When the folks at Benchmark suggested a review of their HPA4 for Tape Op, my first reaction was, “Isn’t this aimed more at the audiophile market?” Benchmark’s Rory Rall replied that I should consider its use as a reference-grade monitor controller, with a world-class headphone amp built-in. He recommended I pair it with my Benchmark DAC3 B digital-analog converter and use it to drive my Benchmark AHB2 [Tape Op #111] power amp. Rory’s final suggestion was to set the unit’s gain staging so the DAC3 B uses neither of the jumper-set 10 dB or 20 dB attenuators and instead set the AHB2’s input sensitivity switch to its minimum position. This sets up something akin to an old-school 600-ohm power-matched system where the source component is a relatively high output, and the first output gain stage comes from the HPA4, with the AHB2 doing less heavy-lifting. Benchmark says that setup allows for the lowest noise floor with a nearly unmeasurably low distortion.
When I previously reviewed the Benchmark AHB2, I described it as being as close to a straight wire with gain as I’ve ever heard – “a silent transmitter of musical energy.” I stand by that description after four years of living and listening to it. With the DAC3 B and HPA4 ahead of it, the whole signal chain runs silently and transparently. The digital bits come in, the DAC3 B strips out the jitter, and then the HPA4 becomes the signal director. I’ll get to the headphone amp portion in a bit.
The HPA4 is built on the idea of avoiding any sort of bit-stripping volume control in the digital realm. So a full-output DAC sends it line-level audio (in the case of the DAC3, with no attenuator resistors jumpered in, that’s quite a hot output level). Inside the HPA4 is a precision relay-controlled analog attenuator, with exact channel-to-channel matching and a measurable/repeatable gain or attenuation (expressed in dB on the large color touch-screen front panel display). The volume knob is a 256-step precision encoder, driving relays that engage resistors, in 0.5 dB steps.
There are two XLR balanced inputs, designed to accept true +24 dBu = 0 dBFS sources. There are two RCA unbalanced inputs, which are boosted +15.8 dB to match pro-level input levels. There is a +/-10 dB adjustment available through the menu system on the touch-screen. With the volume control set to maximum (0 dB attenuation), output is at unity gain to the balanced inputs. You can memory-set a gain level for each input, so for instance, if you wanted to compare an analog source to a digital transfer, you can precisely level-match. Or (like I often do) measure an original LP release against a digital remaster. Or set your mixer’s output with what’s coming out of your digital rig. The possibilities are limited only by your line-level sources and the HPA4’s four inputs.
As for outputs, there are stereo balanced line-level pro-grade XLR sockets, plus a Mono XLR jack. Benchmark explains this mono/sum jack can be used to drive a powered subwoofer – clever! There is also a stereo unbalanced RCA output, however, the control system doesn’t allow you to switch between the outputs, which is a handicap for use as a monitor controller. Maybe Benchmark can address this with a firmware update?
Finally, in what may be a case of “burying the lede,” allow me to rave about the HPA4’s built-in headphone amp. Designed using similar technology as the AHB2 and licensed from THX (Benchmark calls it the THX-888 Amplifier), this is the cleanest, quietest, most neutral headphone amp I’ve ever heard. I love my Little Labs Monotor [#117], but the HPA4 offers a different level of neutrality and power. It might be too much neutrality for pleasure listening to all music on all headphones, but this type of clarity is important for a modern mixing and mastering setup. We have to turn out audio that sounds good on headphones, and even earbuds, because it’s how a lot of people listen nowadays. In the audiophile world, deluxe headphone-only systems are a growth market, and top-drawer headphones are getting better (and more expensive) all the time.
Right now, my go-to headphones are Neumann NDH 20 closed-backs [#132] and Sennheiser HD 650 [#43] open-backs. They are different-sounding, but both represent my kind of music, and the human voice, very well. Neither one of them is particularly hard to drive, but they’re not as easy to make loud as an Audio-Technica ATH-M50 [#63], for instance. The HPA4 has no trouble playing music dynamically and precisely through any headphones I own (including the 600-ohm version of the AKG K240). For example, listening to music at comfortably loud levels through my Neumanns with a USB input on the DAC3 B, I don’t turn the HPA4 any louder than -30 dB, and lower than that for rock music. Putting it in numbers, the THX-888 headphone amp is capable of peak-to-peak voltage swings of 25V and an output of 1.5 amps. It puts out 6 watts into 16 ohms! This should give you the idea that this thing will drive any low-efficiency cans you want to plug in. On the front panel are a standard TRS headphone jack and a newer style 4-pin XLR connector.
For all this silence and neutrality you have to pay a pretty penny, but is it worth it? As with all pieces of your system, it depends. For a precision monitoring setup for mastering, detailed mixing situations, or even a top-shelf listening system, you’ll have to look hard (and maybe spend more) to get this close to the fabled “straight wire with gain” sound. However, at this price level, I do wish the HPA4 had flexible output switching and level-trim memory for the outputs. It would also be good to switch that mono/sub output on and off via the front panel. The remote might also be reworked to allow push-button output switching. That said, especially when working or listening with headphones, I can’t think of anything else that will get you closer to the sound coming out of your sources. Combined with Benchmark’s DAC and power amp with excellent speakers and headphones plugged in, this is quite the monitoring chain. When playing music you love, it will fire all the pleasure synapses. Available in black or silver: With or without remote.
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