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HPA4 Review - Kevin Fiske, the-ear.net

by Benchmark Media Systems September 12, 2021 4 min read

HPA4 Review - Kevin Fiske, the-ear.net

"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 aim is a purity, a veracity of reproduction"

"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."

System Considerations

"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."

Audibility of THD

"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."

"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."

Four Key Elements

"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."

"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.”"

Listening with the Audeze LCD-5

"It had been the arrival of Audeze’s latest flagship planar headphone the 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."

"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."

"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’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."

-Kevin Fiske, the-ear.net

Read the full review here →

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