Benchmark HPA4 (£3,499)​

Pre Amp​

Reviewed by Mr Underhill



A few months ago I had the pleasure of spending time with the Benchmark AHB2 power amp. This was a superbly transparent amplifier that I found especially impressive in my near field system.

The power amp includes the THX-AAA technology that I discuss in the hyper-linked review above. I immediately wondered whether this technology was available in a pre-amp. The answer was yes, but cheaper ones, with the exception of the Benchmark HPA4. I mentally rubbed my hands and spoke to my contact with great anticipation, only to be rapidly deflated, the THX-AAA is used on the headphone circuit only. However, my respect for Benchmark is such that I asked to borrow one and in due course the well packaged unit arrived.

The Benchmark HPA4 is what might be described as a utilitarian black box with an inscribed flourish of the Benchmark name, I love it. Additionally there is a three inch by four inch screen for the highly intuitive Graphical User Interface (GUI), which was my main control mechanism when used with headphones or my nearfield system. A remote control can be supplied for an extra fee of £100.

The HPA4 is supplied with a voluminous ring bound manual which I found to be superfluous, the GUI was easy to navigate, the needed functions were easily found and the unit was a joy to set up. Excellent design.

I set up what I felt to be a set of stiff tests, comparing the Benchmark HPA4 to:
  • The headphone amp to that on my Chord Hugo TT;
  • The pre-amp to the Chord Hugo pre-amp on my Hugo TT; and
  • The pre-amp to my Icon 4 passive pre-amp in my main system.



The Benchmark HPA specifications can be found HERE.

The pre-amp comes with two fully balanced inputs and two single ended inputs. These can be renamed and have their levels set and saved, allowing you to set up any input as an AV Bypass; a facility of which I made use.

In addition to the single ended and balanced left and right outputs Benchmark provide a summed mono output for use with a sub-woofer.

I won’t retread the ground I covered in the Benchmark AHB2 review. Suffice it to say that the THX-AAA technology used in the power amp is also used in the headphone section of this pre amp. That is not to say that there has been any lack of attention to the line-stage pre amp part of the HPA4 equation, far from it, as you can read HERE.

I asked Benchmark why THX-AAA is not used as a part of the pre-amp, the chief designer John Saui replied:

The THX AAA technology is only applicable to power output stages. Low power input and buffer stages can be controlled with conventional local feedback and Class A biasing. These low-power stages have plenty of speed and this enables the effective use of local feedback around each individual stage. In contrast, power output stages use large devices that tend to be much slower. When the stage is slower, feedback is less effective. These output stages must also be capable of driving reactive loads. These reactive loads slow down the response of the output and this limits the amount of feedback that can be applied before the output stage will become unstable. The THX AAA technology solves these problems by feeding a correction signal forward to the output. This correction signal removes virtually all of the distortion caused by the output stage as it transitions the threshold between the Class A and Class B operating regions. High power output stages are too slow to be fully corrected by feedback, especially when loaded with reactive loads. The THX AAA system provides feed-forward correction so that distortion can be precisely canceled in real time. Heavy loads, reactive loads, and slow output devices do not have any impact on the speed or effectiveness of the THX AAA feed-forward correction.

Our goal was to put the best-possible front end ahead of the THX headphone amplifier. In most applications, this stage would need to include an analog volume control. The volume control is the weakest point in most audio systems, so we set out to build the best-possible solution. No solution is better than relay-controlled resistor attenuators. We designed a state-of-the-art 256-step fully-balanced attenuator that is integrated with a 30-step relay-controlled gain stage. Our attenuator is completely different from other designs. It has far more steps than any competing design and it is fully buffered on both the inputs and the outputs. Because of the buffering, we are able to use very low impedances within the attenuator. The low impedances keep the total noise much lower than that of fully-passive designs. We also have a high-speed FPGA that times the relay closures to create a precise make-before-break timing to keep the audio free from any noise related to relays switching. This system is so effective that we can implement a soft fade (-20 dB dim button) that makes 40 gain steps in about 1 second without creating zipper noise.

I cringe when I see a conventional analog pot in front of a THX AAA headphone amplifier. The THX-888 headphone amplifier deserves the best-possible volume control and input state. That is what the HPA4 delivers.

Benchmark make the claim that the primary source of noise and distortion in a pre amp is from the passive components. Benchmark state that low noise requires low impedances, I won’t simply reiterate their position but I would encourage you to go to the hyper-link above and read the page for yourself. As I use a passive pre amp with autoformers, which have varying impedance characteristics, I found Benchmark’s position interesting. Later you will be able to read my aural findings.

As always the Benchmark specifications are refreshing to see, the ultimate question is whether these stellar measurements are reflected in what is heard?

Test Tracks​

The tracks were selected to allow:

  • Comparison of local and Qobuz sourced versions of the same tracks;
  • Comparison of standard and remastered versions of the same track;
  • The presentation of uncompressed LP rips served digitally;
  • Comparison of older and modern tracks, with their different mastering priorities.

This playlist is a subset of the music I listened to:


My headphones are the Sennheiser HD250 Linear IIs. These were well considered in the early noughties and I have felt no urge to update them since, I have found they work excellently with the Chord DACs I use, the Poly/Mojo and the Hugo TT.

I volume matched the Benchmark and Chord amps. My impression through the Sennheiser’s was that the these two amps are within a nat’s hair of each other.

Opium, Dead Can Dance (Qobuz 44.1/24)

This is a superb track with excellent bass, varied drum work, wonderful vocals and detailed high frequency percussion.

Chord Hugo TT (TT)

The presentation was a tad to the brighter side of neutral but very detailed. Percussive backing was dynamic and engaging.

Benchmark HPA4 (BM)

No lack of detail and with slight added weight in bass and with the individual instruments, this presented as being slightly warmer than the TT but perhaps simply musically truer.

Unforgettable, Aretha Franklin (Qobuz 96/24)

My top female vocalist for decades was Ella Fitzgerald, but Aretha has managed to wheedle her way into my conciousness over the years to the extent that I am now leaning toward her. Wonderful voice, phrasing and control.

It was damned difficult to pick differences between the BM and the TT.

Train Song, Holly Cole (Qobuz 44.1/16)

Another superb piece of music with wonderful bass and percussive effects moving around the sound stage. This was slightly better resolved and presented through the BM. High frequency effects, such as a brushed cymbals, were more evident via the TT.

Sand, Nancy Sinatra (Qobuz 44.1/16 – Problem Track)

Nancy Sinatra & Lee Hazelwood’s voices can move into the sibilant, especially Sinatra’s, this can make the track hard to listen to without reaching for the volume control. Via the Sennheisers this issue was reduced but any remaining effect was better controlled through the BM.

Let me be clear, picking differences between the Benchmark HPA4 and the Chord TT was not an easy task. The differences were minor, the music excellent and the task of trying to separate the headphone amps via the Sennheiser’s felt like a fool’s errand. I enjoyed both and could live with either.

Nearfield System​


Why on earth would you want to introduce another component between a DAC and the power amp? Surely another fool’s errand? My experience has been, no. A decade ago I bought a Bel Canto DAC3.5 VBS in part due to its inbuilt volume control and line level input. Here was a chance to simplify my setup AND improve the sound quality. During my initial evaluation it quickly became apparent that this was not the case. When I slotted in the Bel Canto Pre 3 VBS the sound quality went from lack lustre to excellent. Time to try the Benchmark HPA4’s line-stage and volume control up against the Chord TT’s inbuilt pre amp.

When I reviewed the Benchmark AHB2 I noted:

My immediate impression was of the sheer detail being presented. I can understand why this amplifier is valued by professionals, it is superbly informative. The Benchmark controlled my Sound Artist LS3/5a in a vice-like way. The control of the bass was such that I almost felt the speakers were working downwards by an extra octave.

My first impression of the Benchmark HPA4 was balanced detail. The bass seemed to gain a fraction of bass bloom, that ability to grow and decay, and voices were a tad warmer, never tipping towards the sharp but neither becoming thick.

The Chord Hugo TT’s bass seemed a bit more impactful and the music slightly more dynamic, but voices could more easily move toward sibilance and sharpness.

Lets discuss some specific music:

Tea for the Tillerman 2, Where Do the Children Play, Cat Stevens (Qobuz 44.1/24)

This is a modern re-imagining of Cat Steven’s original 1970 with Alun Davis reprising his efforts. It was released to middling reviews, I have to say that I appreciate the new mix and listen to it more often than the original album. I love both.

This recording has some added fullness in the bass that I find nicely balances the music and Stevens voice, which through the TT can be a bit cooler, is more attractive and human. The TT appears a touch more detailed, but I am aware that its tilt to the upper frequencies can aid this impression.

Hjaltalin, Chestmark, Hjaltalin (Qobuz 44.1/16)

Hjaltalin is an Icelandic group that formed in 2004. This track track opens with some conga/bongo drum work that is then backed with synths which leads in to some low level vocals. With the BM the underpinning bass was well rendered without masking any other detail, effects and backing vocals. The voices are warm and human.

The TT was a touch more detailed, but lacked that scintilla of vocal warmth. It was perhaps a tad more dramatic in its presentation.

Nancy & Lee, Sand, Nancy Sinatra (Qobuz 44.1/16 – Problem Track)

Nancy & Lee were favourites of my parents and so this music was inculcated in me from a very young age. As such I want to listen to their music to best effect. In the digital age this has turned out to ne somewhat more difficult that I would have anticipated.

As noted above Nancy Sinatra’s voice in particular can move into the sibilant and this is still evident through the LS3/5a, but marginally. The balanced nature of the BM in this system is even more enjoyable here than through the headphones.

Through the TT Hazelwood and Nancy’s voices were more clearly rendered, but a touch edgier.

Grace, Mojo Pin, Jeff Buckley (NAS 44.1/16)

This wonderful track is superb, but as with Sand can tip into the sharp if not properly handled. Yet again the BM’s slight vocal warmth assists in assuring that the voice never moves into being too hot, it makes the vocals magnetic drawing you into the song. The underpinning bass drum is produced with a good verisimilitude. There was no trace of sharpness and allowed me to simply love the music and relax.

The TT presented as being slightly more detailed; but, Buckley’s voice was a touch less inviting.

All Things Must Pass, Wah-Wah, George Harrison (Qobuz 96/24)

This is a track that can sound like a merry old mess. Not with the BM in place. I made a note stating that all was now rendered transparent, clearly delineated and musically coherent. This was the best I had heard this reproduced, it was music to my ears.

The TT rendered the opening lead guitar on the left channel more stridently. This can be interpreted as greater dynamic excitement, but my experience is that this becomes somewhat strained over time. Somewhat akin to the effect of noise in a digital chain. The music was more detailed but less cohesive.

This was an interesting judgement. The Chord Hugo TT reveals a bit more detail but is less musically satisfying to me. I know that with the Benchmark HPA4 in circuit I could listen to a wider range of music for longer through my near field system and at a higher volume, should I have so wished. I simply found the Benchmark HPA4 more musically enjoyable.

Main System​


Before progressing with my thoughts on the Benchmark HPA4 in my main system I think I need to mention system building and music.

Systems when synergistic can become greater than the sum of their parts, each has its part to play but one aspect in one piece of equipment will be balanced by a facet in another. Slotting in a piece of hardware into a pre-existing system will probably give you a flavour of its potential, but is unlikely to be fully optimised. It is for these reasons that I always include system schematics.

Music plays into system building and reviewing. A well trained voice is far more likely to please, especially if it has been well recorded. Most people who I know have catholic musical tastes, but they still have definite preferences. These can be furthered by technical considerations. Do you prefer the horizonless choices offered by streaming or your own locally ripped files? Do you hoard and play CD or LP? Or are you a bon viveur who wants it all? However you answer this you will have to do that terrible thing, compromise. It is for these reasons I always include lists of what I have been listening to and their source.

Slotting the Benchmark HPA4 into my system in place of my Icon 4 pre-amp I first listened to my dCS Network Bridge fronted digital chain:

Tea for the Tillerman 2, Where Do the Children Play, Cat Stevens (Qobuz 44.1/24)

Through my SBLs big bass was presented which had a nice bloom but no lack of pace. There was nicely delineated detail in the acoustic guitars left and right. At 2.50 there was an engaging step up in the macro dynamics. The individual instruments and cymbal work were well realised with supporting micro dynamics. The backing vocalists were more humanly resolved.

My system usually presents a tad more upper frequency detailing, but is also a bit more prone to letting you know about hot Ts and Ss, the sibilance monster. The BM tames this without sitting on the dynamics or detail.

Nancy & Lee, Sand, Nancy Sinatra (Qobuz 44.1/16)

This is a lighter and brighter sounding track. Sinatra’s Ss are still presented but somewhat ameliorated, no longer distracting, I had no temptation to reach for the volume control. The underpinning bass was invitingly present.

Soundtrack Stardust, Lamias Lair (NAS 44.1/16)

The pulsing bass and wonderful bass drumming was exciting. Great use was made of tom-toms to amp up the energy and engagement. Nicely judged breaks in the rhythmic urgency are used to support the movie narrative before stepping up the beat to re-enter the fray; this prevents this track from just being a rush and adds to its musical interest.

As good as this sounds through the Benchmark HPA4 I am used to a bit more energy and engagement as well as slightly more low bass detailing.

War of the Worlds, Brave New World, Jeff Wayne (NAS 44.1/16)

This is a marvellous bit of music, the Benchmark HPA4 had scale, being musically big and BOLD. The voices of Essex and Burton were well reproduced. The electric guitars left and right were well rendered and their interplay helped drive the track forward. The bass lines were easily followed and underpinned Essex’s vocals well.

I am used to a tad more detail and micro-dynamic punch, but also a bit more sibilant edge, although not to a distracting extent.

Native Invader, Reindeer King, Tori Amos (Qobuz 44.1/24)

This is a great track. The piano has a wonderfully rendered bass with Amos’s overdubbed vocals clearly presented and easily followed. The extensive use of effects were easily followed and in no way distracted from the musical flow.

As well presented as the track is through the Benchmark HPA4 I am used to a slightly bigger bass and a tad more detail. Amos’s voice was highly processed, but there were no issues through either pre-amp.

Testifyin’, Benny Green (Qobuz 44.1/16)

Excellent ensemble piece of piano led jazz. None of the instruments over-rides the others. The double bass plucks wonderfully support the music. The feel is very front row.

The Benchmark HPA4 does not provide as much textural information as I am used to.

The handling of the human voice through the Benchmark HPA4 would make this my preferred pre-amp over the long term. The dreaded intrusion of sibilants stops me relaxing in a track once they have reared their ugly head. I would compromise on other subtle aspects of the Benchmark’s presentation to gain this advantage.

The sibilant issue is NOT one that I find in the analogue presentation of music, time to fire up my Linn LP12, Naim Aro, Linn Troika, Naim Armageddon via my Tron 7 GT Phono Stage:

Nancy & Lee, Sand, Nancy Sinatra

I rescued this album from my father’s, he having had no turntable for a decade or so. Having cleaned the VERY dirty album the surface was hardly pristine, however the music was in NO sense light or bright, it was big and bold with fantastic bass and sound staging. Sinatra’s sibilants were not really present, but you could hear where those vocal effects were being tripped in the digital transfer.

As well presented as this was by the Benchmark HPA4 it was outdone by the added dynamism & detail of my usual pre-amp.

I could run through the various albums and tracks I used, but the findings were consistent. The strength of the Benchmark HPA4 with digital sources was its relative weakness with my analogue front end, which has a remarkable ability to just stay just the right side of too hot. The slight additional dynamics and detail gives a breath taking performance that was slightly down-played by the Benchmark HPA4.


I feel I have put the Benchmark HPA4 through its paces and that it has done well. Benchmark have put an enormous amount of work into their pre-amp and it shows.

With my headphones I feel it is every bit as good as my well considered Chord Hugo TT. Yes, there are audible differences, but nothing that gave one a clear lead over the other via my Sennheisers, just minor variations of emphasis.

In my nearfield system it took an already excellent system up another notch, it enabled me to listen longer to a wider range of music at a higher volume, had I so wished.

In my main system things are more nuanced and it is here that I wish to concentrate some more analysis. Above I emphasised the importance of system building, we never listen to one item in isolation. My system has been built around my Naim SBLs. I listened to these serendipitously when my previous speakers were killed by my cats. These proved to be the best fit of any speakers I have heard in my living room. The main source used to develop these speakers was the Linn LP12, so the synergy here is hardly surprising. That said the issues I have had with sibilants from digital sources have been universal and my current system handles them as well as any other, until the addition of the Benchmark HPA4.

The Benchmark HPA4 enabled me to relax while getting musical enjoyment from my streaming system in a way I truly appreciated.

What were my noted areas that I felt were a tiny bit weaker when using my turntable as a source? Textural information and micro-dynamics. Interestingly these are areas that I noted as strengths of the Benchmark AHB2. It would be interesting to try this partnership with a well matched pair of speakers.

The Benchmark HPA4 is a well designed attractive pre-amp with an intuitive interface. It is an excellent pre-amp that is worthy of both its asking price and a place on your audition list.