"I measured the Benchmark HPA4's performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), repeating some tests with the magazine's sample of Audio Precision's current top-line APx555. As a line preamplifier, the HPA4 performed almost identically to Benchmark's superb-measuring LA4, which Kal Rubinson reviewed in January 2020, so I will refer you to that review. This report therefore focuses on the HPA4's behavior as a headphone amplifier."
"The gain for the balanced inputs to both the headphone outputs with the volume control set to "+15" was exactly 15dB. Reducing the control to "0.0" resulted in a gain of 0dB, ie, the output voltage was the same as the input voltage."
"The HPA4 preamplifier preserved absolute polarity (ie, was noninverting) with both balanced and unbalanced inputs and outputs."
"The HPA4's output impedance from both headphone outputs was extremely low, at 0.6 ohm at 20Hz and 1kHz, rising slightly to 0.83 ohm at 20kHz. (Both values include the series resistance of the adaptor cable and 2m of interconnect.)"
"The headphone amplifier's frequency response was flat in the audio band and down by just 1 dB at 200 kHz. With the very low output impedance, the response into 300 ohms was identical to that into the high 100k ohms load, and there wasn't any audio band rolloff with impedances as low as 10 ohms."
"Both the response and the superb channel matching were identical at lower settings of the control and from the unbalanced headphone output."
"Channel separation was superb, at >130dB R–L and >138dB L–R below 2kHz, decreasing to 110dB in both directions at 20kHz."
"From balanced inputs to balanced headphone output, the Benchmark had extremely low noise, with the power supply–related spuriae in its output close to –130dB, even with the volume control set to +15dB. With the volume control set to unity gain, the noise floor was 12dB lower!"
"We define clipping as the output voltage where the THD+N reaches 1%. However, as this graph shows, the Benchmark's protection circuit muted the output when the voltage reached 11.27V, when the THD+N was just above 0.0002%. Reducing the load to 300 ohms gave an almost identical picture, as did repeating the test with the unbalanced headphone output."
"The THD+N percentage was extremely low throughout the audio band into both 100k ohms and 300 ohms."
"I concluded my report on Benchmark's LA4 preamplifier by saying it was the widest-bandwidth, widest-dynamic-range, lowest-noise, lowest-distortion preamplifier I had encountered at that time. To those virtues, the HPA4 adds equally superb headphone outputs."
- John Atkinson, Stereophile
"Merriam-Webster defines a benchmark as "something that serves as a standard by which others may be measured or judged." This is what the AHB2s has become for me."
"A pair of AHB2s are now part of my dedicated summertime amplification for my Sound Lab 545PX loudspeakers."
- Don Shaulis, The Stereo Times
"I was led to Benchmark Media Systems through recommendations from Roger West, the designer of Sound Lab speakers. Praise from a local Sound Lab dealer and a local Sound Lab owner effectively twisted my arm to the point where I yielded to temptation to try the amplifiers ..."
"The Class-H Benchmarks give me what I wanted from Class-D amplifiers, but with the musical engagement, I found lacking in Class-D. There is the added benefit of a company that has been in business for over 36 years and has a depth of products, and offers an extended warranty."
"The highest compliment I can pay a piece of audio equipment is that it has found a home in my system. A pair of AHB2s are now part of my dedicated summertime amplification for my Sound Lab 545PX loudspeakers."
"The Benchmark performance continues to engage me, and the company gives me confidence in long-term support."
- Don Shaulis, Stereo Times
"I love, I love, I love my Benchmark AHB2 power amps!"
- Theo Nicolakis, Audioholics Online A/V Magazine
Theo uses a pair of AHB2 power amps in bridged mono (480 W into 6 Ohms) to drive a pair of Revel Ultima Salon2 loudspeakers ($20,000/pair). This is his reference amplifier-speaker combination.
Find out why Theo loves his AHB2 power amplifiers: