HPA4 Review - Gary Galo, Audioexpress.com

"You’ll have a tough time finding a cleaner, quieter, more transparent line amplifier and headphone amplifier anywhere, at any price. I found it to be a new reference. Once I heard it, I could not live without it, so I purchased the review sample. The HPA4 deserves my highest recommendation."

"The HPA4 is the latest product from Benchmark Media Systems, an upstate New York audio manufacturer. Benchmark calls it a headphone amplifier, but it’s really two products in one, combining the LA4 Line Amplifier introduced in 2018, with a headphone amplifier. It is also based on the same THX Achromatic Audio Amplifier technology used in Benchmark’s acclaimed AHB2 power amplifier."

"THX’s Achromatic Audio Amplifier topology uses feed-forward error correction to reduce distortion by 20 dB to 40 dB. The crossover distortion produced by conventional Class AB amplifiers is virtually eliminated. With feed-forward error correction, amplifier errors are extracted from the output audio signal, inverted in polarity and fed to the output to cancel the errors."

"Benchmark wanted a headphone amplifier with the same level of performance as the AHB2 power amp, so nothing short of the top-of-the-line would suffice. That top-of-the-line amplifier is the THX AAA888. The 888 is not a module, but rather a circuit topology that manufacturers can incorporate into their own PCB layouts. Benchmark actually participated in the final development of the 888 amplifier."

"The THX AAA technology keeps the distortion levels extremely low even when driving difficult, low-impedance loads. The headphone amp includes protection against DC offset, clipping, short circuits, as well as excessive temperature and output current. Any faults will trigger the output muting relays, protecting your headphones and your ears."

"The signal path in the HPA4 is entirely analog ... The balanced inputs are terminated differentially, to achieve the highest-possible common-mode rejection. Benchmark uses a double differential system, which is essentially differential in and differential out, so internally the amplification circuitry is also fully-balanced."

"Volume control is accomplished with a unique system of precision relays with gold-plated contacts, and low-impedance resistor networks made with 0.1% tolerance precision resistors. To keep the distortion levels as low as possible, Benchmark uses thin-film resistors with low temperature coefficients and conservative power ratings. The relay/resistor networks are actively buffered—the
buffering provides high input impedances and low output impedances. The volume adjustments are accomplished with a hybrid of attenuators inside the feedback loops of the active devices, and passive inter-stage attenuation."

"There are four independent, fully-balanced, 256-step volume controls—two for the line outputs and two for the headphone outputs."

"The attractive touchscreen display controls most of the functions of the HPA4. Channel balance can be separately adjusted for the headphone and line output groups."

"The volume control knob operates an optical rotary encoder that turns continuously in either direction. The encoder includes an acceleration feature, which makes it easy to make large changes in volume very quickly, while allowing precise adjustments when the knob is rotated more slowly. When rotated slowly, each detent results in a 0.5 dB change, so the 256 steps cover a 128 dB range."

"A Xilinx Spartan 6-series Field-Programmable Gate Array (FPGA) ... provides precisely timed, makebefore-break relay connections, eliminating the pops, clicks, and zipper noises often associated with relay switching, no matter how fast the volume control is rotated."

"The construction quality of Benchmark products has always been very high, and the HPA4 lives up to their reputation. This 8-pound chunk exudes quality as soon as you lift it out of the box. The front and side panels are anodized, machined aluminum, and the rear panel and bottom plate are 0.125” (3.18 mm) anodized aluminum stock. The front panel is 0.25” (6.35 mm) thick, matching the appearance of their DACs and power amp; the side panels are 0.84” thick (21.3 mm), which contribute to the robust feel, and also provide effective heat dissipation."

"Benchmark lists an array of impressive specifications for the HPA4. The line amplifier and headphone amp have -3 dB points of 0.1 Hz to 500 kHz, and THD of -125 dB (0.00006 %). The ultra-wide bandwidth ensures virtually perfect differential phase response well beyond the audible spectrum."

"Like the other Benchmark products I use, the line amp and the headphone amplifier achieve exceptional detail without any exaggeration in the treble region. The HPA4 is tonally neutral and does not contribute any brightness or harshness to the sonic presentation—it has provided me with many hours of fatigue-free listening."

"The audio professional will find the HPA4 to be an accurate, reference quality tool for evaluation of their work, whether in the recording studio or in the field. For the audio enthusiast, the HPA4 will provide listening that’s as musically satisfying as the recordings in their collections. The audio fed from your source is what you will hear—no more and no less. And, that’s exactly as I want it."

- Gary Galo, Audioxpress.com

Download the entire review (PDF)