Before she joined Jefferson Airplane, Grace Slick was in a band called The Great Society. They made an LP, Conspicuous Only in Its Absence, the title of which has stuck in my mind ever since I first heard it as a teenager. (The album also contains some killer psychedelic rock riffing, but that's another story for another time.) That album's title is a perfect description of how the new Benchmark AHB2 power amplifier fits into a studio monitoring system. 

You probably know Benchmark for its long-available ADC and DAC units. The ADC1 analog to digital converter has been used by recordists and mastering engineers for years, and the company recently updated its DAC line with the new DAC2 series. More on the DAC2 later, but now more on this new and very different power amp. 

Although many studios, particularly smaller at-home outfits, use active monitors (speakers with built-in power amps), some of us prefer the old-style setup of separate passive speakers and outboard power amps. For one thing, there are many more choices of combinations. For another, almost no powered monitors include really super-quality power amps because of the compromises necessary to fit electronics into a speaker cabinet and meet a price point. While many of the active monitors get plenty "loud," they often sound un-dynamic and harsh, at least to my ears. 

After upgrading my monitors to Amphion Two18 speakers [Tape Op #108], I decided to look into modern alternatives to my circa-1980s Audio Design Associates (ADA) FET300 power amp. That amp, designed by ADA co-founder Albert Langella, is built like a tank and has served me without fail since I built my studio almost 20 years ago. My brother worked as Langella's production manager for years, and I've always considered ADA an unknown powerhouse, especially in regard to power-amp design and build. 

Because they were designed for multi-room systems, Langella's power amps were capable of handling very low impedance loads and were built into massive all-metal boxes. And in this lies the rub: Al preferred to use big fans to keep things cool in the boxes. When the amps are in a closet in a mansion, powered on 24-7 and piping "music to smoke cigars by" all over the joint, big cooling fans in the chassis are just fine. But in a rack in a studio, they are noisy. I've had the amps in various places, but they can't be too far from my monitoring system because they are unbalanced-input, and also because I prefer short runs of speaker cable. Over two decades of working with the ADA amp, I got used to the fan noise, and tuned it out. As far as audio quality, the amp offered very good low-level resolution and plenty of dynamic power. I thought it was the cat's ass and didn't even consider swapping it out, until I started reading descriptions and reviews of the Benchmark amp. 

The Benchmark AHB2 amp uses new technology from THX, the folks who bring you "certified" cinema sound systems. Essentially, THX figured how to tweak a Class AB power amp to run with almost no bias current and yet end up with no switching distortion. The patented feed-forward design incorporates an ultra-clean, low-power amplifier that runs in parallel with the main Class AB amplifier, feeding the main output with an error-correction signal that cancels out crossover distortion. Moreover, the power supply is designed to deliver high current on demand, without the need for capacitive storage. The supply responds instantly to the music dynamics, maintaining a constant, well-regulated supply voltage even when challenged with the most difficult peak-power demands. 

In practice, these technologies result in a small and lightweight amplifier (footprint equals that of Benchmark's DAC line - half-rack width) that runs so cool that the heat sinks are rarely more than lukewarm to the touch. Alongside the power switch on the front panel is an LED indicator, and there are additional LEDs labeled Mute, Temp, and Clip for each channel. Together, the LED lights can display a total of 16 different fault conditions. Instead of relays, Benchmark chose to use an electronic mute circuit on the output stage to avoid the distortion resulting from high-current relay contacts degrading over time. Gold-plated relays are used on the input stage, where the current is near zero and contact wear is not an issue. When the amp is turned on or off, a very quiet click can be heard. 

Benchmark took the THX technology another step by designing the amp to work in a lower-gain mode in a pro- audio setting. This allows for a very low noise-floor, because there is no gain wasted and then attenuated. A switch on the rear offers three sensitivity options for input level, including 22 dBu, the recommended setting when pairing the AHB2 with the DAC2 line, allowing the DAC2 HGC's "Hybrid Gain Control" to operate in its lowest-noise range. Basically, this is a return to old-school gain-staging. In the old days, a studio operated on an impedance-matched, transformer-coupled, power-based system, designed to accommodate +24 dBm peak levels. The old tube power amps provided enough gain from this relatively high line level to drive the relatively efficient studio monitors of the day to screaming ear-damaging levels. In Benchmark's version of gain-staging, the new Class AB solid-state amp isn't expected to do all the lifting between the source and the speaker, unless it has to. (The highest sensitivity setting is for use with unbalanced consumer gear, such as a home hi-fi preamplifier operating at -10 dBV, or with unbalanced "pro-sumer" recording gear.) 

In a series of white papers on the Benchmark website, head engineer John Siau contends that the AHB2 is the only power amp on the market designed for true HD reproduction, because it can accommodate the 110+ dB dynamic range available. This is a good marketing point, but I have yet to encounter recorded music that ever includes true silence within the musical tracks. (Although it's true that digital black can occur between tracks.) The reason for this is simple - no recording space is totally silent; an anechoic chamber comes close, but no one would choose to record music in one of those. So, there is always background noise, even in HD recordings. That said, Siau has a valid point in that the audio system shouldn't add any noise. It should run in the background - a silent transmitter of the musical energy. In my listening tests, the Benchmark AHB2 amp did this. 

One thing I pride my studio on is running quiet. The digital chain runs dead-silent - zero background noise, no hash in the power lines, no crud coming in from the computer. Each piece of analog equipment has been restored and maintained to operate at or below the specified noise floor. I work mostly with older recordings and also lower- fidelity material from archives and private collectors, so the source material generally has all the noise I want to hear. Plus, part of my job is mitigating as much of that noise as is appropriate, in service to the source audio. So it's important that I know what noise is baked into the recording versus what noise is present in my system. Such things as fan noise, from the ADA power amp and also from the studio computers, is distinct enough that I can tune it out, but of course the less to tune out, the better. 

As soon as I unplugged the ADA amp and swapped in the Benchmark, it was clear how nice it was to hear less fan noise. I cranked up the monitor volume all the way on my Lynx Hilo [Tape Op #90] and confirmed there was zero hiss, hum, and other power line hash. I put my ear right to the speakers - still dead silent. The ADA is near-silent but not dead-silent, probably due to 30-year-old resistors and aging power-supply parts. So now, just from being in the system, the Benchmark had taken down the room noise (no fans) and lowered the monitor system noise floor from very quiet to silent. 

I then started listening to music and other audio - projects I recently completed plus favorite albums. I did this for several weeks, and I also took the Benchmark upstate and plugged it into my B&W 805 Diamond speakers there. In every case, I heard the amp as essentially transparent - a silent transmitter of musical energy. Different speakers sound different, different rooms sound different, different albums sound different. This amp doesn't impose any sound on any of it, at least not any that I can hear. It just amplifies the sound efficiently and quickly and drives all the speakers I plug into it. What did stand out was a very quick and precise quality to the bass, as if the amp is quicker on the draw than most, when it comes to moving the piston to move the air. In all the situations I tried it, the Benchmark seemed to "focus" the sound-picture just a little bit sharper than the amp it replaced could. I'll describe the AHB2 as the sonic equivalent of a very sharp plasma display - full-featured, well-defined, and natural-looking, as if looking at a real thing instead of a digital image. 

Benchmark also sent me a DAC2 HGC [Tape Op #97] to test drive, and I coupled it with the AHB2 in my big listening room, driving a pair of B&W 808 speakers. This setup replaced a Benchmark DAC1 HDR and an Aragon 8008 amp. Sound- wise, the AHB2 had the same clarifying effect as it had in my other setups. It sounded quicker and crisper with complex and varied dynamics, and it was equally adept at moving the 808's woofers when big bass was happening. Things like kick drum and plucked acoustic bass strings seemed a bit clearer and more "forward" in the sound field, but not exaggerated. I thought the overall sound quality improved; I could listen at slightly lower SPLs and still hear all the details. I surmise that the Benchmark amp is able to make the speakers "do more" and thus better resolve low-level details because the drivers are moving faster. 

As happened in the studio, when I swapped the former power amp back in, the sound clouded up a bit, the stereo field narrowed a bit, and I found I wanted to turn the volume knob up a bit because things seemed somewhat muddier. To get back to The Great Society album title, the Benchmark AHB2 was very much conspicuous in its absence. My ears missed it! 

The combo of the DAC2 HGC and AHB2 is worth considering if for no other reason because of Benchmark's new 12V interconnected control system. The DAC's remote controls power and muting for both. The 12V control cable is included with both components. And, of course, the pair are designed for ideal gain-staging from source to speakers. 

The DAC2 HGC sounded to my ears just like my DAC1 HDR - like nothing. A dead-quiet, super-accurate converter of digital bits and bytes into audio signals. In that, it's like my Lynx Hilo and probably a lot of other great DACs on the market today. Technology has advanced and design engineers have worked hard to remove the last audible distortions and colorations in the analog stages. As long as there's not some terrible impedance mismatch between a DAC and an amplifier, the modern DAC should be sonically transparent. Benchmark took this further in both the DAC1 HDR and now the DAC2 HGC by working hard on making the volume-level control precise, non-coloring, and accurate. And both units include Benchmark's excellent headphone amps, which can drive all my headphones, from earbuds to the600ΩAKGK240DF. 

As far as sound differences between the two Benchmark DACs, I couldn't hear any on the big speakers in my listening room, but Benchmark claims a number of improvements in the DAC2 HGC: measurably better performance across the board; more digital and analog headroom for handling inter- sample "overs"; and a 32-bit digital volume control that offers perfect channel-to-channel tracking versus near-perfect from the DAC1 HDR's analog volume control. I tried the same comparison in the studio, and lo and behold, the stereo image of an orchestra recorded in a very "live" room did shift a little bit to the left as the volume knob was turned down from normal monitoring volume to too-soft-to-critically- listen volume on the DAC1 HDR, but did not shift at all when the DAC2 HGC's volume knob was turned down. Still, I would say that in most normal listening conditions, it's unlikely most people will hear any differences. 

For me, the compelling case for switching to the DAC2 HGC is its feature set. It includes more inputs than the DAC1, and its front panel clearly shows the actual sample- rate being converted to analog (as opposed to what the file or source claims to be), while the DAC1 has no sample-rate readout. Also, the DAC2 HGC's USB port accommodates 192 kHz PCM and native DSD, whereas the DAC1 offers only PCM up to 96 kHz through USB. With these upgrades, the DAC2 HGC becomes an obvious candidate for mixing and mastering studios. Note that the DAC2 line also includes the DAC2 DX, which forgoes the analog inputs but includes an XLR AES digital input and two analog outputs (one or both of the outputs can be set to bypass the volume control), as well as the DAC2 L, which loses the headphone amp for a $200 price break. 

I liked the feature set on the DAC2 HGC enough to buy it for my big listening room, replacing the DAC1 HDR. I bought the AHB2 power amp for my studio and have started filling the piggy bank to buy one for the big listening room. Paired with the Amphion Two18 speakers in the studio, the AHB2 is half of the new "Dynamic Duo." 

AHB2 $2995; DAC2 HGC $1995;
Tom Fine is at

Tape Op is a bi-monthly magazine devoted to the art of record making.

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