Benchmark HPA4

If you’ve spent money on serious headphones for critical listening, could it be worth investing just as heavily in your headphone amp?

If you use relatively inexpensive headphones (even professional models such as Sennheiser’s HD 25s, for example), or use your headphones only to check stereo imaging occasionally, or to listen whilst tracking to make sure the performance went down well and that there were no unexpected, untoward noises or headphone bleed, you’ll be fine using the headphone amp built into your mixer or audio interface. But if you’ve invested in a great pair of cans and plan to use them for critical listening, so that you can really get into the nitty gritty of your mixes and masters, it’s worth considering what a decent headphone amp could do for your setup.

It’s worth me pointing out that this product category is aimed at both the pro‑audio and the hi‑fi audiophile sectors, so you do have to watch out for snake oil, but there are plenty of capable amps out there, and I’ve used quite a few in my time. SPL, for example, have good models to suit a range of budgets, and I’ve enjoyed units made by Violectric (a sister brand of Lake People) and Manley too.

Overview

Benchmark Media Systems sell to the hi‑fi sector but they’re well established in the pro‑audio sector too, and are known for their unusually high standard of engineering. I recently bought their HPA4, which is identical to their LA4 Line Amp except that it adds a hiqh‑quality headphone amp. Its signal path is entirely analogue, but you can pair it with a Benchmark DAC if you wish; the optional remote control can operate both devices so it’s a seamless partnership.

The HPA4 is available in a choice of silver or black finishes and in two configurations: there’s a 2U 19‑inch rackmount version and a table‑top version. The latter is a little less than a half‑rack space wide and, with its feet, just over 2U high. As well as having a wider faceplate, the rackmount version includes heatsinks to the left and right of the chassis, so it can ‘breathe’ in an enclosed environment (there’s a meaty PSU inside). Whichever version you prefer, the build quality is unassailable: this is a well‑manufactured and nicely designed piece of studio art that exudes quality in an understated manner.

On the front panel are two separate headphone outputs, one being a traditional quarter‑inch TRS jack and the other a 4‑pin XLR, and both are driven from the same (single‑sided) amplifiers. Although 4‑pin XLR connectors are usually associated with ‘balanced’ headphones, that’s not the case here; only the positive sides of the headphone connections are actively driven. Instead, Benchmark use the 4‑pin XLR simply to separate the ground returns from each earpiece (in suitably wired headphones). This provides a better‑quality connection and avoids potential distortion and crosstalk associated with the combined ground return currents via TRS sockets.

A single 256‑step, gold‑contact encoder controls the master level: each step increments by 0.5dB, so you can set the level very precisely. On the left is an LCD touchscreen. Initially I was skeptical, suspecting it might be a gimmick, but those fears were totally unfounded. The screen is used to change various settings and assign different parameters to the encoder knob. You can, for example, change both the input and output levels, assign the encoder to control phones or line volume, and name inputs and select them using an arrow icon. The data‑input style perhaps feels a little ’90s Nokia, but it’s easy enough and once done you can forget about it. Benchmark have also included an auto power on/off toggle in the menu, which I’ve found particularly useful since I have everything on a timed power sequencer. Among other things, you can also push‑mute the headphone or line output, push to dim the signal (‑20dB), and perform a factory reset (useful if you wish to sell it or can’t figure out a weird setting you’ve engaged). You can also turn the screen off when you’ve finished setting things up, or set it to turn off after a specified delay or period of non‑use.

As well as the expected analogue I/O and power inlet, the HPA4 includes trigger inputs, which allow it to switch on/off (or be switched on/off by) compatible devices.

An internal universal switch‑mode PSU connects to the mains supply via a rear‑panel IEC inlet. The rear panel also has two L‑R pairs of analogue inputs on 3‑pin XLRs, three sets of stereo analogue inputs on RCA phono connectors (these are tightly packed, so may not accommodate larger phono connectors), and three analogue outputs on XLRs (L‑R and mono, for a subwoofer). Finally there are two 12V trigger ports, which are fairly common in the hi‑fi world but rarer in pro audio: they enable the unit to start up the power on other devices, such as a DAC (including Benchmark’s own) or power amp, or to be started itself by another device.

In keeping with Benchmark’s reputation, the HPA4’s numbers are impressive. For example, the bandwidth extends between an astonishingly low 0.01Hz to over 500kHz, ensuring precise phase and timing accuracy. The THX‑888 amplifier design is capable of delivering high voltages into high‑impedance headphones, as well as high current into low‑impedance headphones, so it ensures the very best can be obtained from any type or make of headphone. The figures are 11.9V RMS into 300Ω and 6W into 16Ω, so there’s a huge amount of power provided here, but with clever built‑in protection systems to look after both the headphones and the amp.

Sound Opinion

I feed the HPA4’s input with the headphone output of my Crane Song Avocet II monitor controller at unity gain, and the HPA4’s XLR output feeds my Dan Clark Audio Stealth headphones (planar magnetic beauties with a 23Ω impedance), so to control the level of my headphones I use the HPA4’s volume encoder. It sounds great. There’s absolutely nothing in there to distract your ears from the source signal, and I can fully attest to the lack of harmonic intermodulation distortion when driving the HPA4 all the way.

Let me try and bring the sound to life with an analogy. When plasma screens first came along, if you compared them with the LCD technology of the time you’d experience a blackness and depth you’d not encountered before; with a good set of cans the HA4 delivers that same sense of depth and detail. It’s also a bit like stepping up from 16‑bit to 24‑bit recording: you can hear detailed reverb tails that explore a Bricasti’s chambers with the smoothness the designers intended, the tail decay of a well‑honed 808 bass drum becomes physical and visual. Put a guitar through a Vertigo VSM‑3 plug‑in and feel the rasp, as if it were brushed through by a fine‑toothed comb. I can listen to an Anthony Rother or a Carl Finlow electro production and really be swept away. And if a track includes well‑crafted distortion and saturation, then you can hear that and that alone; there’s zero coloration from the amp, no distortion leading you to make the wrong decisions. I could go on: what the HPA4 does, and does with aplomb, is provide good‑quality headphones with all the definition they need for professional mixing and mastering.

There’s absolutely nothing in there to distract your ears from the source signal, and I can fully attest to the lack of harmonic intermodulation distortion when driving the HA4 all the way.

I’ve also listened to lots of commercially released music through the HPA4. I’ll spare you the cliché ‘listening to Pink Floyd’ descriptions (which make me want to leave the hi‑fi shop instantly!) but rest assured that if the material sounds good it will sound great over the HPA4, and this amp will also reveal where a production doesn’t really stand up to scrutiny!

A final positive experience to share is that, as long as I was careful with monitoring levels, I could listen for ages through the HPA4 without becoming tired. That’s something to consider if you work a lot on headphones, which people increasingly do; I estimate that 95 percent of my Haarlem Conservatorium students do almost everything on headphones now.

The rackmount version of the HPA4 is the same except for the wider faceplate and heatsinks on either side of the chassis to aid cooling when placed in a rack.

Wishlist

I don’t have any significant criticisms, but do have ideas for improving the HPA4. On the hardware side, I’d appreciate the ability to update the firmware via SD card or USB. There have been some firmware updates already (my unit is Rev C), but there’s still potential to add useful features. I’d love Benchmark to exploit the screen more, for example by using it to display VU meters or a custom logo, which would be great for branding and security in a studio.

There’s one more thing I’d like to add: a crossfeed function, which partially emulates what it’s like sitting in front of speakers, can occasionally be helpful in a headphone amp. It’s not a dealbreaker — this amp will be staying with me for a very, very long time — but even if hidden in a menu it could prove very useful.

Heading Off

This is a brilliantly designed, almost addictive headphone (and line) amp, with the power to drive high‑quality headphones with the sonic transparency they deserve. It’s like bringing a microscope to the source, revealing every detail, whether you’re soloing an instrument or critically appraising a pre‑master. But it’s not at all fatiguing to listen through during long sessions. The unit is beautifully constructed inside and out too, and everything has a purpose; nothing is superfluous or ever gets in the way. The HPA4 is understated brilliance, and it has the potential to become a trusted and long‑lived studio companion.