by John Gatski
I applaud Benchmark’s efforts to clean up analog amplifier noise. That low-level idle hiss that I hear from numerous vintage and modern amplifiers is, at the very least, annoying, and, at its maximum, detrimental to ultimate resolution.
Siau said that the amp has exceeded his expectations in terms of measured performance. The SNR of the AHB2 was spec'd at 126 dB (A-weighted) and THD is -115 dB, just about as good as the DAC2’s measurements. “The AHB2 power amplifier was designed from the ground up to deliver the full performance of the DAC2 HGC," he emphasized.
As I got into serious listening of recordings, I was impressed by the AHB2’s "smooth factor. Although it uses bipolar output devices, its tonal delivery ease reminds more of MOSFET designs. Drum cymbals and the upper register of the Bernhard album’s piano sounded fantastic without any edge, grit or harshness.
“It was not optimized for the highest possible efficiency that could be achieved with the new THX topology”, Siau explained. “Nevertheless the AHB2 is much more efficient than a conventional Class AB design. Peak power does not vary with AC line voltage (due to the regulated supply). Likewise, power drawn on one channel does not influence the power available from the other channel.”
You can also bridge the amp to mono, via a rear toggle panel switch, to get 350 watts (8 ohms). Thus, buying two stereo amps and bridging mono for each one would be the chosen path for powering the less efficient speakers, or filling big rooms.
|Production AHB2 will get optimized conection access|
Since the amp is not a high-gain amp, ensuring gain match with your source preamp is a consideration. The Benchmark DAC2 HGC and DAC2 D have enough gain in the stock setting on various audiophile recordings. However, if you move a jumper inside the DAC you can get 10 dB more gain to give extra headroom.
The Oppo BDP-105’s variable or fixed output, even at full blast, was just enough gain, but not ideal Benchmark’s Siau said that the production AHB2 will have three gain settings (with virtually the same noise spec), and offers more flexibility for preamp choices than the single (low-gain) setting that was active on the prototype. “This (three-position gain level switch) makes the AHB2 compatible with all preamplifiers, but preamps that do not have high output usually have a poor signal-to-noise ratio, and will, therefore limit the system performance.”
For those who listen with very accurate, discerning speakers, an extra 20 dB of lower noise could be heard — in terms of ultra detail that may be hiding in the noise of an average audiophile amp.
Sources for the AHB2 listening sessions included the Oppo BDP-105, Benchmark DAC2 D, TASCAM DVRA1000 HD master recorder/player, and Apple Macbook Pro feeding a Mytek Stereo192-DSD DAC. Preamps included a Coda, a Pass Labs XP-10 passive preamp, Rogue Audio Model 99 tube preamp, and the line output of the Bryston BHA-1 headphone amplifier.
|Pre-brochure AHB2 Specs|
As I got into serious listening of recordings, I was impressed by the AHB2’s "smooth factor. Although it uses bipolar output devices, its tonal delivery ease reminds more of MOSFET designs. Drum cymbals and the upper register of the Bernhard album’s piano sounded fantastic without any edge, grit or harshness. The dynamics of the So Real recording jumped out at me when listening to the AHB2 — with a deep, wide stereo image and an encompassing presentation of the subtle layers of each instrument and their ultimate interaction with each other. This quality was repeated through numerous other recordings as well.
|The DAC2 and AHB2 make a dandy pair|