HPA4 Review - Herb Reichert, Stereophile

"While removing the Benchmark HPA4 headphone amp, I noticed three things. First, I noticed that the amp had shipped from Syracuse, NY, where it was made. Second, one whole side of the box said "Benchmark. . . the measure of excellence!™" As I cut the clear tape with my boxcutter, I noticed the "THX Technology" logo printed on it and wondered why it was there."

"After admiring the black, 0.5"-thick, brushed-aluminum side panels, I looked at the HPA4's 0.25"-thick brushed aluminum front panel. The first thing I noticed was a 3.5" Color TFT touchscreen and thought, "I hope that screen has a dimmer." (It does.)"

"Looking at the chassis' back, it finally hit me. This thing is not just a headphone amp; it has four (!) line-level inputs: two unbalanced (RCA) and two balanced (XLR)."

"Then I peeked again at the instruction manual..."

The HPA4 is not just a "Reference Stereo Headphone Amplifier"; it is also a "Reference Line Amplifier" with "Relay Gain and Input Control." 

"I didn't know what that last thing was, so I checked Benchmark's website and consulted via email with John Siau, Benchmark's vice president and chief engineer."

Relay-Controlled Gain

"Siau explained, "The Benchmark attenuator is actually a fully balanced relay-controlled gain stage. It applies up to 15 dB of gain or up to 127.5 dB of attenuation for a total range of 142.5 dB. There are 286 volume control steps in precise 0.5 dB increments. Of these, 256 are mapped to the user volume control. The remaining 30 steps are reserved for the balance control and for the input boost and cut functions that allow ... level matching of the four analog inputs. When this is set properly, the user can switch between sources without experiencing a change in volume level.""

Listening with Loudspeakers

"I replaced the hybrid (transformer-tube-JFET) Rogue Audio RP-7 line-level preamplifier with the Benchmark HPA4."

"Digital came from my reference HoloAudio May (Level 3) DAC feeding the balanced inputs of the HPA4. The Benchmark's 30 ohm output impedance fed the 33k ohm input impedance (via unbalanced Cardas Clear Cygnus) of a Parasound A21+ stereo amplifier powering Harbeth M30.2 monitors."

"During my first day of HPA4 listening, I noticed that every digital and analog recording was sounding curiously similar. They all sounded cleaner, better sorted, and more macrodynamic than they had with the Rogue, but strangely, compared to the RP-7, hall resonances and piano note reverb tails seemed shorter to me."

"The illusion of a sound space was enormous, but the HPA4's "enormous" sound space was different: images of performers inside the illusion appeared more distinctly outlined and concrete than I routinely observe with the Rogue. This sense of image concreteness appeared to be caused by, or enhanced by, the fact that the "empty spaces" surrounding these instrument and performer images were now the most air-free, glare-free, nonrefractive environs I have encountered while listening to home audio."

"The HPA4's sharp-focus specificity is something I associate with recording studio sound but almost never encounter in home systems."

THX-888 Headphone Amplifier

"The THX-888 headphone amplifier board "is a unity gain (current) amplifier with feedforward error correction." Feedforward error correction is also used in the AHB2 power amplifier."

"According to Benchmark's website, the HPA4 can deliver up to 11.9V RMS into 300 ohms and 6W into 16 ohms."

"The THX-888 headphone amplifier board in the HPA4 is manufactured by Benchmark under license from THX."

Driving HiFiMan Susvara Headphones

"With most headphone amplifiers, the tires go flat trying to drive HiFiMan's Susvara open-backed planar-magnetic circumaural headphones. The Susvara's low (83dB/mW) sensitivity and flat 60 ohm impedance need at least a few watts, probably a third of an amp, and maybe 7V, plus 15–20dB gain, to play effortlessly and sing with their clearest voice."

"Listening with the HPA4 and Susvara—I used the HPA4's balanced outputs for all my Susvara listening—and playing the full hour of Horowitz in Moscow (16/44.1 FLAC, Deutsche Grammophone/Qobuz) was the first full-length unmitigated musical pleasure I achieved with the HPA4."

"On this recording, with the HPA4, the notes from the master's piano were displayed with a weighty force and overtly pacey momentum that showed me more hammers and less pedal."

"The Benchmark-Susvara pairing moved the performances right along, emphasizing the authority of Horowitz's execution."

"After my HPA4-Horowitz experience, I had a feeling the Benchmark's absolute clarity coupled to the Susvara's legendary resolving powers would finally show me what the album's producers heard."

Powering Abyss TC Headphones

"Sticking with planar-magnetics but moving up the sensitivity ladder (to 88dB/mW), I again played Tango: Zero Hour, listening this time with the JPS Labs Abyss AB-1266 Phi TCs."

"Ultimately, the Benchmark-Abyss combo really clicked for me. It gave me a fresh, intriguing look at my most familiar recordings."

"The HPA4 exhibited no difficulty driving low-sensitivity, low-impedance planar magnetics."

Listening to ZMF Vérité Dynamic Headphones

"Next, I wanted to see how the THX 888 would fancy the 99dB/mW-sensitive, 300 ohm ZMF Vérité closed-backs."

"If you are considering buying or already own an HPA4, I recommend you audition the Zach Mehrbach–designed Vérité closed-backs. They were my favorite music-listening headphone with the HPA4."

Driving Focal Stellia Headphones

"The ZMF Vérité and the Focal Stellia are my references in the closed-back category."

"Of all the headphones in this test group, the 35 ohm, 106dB/mW Stellia present the easiest-to-drive load."

"Like high-quality, high-sensitivity floor speakers, the natural ease with which the "sound of music" flows out of them is the main reason I keep the Stellia in my daily-driver desktop system."

"The Stellia's ease of flow was not diminished while being driven by the Benchmark HPA4."

"Who would have imagined?"

"My romantic-dreamer mind adapted surprisingly well to the Benchmark HPA4's presentation."

"The more I used the HPA4 to drive high-resolution headphones, the more comfortable I felt with its precise, pro-audio recording-studio aesthetic."

"Spending a month with the HPA4 forced this old triode-tube lover to first acknowledge, then accept, and finally enjoy a new type of engagement, one that satisfied my engineering mind and my audiophile lust to look and listen into the hidden nooks and distant corners of every recording. Thank you, Benchmark."

- Herb Reichert, Stereophile

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