Grado HP 1 headphones Page 3
A battery-powered amplifier can't be expected to perform as well as one driven from a well-regulated, low-impedance, AC-powered supply. I realize that this runs contrary to the views of many others, so a bit of explanation is in order. Many electronic designers hold the view that batteries have an output impedance of 0 ohms, and therefore must be better than an AC-powered supply. If a power source has a true 0 ohm output impedance, its output voltage will remain constant regardless of the current drawn by the load. Although a 12V Sears Die Hard undoubtedly has much lower output impedance than any AC-powered preamp supply (I don't know of any preamp power supply which could start your car), a 9V alkaline battery doesn't come close. The voltages at the battery terminals on the HPA-1DC amplifier drop approximately 30mV when the unit is turned on, and another 200mV when the amplifier is driven from idle to full output. In addition, the light-gauge battery clip wires increase the supply output impedance even further. The limited current capability of the batteries necessitates using an unbuffered op-amp, since a high bias-current buffer would drain the battery in a very short amount of time. If larger batteries were used, the amplifier would no longer be portable.
In theory, a battery supply will have a lower noise (ie, hum) level than an AC-powered supply. In practice, however, this really isn't true. A well-designed regulated supply will have extremely low ripple and, since all modern solid-state designs use dual-polarity supplies, hum at the output will be virtually nonexistent if the amplification circuitry has good common-mode rejection. It is true that a hum field can be radiated by the power transformer, but careful transformer positioning and shielding eliminates this problem as well. I think most designers and readers will agree that modern high-performance audio circuits driven by AC supplies do not suffer from hum problems. If they do, they must be considered defective designs.
Joe Grado's goal was to build a portable amplifier for the field recording engineer, and using a battery supply kept the size and cost within reason. His amplifier performs far better than I would ever expect, and easily outperforms any headphone jack I've encountered on tape recorders, CD players, and preamps. But I'd like to see someone manufacture a no-holds-barred, AC-powered headphone amp, which I believe is necessary if the full potential of high-performances headset such as the HP 1s is to be realized. One performance advantage offered by a battery-powered amplifier is its immunity to the sonically degrading effects of dirty AC power lines. Any type of AC-powered audio equipment will benefit from power-line filtering, whether internal or external. An AC-powered headphone amplifier could employ an electrostatically shielded power transformer to minimize these problems.
When I received the Grado HP 1s, I called Tom Norton and asked to borrow Stereophile's Stax Lambda Pro 3 headphones for comparison. In a short time, the headphones arrived along with the Stax SRM-1 Mk.II amplifier and the ED-1 Diffuse Field Equalizer. I find myself in agreement with most of the comments made on the Lambda Pros in previous issues of Stereophile (Vol.7 No.5, Vol.10 No.9, Vol.12 No.4, and the summary in "Recommended Components"). As Bill Sommerwerck noted, the Lambda Pros do, indeed, have "oodles of detail," and a very extended high end, in common with other high-performance electrostatic transducers. I also agree with various comments made on the upper-midrange suckout, giving the Stax headphones a laid-back character in this region, along with an upper-bass and treble emphasis. The HP 1s, as mentioned above, are also somewhat recessed in the midrange, but the treble is not accentuated relative to the midrange, and the bass/midrange balance can be altered using a trick recommended by Joe Grado. The HP 1s are supplied with eight slots cut into the inside edge of each of the foam cushions. Cutting four additional slots into the rear edges of the cushions reduces the bass coupling to the ear, providing a more natural balance between bass and midrange.
The Stax ED-1 Diffuse Field Equalizer does improve the midrange balance of the Lambda Pros, but also seems to narrow the soundstage. To my ears, the midrange boost is a bit excessive, giving the music a slightly nasal character which I find detectable even after prolonged listening. In addition, the portion of the frequency spectrum given the greatest amount of boost moves toward the center of the stereo image, reducing the sense of spaciousness. With the equalizer in the signal path, but switched out, the Lambda Pros produce a reasonably consistent soundstage throughout their frequency spectrum. Switching the equalizer in moves the upper midrange toward the center of the soundstage, giving a sense that this portion of the spectrum has been squeezed toward the center of the listener's head. I find this to be a disturbing side effect of the equalizer.
Despite the excellent detail of the Lambda Pros, and their extended high end, I do not find the midrange and treble to be as sonically clean and pure as the Grado HP 1s driven by Grado's amplifier or my own. To my ears, the colorations are more electronic in nature, and not what I would normally associate with electrostatic transducers. So I decided to investigate the Stax electronics, beginning with the ED-1 Diffuse Field Equalizer. I connected the ED-1 between my CD player and the Grado HPA-1DC amplifier to see what sonic effect it would have on the Grado headphones. I had no intention of equalizing the HP 1s (the EQ would be incorrect anyway), but instead was interested in determining what sonic effect the ED-1 would have on the sound. Bear in mind that the audio signal still passes through some of the ED-1's circuitry even with the equalization switched out.
I was extremely disappointed in the performance of the ED-1. The equalizer added a layer of sonic grundge to the midrange and an edge to the treble. The smoothness and purity of the HP 1s were substantially degraded, the soundstage was narrowed, and the spaciousness reduced.
I pulled the cover off the ED-1 and found a pair of sonically dated LF-353 op-amps and a power supply I can only describe as inadequate. The LF-353 is a dual version of National Semiconductor's LF-351. These Bi-Fet input devices offered fine performance in 1980, but they have been substantially outclassed in recent years. In my opinion, these op-amps no longer have any business in circuitry claiming high-end performance. (I believe the same is true of the 5532, but that hasn't stopped dozens of manufacturers from continuing to use this device, often in very expensive equipment.) The ED-1 has a hefty raw supply, with a large power transformer and a pair of 4700µF input filter capacitors. The output side of the three-terminal regulators is not nearly as impressive, being bypassed by a pair of 47µF caps. Local bypassing consists of only 0.1µF film capacitors, sufficient to prevent the op-amp circuits from oscillating, but grossly inadequate in terms of keeping the supply output impedance sufficiently low.
The Stax solid-state headphone amplifier is not representative of current design practice either. True, it is a class-A design (after an hour of use the heatsinks are so hot that they are painful to touch), but both the driver and output stages are quasi-complementary. I don't have a schematic, but each channel consists of a pair of identical output transistors as well as a pair of identical driver transistors. Quasi-complementary topologies were used in the early 1970s because of the problems in finding pnp power transistors that matched the more common npn types but are now rare. [I believe the Naim power amplifiers are still quasi-complementary.Ed.]
The Stax Lambda Pros are excellent headphones, but I believe that their performance is compromised by the dated Stax electronics. Although the Lambda Pros certainly deserve a Class A rating as headphones, the associated solid-state electronics fall far below that point. [The Stax Lambda Pro tube amplifier should be regarded as mandatory to drive these headphones.Ed.] In fact, I wouldn't rate the ED-1 equalizer or the SRM-1 Mk.II amplifier higher than Class C, with the amplifier toward the high end of that category, since I believe it fares a bit better than the equalizer. Given a choice between the Stax Lambda Pro/SRM-1 Mk.II/ED-1 as a headphone system, and the Grado HP 1 with a high-performance amplifier, I would have to opt for the Grados primarily because of their lower sonic coloration, slightly more extended bass, more spacious sound, and superior dynamics. The virtues of the Grado system, for me, outweigh the slight improvement in detail and high-frequency extension offered by the Lambda Pros. In fact, the superb detail and extended highs of the Stax headphones only magnify the deficiencies in the electronics. If the Stax were powered by electronics worthy of a Class A rating, it might be a different matter.
The Grado HP 1s are a powerful analytical tool for the recording engineer and electronic designer. If it seems unusual to devote this much space to a single headphone review, bear in mind that reviews of high-performance loudspeakers are often even longer. To the recording engineer working "on location," an accurate set of monitoring headphones is at least as important as the loudspeakers back in his/her studio or living room. The HP 1s allow the recording engineer to hear exactly what cables and equipment are doing to the sound, and their extremely clean performance makes it easy to spot such sonic menaces as not-quite-perfect microphone connections. Designers of high-end electronics will find the HP 1s useful tools for evaluating the sonic characteristics of individual components, such as capacitors, resistors, switches, potentiometers, and wiring. The HP 1s deserve a solid Class A rating; I recommend them highly.
One final commentalthough the HP 1s look as though they would be uncomfortable, they are not. I can wear them for long periods of time without any discomfort, and their eyeglass compatibility is excellent.