the Sony V-6 cans for years, they are also known as the MDR-V7506, perhaps the most popular because of their price / performance ratio. In between, the Sennheiser HD-280 pro cans are designed for isolation, sonically similar to the Sony with perhaps a bit more bottom. The Fostex T20 cans are warm and comfy, their midrange smooth but with significantly less top. They are a similar vintage to the AKG-240 cans, which have less low end and more top than the Fostex, not extended like the V6 and more like the AT D40fs. Like the HD-280, the ATM40fs cans are designed for isolation and level. Unlike the D40fs, the top end is almost odd and not nearly as nice as the D40fs.

Whether listening with headphones or loudspeakers, the ear’s Loudness Curve is quite the moving target. ( For more info on the topic of Bass Management, click here .)  Just from this listening test I noticed how different a headphone’s midrange can seem based on what the ear perceives as "louder." For example, at the top of the list, the Sennheiser HD600 cans have such "extended" top and bottom that their midrange never seems painful even when cranked up. At the bottom of the chart, the more mid-rangey cans seemed capable of more "pain" in that region because the lack of top and bottom made them seem less loud than they actually were. A headphone’s spectral response above 4kHz also effects the imaging quality substantially.


SIDE BAR: Bench Perspective

From my own experience, the disparity between headphone and line outputs on DAT machines can be detected on Auratones so it stands to reason that all headphone amplifiers should come under similar scrutiny. I learned this using the headphone out of DAT machines to drive an amp and speakers during post-service testing. Figure-1 shows the sonic trail from Square Wave Oscillator through Digital Conversion to Headphone Out as is typical for Panasonic DAT machines like the SV-3700 and SV-3800. The ringing on the center wave (dc) is typical of conversion at 44.1kHz, the rounding of the leading edge of the bottom wave is neither typical or acceptable, resulting in an obvious lack of high frequency response. 

FIGURE-1: Following the sonic trail from Square Wave Oscillator (swo) through Digital Conversion (dc) via XLR to Headphone Out as is typical for Panasonic DAT machines like the SV-3700 and SV-3800. 

The headphone jack on any piece of equipment is, at minimum, a convenience feature used to confirm signal presence. It is only when asked to deliver a more qualitative analysis of the signal that the amplifier behind the jack falls short. Some may have noise, either in the form of hiss or interference, the latter from the display or any number of internal clocks. Others may have inadequate headroom or an obvious sonic deficiency as shown in Figure-1.

Key components in the Grace 901

  • AD 815 AY headphone amp
  • CS 43122 crystal converter
  • INA 103 opamp
  • AD 712 (DC servo control) for the AS815 headphone amplifier.