The AHB2 Power Amplifier was Introduced in 2016 on Benchmark's 33rd Anniversary. Here is the full story behind this award-winning amplifier:
Over 33 years ago (in1983), Benchmark's founder, Allen H. Burdick, began building analog audio distribution amplifiers for television networks. This specialized application demanded an audio power amplifier with very low noise, very low distortion, and a very wide frequency response. The Benchmark DA101 distribution amplifier had a 160 kHz bandwidth, THD+N of 0.00044%, and a SNR of 130 dB. The power output was about 40 watts in bridged mono. These amplifiers were designed to simultaneously drive 10 balanced audio lines while withstanding short circuits on as many as 1/3 of the 30-Ohm outputs. This requirement meant that the amplifiers needed to be capable of driving an 8-Ohm load. The DA101 distribution amplifiers were not intended for driving speakers. Nevertheless, they were capable of providing spectacular performance when driving 8-Ohm speakers. For many years, we used our DA101 distribution amplifiers to drive the speakers in our listening room. We couldn't buy an amplifier that came close to the performance of our distribution amplifiers, so we used them in all of our critical listening tests. Allen had often talked about building a bigger general-purpose power amplifier that could match the performance of the DA101.
Allen retired in 2006 due to health issues, and in 2011 we began to talk about making Allen's dream for a power amp a reality. The goal was to meet or exceed the performance of Allen's DA101, while scaling up the power by a factor of 10. One obstacle was that push-pull crossover distortion can be hard to manage in a large amplifier. We began to look at unconventional solutions to this problem, and had a design code-named "PA1" in progress by June of 2011. It was to have a switched-mode power supply and a unique servo-biased push-pull output stage. We were going to have to think out of the box if we were going to reach our performance goals.
In November of 2011, I was attending the AES conference and I ran into Laurie Fincham and Jayant Datta, some good friends from THX. We met for lunch and they began to tell me about a new amplifier topology that they were working on. They had applied for patents and were looking to turn their ideas into a product. This was the classic lunch with schematics and numbers scribbled on napkins. We realized that our two companies had complementary skills that could make a joint project a success. Their patents offered an elegant solution to the crossover-distortion problem and we were already planning to build an unconventional power amplifier with ground-breaking performance. By the time we finished lunch, we had outlined the performance requirements for the new amplifier. These specifications, written on a napkin in 2011, are almost identical to those of the production AHB2.
Much of 2012 was spent testing and improving low-power THX prototypes. The THX team was split between California, New York, Canada, and England. Benchmark's team was located at our headquarters in Syracuse, NY. It was an international effort tied together with email, Skype, air travel, and a common love for music and great audio.
In November of 2012 we set the final specifications and code-named the project the "PA2" (Benchmark's second power amplifier - Power Amp #2, or PA2). The photo at the left is an early concept drawing. We soon completed the designs for the casework and custom transformers. We then began the final circuit designs for a full-power prototype.
On September 27, 2013, everything was ready, but we still didn't have a name for the new amplifier. The faceplates were machined and finished but needed to be printed with the product name. I began thinking about the technology in the amplifier and how this could be incorporated into the name. The amplifier delivered class-A performance, had class-H tracking rails, and a class-AB output stage. It then struck me that the letters A, H, and B were Allen H. Burdick's initials. Instantly it was clear to me that the new amplifier should be named after the man who inspired it! At that moment, the PA2 became the AHB2. I announced the name to my staff, and we placed the order for the printing. Less than an hour later we got a phone call with the sad and shocking news that Allen had passed away. I am still stunned when I think about the coincidence of these two events.
In October 2013 we demonstrated the first working AHB2 prototype at the AES show. We then completed a 20-unit pilot run in early 2014, and spent several months completing compliance and safety testing. By October of 2014 the first production run was nearly ready, and we hoped to take the very first unit to the 2014 AES show.
Through heroic efforts, the first production AHB2 was ready just in time for the 2014 AES show, where an amazing coincidence occurred. Through similarly heroic efforts, Audio Precision completed their first production APx555 audio test station just in time for the same show. This last-minute success was important for AP because they were planning to celebrate their 30th anniversary on the show floor. They invited us and our new AHB2 to their booth for their 30th anniversary celebration. Top engineers from Audio Precision, THX and Benchmark all gathered around the test station to see APx555 run tests on the AHB2. The group included Laurie Fincham, and Jayant Datta (THX), Bruce Hofer and Tom Kite (AP), and John Siau (Benchmark). Everyone was so impressed by what they saw, that the festivities faded into the background. The APx555 was the best audio test station we had ever seen, and the performance of the AHB2 was pushing the limits of the test station. It was hard to say who was the most excited.
- John Siau, March 17, 2016
The Benchmark AHB2 power amplifier and HPA4 headphone amplifier both feature feed-forward error correction. This correction system is an important subset of the patented THX-AAA™ (Achromatic Audio Amplifier) technology. It is one of the systems that keeps these Benchmark amplifiers virtually distortion free when driving heavy loads. It is also the reason that these amplifiers can support 500 kHz bandwidths without risk of instability when driving reactive loads.
This paper explains the differences between feedback and feed-forward systems. As you read this paper, you will discover that you already understand the benefits of feed-forward correction because you use it instinctively to improve a feedback system commonly found in your automobile. If feed-forward correction can improve your driving experience, it may also improve your listening experience!
If you look at the back of any Benchmark product, you will find balanced XLR analog-audio connectors. As a convenience, we also provide unbalanced RCA connectors on many of our products. In all cases, the balanced interfaces will provide better performance.
We build our unbalanced interfaces to the same high standards as our balanced interfaces, but the laws of physics dictate that the balanced interfaces will provide better noise performance.
This application note explains the advantages of balanced interfaces.
Benchmark has introduced a new analog-to-analog volume control circuit that features a 256-step relay-controlled attenuator and a 16-step relay-controlled boost amplifier. The volume control has a +15 dB to -122 dB range in 0.5 dB steps and is a key component in the HPA4 Headphone / Line Amplifier.
Our goal was to produce an analog-to-analog volume control with the highest achievable transparency. We wanted to be able to place this volume control in front of our AHB2 power amplifier or in front of our THX-888 headphone amplifier board without diminishing the performance of either device. Our volume control would need to have lower distortion and lower noise than either of these amplifiers. Given the extraordinary performance of these THX-AAA amplifiers, this would not be an easy task!
This application note discusses the engineering decisions that went into the development of this new analog volume control circuit. The end result is a fully buffered volume control with a signal-to-noise ratio that exceeds 135 dB. THD measures better than the -125 dB (0.00006%) limits of our test equipment.