Benchmark HPA4

Hardware Review

Benchmark HPA4
Thursday, December 9, 2021
headphone amplifier & preamplifier
Kevin Fiske

Benchmark’s HPA4 is the first item of audio kit I have encountered that peels apart the ying and the yang of audio to expose the fault line between those of us for whom measurements are all that matters… and those of us that approach audio from a more emotional, less scientific direction. But in another sense it might also bridge the divide. 

The Benchmark HPA4 is a single-box headphone amplifier and line stage from a New York State company, whose products are widely used in recording studios and other professional settings, that seeks to push the artefacts hostile to good audio reproduction way down below the limits of human hearing. The aim is a purity, a veracity of reproduction, that makes Benchmark’s sonic truth, and the version of truth we might hear from some other brands, somewhat different.

The frequency response of the HPA4 is ruler flat between 10Hz and 200kHz with a -3 db point at 0.1Hz and 500kHz. THD with a 16 Ohm headphone load is -125dB or 0.00006% and -126dB on the line stage balanced output. With a 1kHz test tone, and set to deliver a 22dBu output, the line stage of the HPA4 prints the third harmonic better than -124dB and the fifth better than -141dB (0.000009%). The unweighted SNR 20Hz-20kHz is >131dB on the headphone output and >135dB on the line stage output. Be in no doubt, these are exceptional test results.

The HPA4 handles headphones from low impedance to high impedance with equal ease, giving 6 Watts and 9.8 Vrms into 16 Ohms and 440 mW and 11.5 Volts into 300 Ohms. There’s in-built protection from DC, over-Voltage and over-current, but no protection from the law of natural selection; if users are daft enough to exploit all that grunt and wind the volume to brain-melting level, then they are free to do so.

Open the user manual and after the pages describing setup we find 18 Audio Precision AP2722 test system graphs, plus accompanying technical notes. If nothing else this remarkable level of disclosure demonstrates the maker’s confidence in its engineering. It also stretches the comprehension of most buyers beyond breaking point. Really, it’s a banquet of specs that only electronic engineers will be able to savour.


There’s an important caveat to this technical Bacchanalia of course. The HPA4’s measured excellence cannot rectify errors fed to it or created after it. If the upstream or downstream components in the audio chain don’t support the HPA4’s unswerving approach to accuracy then the sonic result may well be disappointing. Commercial self-interest apart, that’s why Benchmark would prefer us to use the HPA4 to drive its AHB2 stereo power amplifier and employ one of its DACs as a source. All the components in the company’s eco-system of products employ the same uncompromising approach to measured accuracy with the aim of preserving signal integrity end-to-end.

Is this relentless drive for measured perfection worth the effort? At what level does THD become inaudible? Does Benchmark’s pursuit of better than three zeros to the right of nought-point actually make sense? Benchmark’s chief engineer John Siau says yes, absolutely. If total distortion is below the 0dB SPL threshold of hearing, we can guarantee that it will be inaudible. He points out that this level of performance is achieved, we are not relying on the music to mask the distortion; it is inaudible because it is reproduced at levels that are below the threshold of hearing. However, this changes depending upon the SPL we want to experience. At 80dB, peaks will reach about 100dB. Here, distortion must be lower than -100dB (0.001%) to be inaudible. For listening at 90dB, peaks will reach about 110dB and distortion must be lower than -110dB (0.0003%) to absolutely guarantee that it will be inaudible.

Under The Hood
The relatively small form-factor of the HPA4 – it is just 22cm wide – doesn’t provide a lot of rear-panel real-estate. Even so, the HPA4’s four stereo inputs, two balanced and two single-ended, will likely be sufficient for most users. There are also two stereo outputs, one single-ended and the other balanced, as well as a mono XLR output and two 12 Volt trigger sockets. 

On the front, the HPA4, particularly in its black finish rather than the alternative silver, is expressionless. There is no labelling to the knurled volume knob, the on/off button or the 4-pin XLR and ¼ inch Neutrik sockets. The colour touch screen is also black until a press on the on/off button causes it to blink into life to let you know the HPA4 is going through its start-up sequence. The touch screen allows source, input gain, channel balance and output mute to be controlled, and displays volume and mute status. 

To achieve its stellar stats the HPA4 employs four key elements of technology: an ultra-quiet switched power supply, an extremely high performance line input stage and an analogue volume control with 256 half dB steps, all three of which are Benchmark proprietary technology. The fourth element is a pair of THX-888 feed-forward amplifiers, built by Benchmark but licensed from THX. Like other Benchmark products the HPA4 is designed to handle the +28 dBu signal levels used in studios and other professional settings, as well as typical domestic-level signals that can be as much as 10 dB lower.


I asked Siau if he would shed more light for readers of The Ear on how the volume control and the other the key elements combine to produce such a notable technical performance. He responded: “The input stage delivers a pristine signal to the THX amplifier and the THX amplifier delivers a pristine signal into the complex loads presented by headphones or loudspeakers. 

“The THX amplifiers are really two amplifiers running in parallel. The first amplifier delivers the bulk of the output power while the second amplifier provides feedforward error correction to cancel the distortion produced by the main amplifier. These two amplifiers are passively summed at the output terminals. The feedforward correction is instantaneous, and it has a very high correction bandwidth. In contrast, feedback systems rely on recursive passes through a feedback loop, the correction is not instantaneous, and this limits the correction bandwidth. Feedback systems can also become unstable when an amplifier encounters a highly reactive load. The THX feedforward system remains stable when the HPA4 is driving difficult loads.”

The HPA4 input stage uses very high precision 0.1% and 0.01% metal film resistors in a fully balanced low-impedance (300-Ohm) binary weighted eight-stage relay-controlled passive attenuator. This is driven by an eight-step 4-stage relay-controlled gain amplifier, allowing volume settings over a range of 143 dB. The stepped gain is built into a balanced pair of differential amplifiers constructed from discrete precision resistors and op-amps and connected through relays directly to the input XLRs.

Siau says this arrangement allows exceptional common mode noise rejection at all gain settings. “As far as we are aware there is no other analogue volume control that comes close to replicating the transparency we have achieved. It has the lowest distortion, lowest noise, most precise L/R matching, and widest control range of any analogue volume control available at any price.” 

Sound quality
It had been the arrival of Audeze’s latest flagship planar headphone the £4,000 LCD-5 that had prompted me to ask if I might borrow an HPA4. With the LCD-5 being a statement 14 Ohm/90db design, I felt I owed it to Audeze and myself to try driving the headphone with an amplifier whose specs suggested that it could easily cope with the requirement for current, and which pound for pound, was in the same price band as the headphone. My own Graham Slee amplifier had enabled me to write a confident evaluation of the LCD-5, but it retails at less than £700.


Benchmark says its products do not require a burn-in period, measuring identically straight off of the production line, or after months of usage. That was not my finding. New out of the box and tried back-to-back with two other headphone amplifiers, the sample HPA4 sounded dark, shut-in, with an almost mono presentation, so much so that I actually put it aside intending to ship it straight back to the distributor. A few days later curiosity made me retrieve it and set it running in order to put a further 100 or so hours on it. 

On being compared for the second time with the other two alternatives the subjective experience now accorded with the measurements: the HPA4 had developed a prodigious apparent bandwidth with the most cavernous and detailed soundstage I had yet to encounter in any headphone amplifier. Clearly, it was going to be worth extended listening after all.

The LCD-5 and HPA4 made an impressive partnership, the ultra-low noise and distortion of the amplifier enabling the LCD-5’s similarly rigorous standards of engineering to shine, resulting in high resolution with impeccable neutrality for what can only be described as reference-standard playback.

Familiar material such as Marcin Wasilewski’s reflective and beautiful album Spark of Life sounded at once more detailed and tonally pure than I had heard it many times before. I am generally an admirer of ECM producer Manfred Eicher, but have never felt his treatment of piano to be up there with the best that can be done with this, surely the most difficult of instruments to record. Spark of Life has the ECM sonic imprint, with Wasilewski’s piano sounding oddly damped yet at the same time hot with intermodulation that tends blur the notes. Of course, the HPA4 teamed with the LCD-5 could not right the recorded wrongs, but it did open a cleaner window into the recording so that saxophonist Joakim Milder’s warm and sensitive tone could be heard weaving around and inside Wasilewski’s piano phrases with notably greater clarity than I have heard before. 

On more energetic material, such as Marcus Miller’s M2 album from 2001, and the aptly named track Power, the LCD-5 showed that the HPA4 couples deep reserves of low-end grunt with a good level of dynamic expression. At loud but not silly volumes, the boom and snap of Miller’s slap bass technique had timing and tonal qualities that to my ears – alas, I do not have Miller’s talent but I do play bass – sounded uncommonly real through the LCD-5 headphone.


I tried the HPA4 as a line stage, driving the household Bryston 4B3 power amplifier and PMC MB2se speakers. The Benchmark gave me one of those rare moments in audio when a lucky collision between two brands and two different sonic approaches delivers a result better than the sum of the parts. The Bryston and the HPA4 absolutely loved each other; the HPA4’s transparency and lack of audible distortion combining with the amp’s iron fist in a velvet glove to give performances that had heads nodding, feet tapping and faces wearing wide grins. What we heard was music. No apologies or explanations required.

Sound stages were served up to a level of resolution that made listening to good recordings almost an audio-visual experience. Benchmark’s channel phase accuracy is not an academic metric but a key element of technical performance that allows sonic images to have a locked-in-space quality. On the track Flight Of The Falcon from Bill Evans’ album The Alternative Man, the soundscape had a precision in its front to back layering, and the left to right location of performers, that made it easy to conjure up a visual imagine of the scene in the studio at Planet Sound in New York. 

An even better recording is Translation by the Dylan Howe Quintet. Son of former Yes guitarist Steve, Howe the younger is, I think, among the most complete jazz drummers currently working. Not only is the recording’s capture of Ross Stanley’s piano playing a real treat for the ears – ECM take note – but Howe’s drum kit was rendered by the HPA4 in spotlit sonic 3D, so deeply and accurately was it possible to hear into the recording space. 

The HPA4’s transparency and sound-staging abilities made listening to vocals an arresting experience too. Étienne Mbappé, the Cameroonian bass player who has worked with John McLaughlin and many other luminaries, released his album Su La Take in 2008. It is one of my go-to discs if ever I need a cheer-up, so infectious and joyful are its west African rhythms. On the track Musango, which translates as Let Peace Be With You, the HPA4’s transparency showed that Mbappe’s voice has an almost luminous, lullaby-like quality, and it rendered his closely-miked gentle voice so grain-free and so intimately that I was blindsided by a teary wave of emotion.

Turning to orchestral music with constantly shifting multi-instrument layers that pose a stiff test of timing integrity, I played Dvorak’s Eighth Symphony and the 1999 recording under Sir Colin Davis. I have played it many times before, but the HPA4’s resolution drew my attention for the first time to the noises off. Nine seconds into the adagio, someone – I assume Sir Colin Davis – can be heard taking a sharp nasal breath before humming along as the mass weight of the strings leans in for the first time. Listen on and it becomes clear the whole recording is replete with various noises off; score pages being turned, restless feet and so on. Oddly perhaps, I did not find this distracting, but actually found it a welcome you-are-there addition to my experience of a wonderful recording.

Might this resolution might come at a price, though? What I was hearing from the HPA4 both as a line stage and as a headphone amplifier, along with the forensic level of detail, was on some material a somewhat less rich degree of tonal density than I was used to. On Hans Zimmer’s Live In Prague double album, the HPA4 was transparent to a fault, peeling apart and presenting in a satisfying way the complex layers when the chorus and the instrumentalists combined. Still, and despite Zimmer’s fondness for a Spector-esque wall of sound underpinned by keyed infra-bass, for example on the album’s medley from Pirates of the Caribbean, the Benchmark made the recording somehow less wall-like than before.


Siau said that my reaction is very common, and that what I was experiencing was a result of the HPA4’s ultra-low noise floor and perfect phase, coupled to the suppression of distortion to a level that puts unwanted harmonics quite beyond human hearing. “…If distortion has always been in your playback system [to a greater or lesser degree] the music doesn't sound right when the distortion is missing,” he said. “But, once you lose your taste for harmonic distortion, it is hard to go back.”

What Siau is claiming – and the measurements confirm – is that the HPA4 leaves no audible artificial harmonic wash over recordings, no matter where we set the gain control. What we hear from the HPA4 is only the natural, recorded harmonics, so, says Siau, the presentation can seem less tonally full, dryer, less warm, if we have been used to subtle euphonic colouration.

This touches on a contentious area of audio design. A sizeable cohort of amplifier manufacturers, mostly those using valves as gain devices, seeks to persuade us that some audible harmonics are harmless. The second cohort is those designing with solid state devices which, as Benchmark has shown, can enable harmonic artefacts to be suppressed to the point where they are not just inaudible but actually so low they are difficult to measure. Here, some allow a level of harmonic artefacts in order to achieve what might be perceived by buyers as a warmer, or more analogue sound. No shame there, I think. As Nelson Pass has observed, this is the entertainment business after all.

Serving professional users as well as domestic market, Benchmark’s schtick is a little different. It boils down to this: if you want to make a perfectly rational and valid consumer choice and opt for music replay that is coloured by the amplification chain? That’s fine with us. However, if you want to hear the sonic truth then, as they say, you know who to call.

So, what to make of the Benchmark HPA4, is it an emotionally dry engineer’s delight? It is certainly that, but as a paid-up romantic myself I’d encourage anyone in the market for a high-end headphone amplifier to audition an HPA4. It is a device that achieves stellar measurements, yet sounds excellent too; evidence that the science-based approach can deliver high musicality. The HPA4 can drive pretty much anything, its resolution is truly remarkable, and you will likely hear your music with an acuity as never before. And, as an ultra-low distortion line stage feeding the Bryston 4B Cubed, the Benchmark HPA4 also made a compelling case for itself. With the rider that careful matching with other choices of power amplifier may be required, the Benchmark HPA4 can be confidently recommended. 


Type: headphone amplifier and preamplifier
Analogue inputs: 2x single ended RCA, 2x balanced XLR
Headphone outputs: 1/4” (6.3mm), 4-pin balanced
Preamp outputs: single ended RCA, balanced XLR, mono XLR
Headphone Impedance range: 16 – 300 Ohm
Gain settings: variable
Output power: 6W into 16 Ohms, 440mW into 300 Ohms
Finish: anodized black or silver 
Dimensions HxWxD: 98.5 x 220 x 212mm
Weight: 3.6kg
Warranty: 2 years

Manufacturer Details: 

Benchmark Media Systems

Distributor Details: 

T 03301 222 500