It took a while, but Pass Labs is finally selling a headphone amplifier. One can only guess why Pass hasn't introduced one until now. I doubt it is because they felt that there really was no reason to make one, as Pass Labs has a reputation of communication with their customer base, and I can't imagine them not being aware of the fact that many of their customers are headphone devotees. So, perhaps Pass Labs didn't want to introduce a headphone amplifier until they got it right; that is, wanted to wait until there were sufficient models on the market to find which design parameters they would pursue, along which what features they would make available.
And so, here we have it, the Pass Labs HPA-1 headphone amplifier, which Pass Labs claims: "Like the XP preamplifiers, the HPA-1 is the result of great attention to detail". There isn't much to go on that vague claim, other than to correctly assume that the HPA-1 has been designed as a power amplifier rather than a headphone amplifier. Therefore, it can drive headphones with a very low impedance, such as some planar models. This unit is built more along the lines of a two-input preamplifier than most headphone amplifiers on the market. As you'd expect, this is not just any preamplifier, but a Pass Labs preamplifier.
Technically, the HPA-1 is a rather simple device. It's built like a tank, but still relatively simple, at least when compared to one of Pass Labs' full blown preamplifiers. Those who are familiar with the brand are well aware that Pass Labs doesn't make things more complicated than they really need to be. And so, the HPA-1 is a headphone amplifier and a headphone amplifier only. Although it has a preamplifier output and can easily be used as a two-channel preamplifier, unlike many headphone amplifiers on the market it does not include an internal digital-to-analog converter. Nor does it have a remote control, and it is not Bluetooth compatible. It does not have an iPhone/iPad input, and it only has unbalanced inputs and outputs.
In Pass' literature they stress the importance of the HPA-1's power supply. When the AC enters the cabinet the high frequency noise is filtered from the AC line. It is then isolated before it reaches its toroidal transformer that is shielded by Mu-metal to lower magnetic noise, and electrostatic noise is filtered by a separate Faraday shield, which is grounded. Then things get complicated enough for me to quote Pass directly: "This is followed by fast/slow recovery rectifiers, generous CRC passive filtering and sophisticated discrete DC regulators and then more passive RC filters". Pass Labs uses a switching system in the HPA-1 that uses relays that are spec'd to be able to be used to switch moving coil levels. The HPA-1's volume control is "the highest quality Alps dual potentiometer". The circuitry of the HPA-1 is a "simple Class A two-stage CFA topology using cascoded ultra-low noise Toshiba JFETS driving complementary Fairchild power MOSFETS". Pass concludes their promotional spiel by simply saying "The measured performance is superb, and the sound even more so". All switching and mute functions are handled by a custom programmed microprocessor and in the event of a loss of mains power the unit will revert to the state it was before the power loss occurred.
The HPA-1 can drive headphones with impedances as low as 15 Ohms and as high as 600 Ohms. It is a Class A design with a direct coupled MOSFET output stage. The Pass Laboratories HPA-1 headphone amplifier's cabinet is rather large. At 11" wide, 13.5" deep, and 4.25" inches high, it will never be mistaken for a portable unit. On its front panel it has a Neutrik locking headphone connector, which is seen more and more on components designed for home use, but have been used in the broadcast world for quite some time, where a disconnected headphone cable can be disastrous. This Neutrik connector has a locking mechanism which maintains the signal integrity between the headphone amp circuitry and the headphone's cable. It has silver contacts that are more reliable than the standard contacts that are usually used, and this has the potential to be better-sounding and more trouble-free. Insertion of the headphone jack is relatively normal, but removing the jack requires a bit of finesse. Which is good, as it pretty much guarantees that the jack will not be accidentally disconnected. One must press the red button on the front of the connector and held in as the plug is removed. I became accustomed to this rather quickly, as I pushed in the button with my thumb while hold the plug with the thumb and forefinger at the same time.
The front panel layout of the HPA-1 headphone amp will be instantly recognized by many as a Pass Labs product. On the far left are three buttons that are used as input selectors and the preamp out. In the center of the front panel is the headphone jack, and slightly to the right of that is a large volume control. That's it. On the HPA-1's rear panel, from left to right, is the power switch and the fuse, an IEC power cord input, the stereo preamp output, and inputs for two sources, that is, two pairs of unbalanced inputs. As the power switch is located on its rear panel, I took that as a hint that Pass Labs would recommend that I leave the HPA-1 powered 24/7. They did, so I did.
For a time during the review period I was using a Luxman C-700u control amplifier, so I took advantage of its "rec out" (record output) on its rear panel. To this output I connected a long run of older Cardas Quadlink interconnect and connected it to the input on the Pass Labs headphone amp. The Auralic DAC has both unbalanced and balanced outputs that work simultaneously, so when the Luxman was no longer in my system, I connected the unbalanced Cardas cable directly to the DAC. The Pass Labs phono preamp has the same output scheme, so when playing records, I simply move the cable from the DAC to the spare unbalanced output of the phono preamp.
Headphones used for this review include Grado's top-of-the-line SR-1000e. Its level of transparency, exquisite frequency response, low distortion, and excellent transient characteristics make it a perfect choice for a headphone amplifier review. I also have the Oppo PM-1 headphone, which offers a different sonic perspective due to its planar magnetic driver. The larger area of the driver that covers the interior of the headphone gives the appearance that the instruments and voices are farther apart from one another, and a "soundstage" that is unique to this type of headphone. It is also an extremely comfortable set of cans, so longer listening sessions are possible. Last but not least, I also used my vintage Sennheiser HD-600 that I purchased in 2007 and had re-built by Sennheiser USA last year. At one point this headphone was called "The best in the world" by at least one large audiophile publication. Sure, times have changed, but these headphones are still being sold, and remain at the top of the list for many listeners – and not only because they are such a bargain.
There are some who are going to be quick to point out that the Pass Labs HPA-1 is a solid-state unit, and because they might claim there will be limitations to its sound in the areas of "sweetness" and "naturalness". I'm sure if I delve into the body of work that Nelson Pass has written on the subject I'm sure I'll eventually come across his many treatises where he discusses how he's discovered how to get his solid-state components to exhibit the qualities of tube units without any of the sonic disadvantages that come with it, and therefore construct a unit that is neither solid-state sounding nor tube sounding. Pass Labs isn't the only one these days designing components that don't exhibit the stereotypical sound of their innards. To me, Pass Labs is one of the leaders in this field. And so, the Pass Labs HPA-1 headphone amp doesn't sound like a typical solid-state unit, nor does Pass Labs deliberately voice the HPA-1 to sound like a tube unit. It just sounds like a great headphone amp. Period.
Afterwards we'll talk, and they'll explain to me the same things that I found when listening to this selection through the HPA-1: that instruments and groups of instruments occupy spaces within a sort of aura around our heads. These spaces are occupied by extremely realistic sounding reproductions of what is on the recording. But these instruments and groups of instruments are connected by an unexplainable thread, that is pace, timing, dynamics, or what some might simply call music, the space between the notes. The same type of experience (sorry) can be had with Jimi Hendrix's 2nd studio LP, Axis Bold As Love, the latest all-analog pressing on Sony/Legacy. During the second track "Up From The Skies", Jimi's voice enters in the right channel. I've witnessed guests almost injure themselves as they quickly whip their head around, looking for the person who has invaded their personal space. Of course this sensation lasts only a micro-second or two. Imagine becoming accustomed to this sensation? The beauty of a great headphone and headphone amplifier combination is that the audiophile is able to "see" into the recording to hear details that are lost when listening to speakers that are placed in an environment totally unlike the room in which the recording was made. I'm lucky in that I was able to use headphones good enough to pair with the Pass Labs HPA-1, a headphone amplifier which is an excellent component, one which excels at being able to translate the electrical signals it receives to the headphones.
One might desire more features in a preamplifier, but when I inserted the HPA-1 between my sources and my power amplifier I was floored. All the sonic qualities that the makers of the HPA-1 should be proud of when used as a headphone amp are present when used as a preamp. I wish it had a bit more gain for my beastly speakers, and balanced inputs would be nice. I've reviewed and otherwise auditioned many other headphone amps where the manufacture claimed the headphone amp can double as a preamplifier. Simply being able to function as a preamp is often all they could do. The ability to switch between a couple of sources, provide attenuation, and have a circuit that can provide a bit of gain does not often translate into real world functionality. The Pass Labs HPA-1 on the other hand, can provide these functions plus sound good whilst doing so. I drove a long length of cable between it and my reference power amp, and spent quite some time enjoying both digital and analog sources to the fullest. No, it can't compete with a $10,000 preamplifier designed exclusively for the task, but sound-wise, it comes pretty damn close. Amazing.
This is not to say that the HPA-1 sounds like a scientific instrument to dissect our favorite recordings. The HPA-1 excels in every area of what makes being a headphone loving audiophile so attractive in the first place. It places us within the recording, makes us part of the recording, and lets us hear what the artist intended for us to hear -- with scary-real reproduction of acoustic instruments and voices, and with the highest fidelity available to reproduce the sounds that can only be made in the studio or electronically. Kudos to Pass Labs for making the HPA-1 headphone amplifier available to all those that can afford one. I highly recommend an audition, and especially recommend a purchase.