Grado HP 1 headphones Page 2
Most preamps, tape recorders, and integrated amplifiers contain an "all-purpose" headphone jack, often driven by a 4556 or 4560 op-amp. These op-amps are hardly "state of the art," but they will drive a 40 ohm load. Most of these headphone amplifiers also contain a resistor in series with the output, often as high as 600 ohms. This prevents very-low-impedance headphones from exceeding the current capability of the op-amp, but since most of the amplifier's output is dissipated by the resistor, performance is seriously compromised. If a headset has a non-uniform impedance curve, the series resistor will also cause errors in the frequency response. At the time I received the HP 1s, I did not possess an amplifier which I felt could do justice to them, so I built an amplifier specifically for the 40 ohm load provided by the Grados. A few weeks later I visited Joe Grado in Westfield, NJ and received a pre-production prototype of his new HPA-1DC amplifier. Joe had also supplied me with an adaptor cable for connecting the HP 1s to a power amplifier, with suitable resistive attenuation, but I found this to be the least satisfactory method for powering his headphones.
I used several familiar commercial recordings to evaluate the HP 1s, and spent considerable time using them during my live concert recordings at The Crane School of Music. It takes very little listening to realize that the HP 1 represents a significant advance in dynamic headphone design. Nearly every headphone I've ever used has been too bright in the high frequencies. The most immediately appealing characteristic of the Grado HP 1s is their very neutral tonal balance. They are completely free of the midrange or high-end peaking which plague so many other designs. If anything, I find the upper octaves somewhat laid-back compared to the rest of the spectrum. But the response is not bumpy as in many designs; it is extremely smooth.
The HP 1s have an excellent low end. It is clean, extremely well-defined, and extended. Subjectively, I would estimate that they begin to roll off around 40Hz. Even Telarc bass drums are rendered with authority and lack of strain. A low end as fine as that of the HP 1s is unusual in headsets that do not isolate the listener from outside sounds. The foam cushions are denser than those found on "open air" headsets, but the coupling to the ear is not nearly as tight as it is with headphones that provide a firm seal around the ears. The cushions would appear to provide a sort of aperiodic damping at low frequencies, which may account for the very smooth low-end response.
Most headphones confine a portion of the soundstage inside the listener's head. The HP 1s are incredibly spacious, with instruments in the center of the soundstage appearing above the top of the listener's head. They have superb resolving powers, and are able to capture low-level details with ease, especially hall ambience. The excellent ambience retrieval of the Grados undoubtedly accounts, in part, for their extremely spacious sound. The HP 1s are remarkably low in coloration, and when driven by a high-performance amplifier (more on that below) the sound has a purity and cleanliness not equaled by other headphones I've auditioned. The HP 1s excel in the area of dynamics. With a suitable amplifier, dynamics remain uncompressed even at very loud playback levels. They are also able to maintain their sonic purity at loud levels, a characteristic of their performance which will certainly appeal to field recording engineers.
Having heard so much about Joe Grado's abilities as an operatic tenor, when I visited him in New Jersey I brought along a friend who is a fine pianist (and as fanatically devoted to great singing as I am). With my friend seated at Joe's 7' Bösendorfer, I was treated to quite beautiful performances of di Capua's O sole mio and Gastaldon's Musica Proibita. After the vocal demonstration was completed, we descended into Joe's basement laboratory to hear the HP 1s driven by a pre-production prototype of his new HPA-1DC headphone amplifier.
Bearing in mind that Joe has spent most of his life designing and manufacturing equipment for playback of LP records, it may come as a surprise to find a complete absence of analog playback gear in his lab. A CD player and DAT recorder provide the only source material. (Analog lovers, fear notJoe still works on phono cartridges, but in another room.) I sensed Joe's enthusiasm for digital audio (in its current stage of development) during a phone conversation we had a few weeks prior to my visit. I commented that I was surprised to hear a manufacturer of analog playback equipment rave about digital audio, to which he replied, "I'm not interested in living in the past." I noted that his attitude was refreshing, since I've heard other phono cartridge designers badmouthing CDs. Joe replied, "They just don't want to get off their butts and look to the future." LP lovers, please note that these comments come from someone who should have an axe to grind regarding the death of analog. Joe clearly does not.
Joe's demonstration focused on the HP 1s' abilities as an analytical tool for evaluating microphones, cables, and other equipment recording engineers normally encounter in the signal path. His headphone amplifier is a portable unit using a dual operational amplifier, powered by a pair of 9V alkaline batteries: the ±9V supply rails are each locally bypassed with a 330µF capacitor. Sid Smith, famous for his classic Marantz designs, developed the circuit topology, while the selection of internal wiring and components was done by Joe using the HP 1s. No expense has been spared in selecting parts for this amplifier. Roederstein metal-film resistors, Grado's own proprietary wire, and the most expensive potentiometer Alps manufactures combine to produce an amplifier with very low sonic coloration. The Grado headphones, used with the HPA-1DC amplifier, are sonically so clean that they reveal subtle differences in electronics and cables in a fraction of the time normally required when using loudspeakers.
Upon returning to Potsdam, I immediately put the HP 1s, driven by the prototype HPA-1DC, to work as a monitoring system for some live concert recordings, the first being a concert by The Crane Symphony Orchestra. During the concert, I passed the HP 1s around to my student recording assistants and our Concert Halls Manager (all of whom are well-acquainted with the sound of live music, given their almost daily exposure to the real thing). There was a universal consensus that Grado's HP 1s were exceptionally faithful to the sounds of the live instruments in the hall. Joe's amplifier received similar accolades when compared with the headphone outputs on my mixing console and my Panasonic SV-3500 DAT recorder. The HPA-1DC put them all to shame, delivering sound which was much more spacious, more detailed, and more extended at both ends of the frequency spectrum than either of the other headphone amplifiers. The SV-3500's headphone output produced a somewhat edgy high end, a congested midrange, and a narrow soundstage lacking in realistic hall ambience when compared with the HPA-1DC. The headphone output of my mixing console fared even worse.
Later, I compared the headphone output on an Adcom GFP-565 preamp, used in my editing and production studio, with the HPA-1DC. Although the Adcom headphone amplifier is considerably better than that of the DAT recorder or mixing console, it was still outperformed by the Grado. The HP 1s are so low in sonic coloration that they reveal any deficiencies in the amplification used to drive them. The all-purpose headphone outputs provided with most preamps and tape recorders are simply inadequate if the full potential of Grado's headphones is to be realized. The HPA-1DC has been designed specifically for driving the HP 1 headphones, and as such it performs extremely well. I would normally not expect such a high level of performance from a battery-operated, unbuffered op-amp driving a 40 ohm headset.
An impedance and phase-angle graph supplied by Joe Grado provides the answer. The HP 1s are an extremely easy load to drive. The phase-angle hovers around 0° from 10Hz up to 1kHz, indicating a primarily resistive load. Above 1kHz, the phase angle increases due to the normal inductive reactance of the voice-coil, but even at 20kHz it is only 25°. A headset providing a highly reactive load, particularly in the low frequencies, would be far more difficult for an op-amp to drive. The impedance curve is extremely smooth, rising only a few ohms above the nominal 40 ohm rating at resonance and still being less than 55 ohms at 20kHz. But, as with loudspeakers, the impedance curve alone does not indicate the nature of the load presented to the amplifier. A reactive load is always more difficult to drive than a resistive one, and only a phase-angle graph can provide this information.
Since I believe that an AC-powered headphone amplifier offered potential performance advantages over a battery-powered device, I spent considerable time comparing the HPA-1DC prototype to my own amplifier. I note that I am not the first Stereophile reviewer to use a custom-built amplifier for a headphone review. High-performance headphone amplifiers are virtually nonexistent, a fact which prompted John Atkinson to use an amplifier he built for evaluating the Beyer DT 320 Mk.II and DT 325 headphones in Vol.12 No.4. Conceptually, the design built by John, from Electronic Design & Wireless World magazine, appears to be quite similar to mine, at least from the description he gave in the Beyer review.
My amplifier uses a recently introduced op-amp from Linear Technology, the LT-1122CN8, which is buffered by an LT-1010 IC buffer amplifier. The buffer amp has an output impedance of 6 ohms and has an external bias adjustment for the output stages. Needless to say, I run mine very rich class-A. I also bias the output stage of the op-amp for class-A operation using a current source between the output and the negative supply rail. A tightly regulated, low-impedance power supply, the same as I described in the Oct. 1990 issue of Audio Amateur (footnote 1), provides the rail voltages, and heavy local bypassing is used for the op-amps and buffer amplifiers.
Footnote 1: Galo, Gary A., "Preamp Power Supply," in "Ask TAA," The Audio Amateur, Vol.21 No.4, October 1990, p.47.