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by John Siau July 30, 2014 7 min read
The Benchmark AHB2 is a radically different audio power amplifier. It delivers the lowest distortion and lowest noise of any commercially available power amplifier. The THD+N and SNR specifications are 20 dB better than many high-end hi-fi power amplifiers. The unique high-efficiency design delivers hundreds of watts from a relatively small, passively cooled chassis.
It seems like everyone is building class-D amplifiers, and some of them look very good on paper until you take a deeper dive into the specifications. Many class-D deficiencies are obscured by the fact that 20 kHz brick-wall filters are always used when measuring class-D amplifiers. These class-D test filters improve the measured performance, but they are not present when you connect a class-D amplifier to your speakers. Tweeters are not ideal low-pass filters and the class-D ultrasonic switching noise can produce audible effects when it interacts with the tweeters.
Class-D amplifiers also suffer from poor THD performance at input frequencies above 1 kHz. Some of the newest Class-D amplifiers have impressive specifications at 1 kHz, when measured through a brick-wall 20 kHz lowpass filter, but this performance quickly degrades at higher audio input frequencies. It also degrades when driving low impedances and difficult reactive loads.
For these reasons, Benchmark does not believe that class-D amplifiers are the best choice for reference-level performance. Nevertheless, class-D amplifiers are a great choice for cost-sensitive applications.
Our goal was to build an amplifier with a SNR that exceeded 130 dB while delivering full output power at a THD+N better than 0.0003% (-110 dB), with a noise-free bandwidth exceeding 200 kHz.
It was clear that our goals could not be achieved with class-D technology. It was also clear that we could not reach these goals with a traditional class-AB or class-A linear design. We would need to throw out all of the established "rules" for building power amplifiers and start from scratch. We would need to take a radical approach and question all of the existing rules. Looking back, it is surprising how many rules we broke.
The performance of the AHB2 would not have been achievable without taking a radical approach to power amplification. In many ways, the AHB2 is a complete 180 degree departure from traditional high-end amplifier designs.
In summary, Benchmark has taken a radical approach to audio amplification, has leveraged two patented technologies (licensed from THX Ltd.), and has reversed many common practices. The result is an amplifier that is significantly quieter, cleaner, smaller, and more efficient than any traditional design. There is nothing ordinary about the Benchmark AHB2.
The power supply responds to audio peaks in real time and does not rely upon capacitive energy storage. The small capacitors at the bottom of this photo comprise most of the capacitance on the secondaries. This is a very small amount of capacitive storage for a power amplifier of this size. This is a radical departure from traditional designs. High-speed active regulation maintains constant DC voltages on all of the DC rails.
The bipolar power transistors are mounted directly to the heat sinks (8 transistors per channel). The mounting clips allow thermal expansion without stressing the transistor packages. There are no fans or vents in the chassis. The output transistors and power supply heat-sink bars place a uniform thermal load on the external heatsinks. The temperature differential between transistors is very small.
The large chip in the front is an FPGA. This device monitors output voltage, output current, output power, output distortion, and the temperatures of critical components. The FPGA will shut down the power supply, the amplifier board, or both, before a fault can cause damage to the amplifier or speakers.
The relays in the back control the gain of the balanced line-level inputs. There are no relays in the output path.
From top to bottom:
by John Siau June 06, 2023 2 min read
At the 2023 AXPONA show in Chicago, I had the opportunity to see and hear the Hill Plasmatronics tweeter. I also had the great pleasure of meeting Dr. Alan Hill, the physicist who invented this unique device.
The plasma driver has no moving parts and no diaphragm. Sound is emitted directly from the thermal expansion and contraction of an electrically sustained plasma. The plasma is generated within a stream of helium gas. In the demonstration, there was a large helium tank on the floor with a sufficient supply for several hours of listening.
While a tank of helium, tubing, high voltage power supplies, and the smell of smoke may not be appropriate for every living room, this was absolutely the best thing I experienced at the show!
- John Siau
by John Siau June 04, 2023 1 min read
We have added an "Audio Calculators" section to our webpage. Click "Calculators" on the top menu to see more like these:
by John Siau June 03, 2023 13 min read
If an audio system is composed of multiple components, we may have detailed specifications for each component, but we will not know the performance of the combined system without doing some calculations. You may have questions such as these:
Use Benchmark's online audio calculators to find answers!
For example, if we know the output power of an amplifier, as well as the sensitivity and impedance of our loudspeakers, we can calculate the maximum sound pressure level that our system can produce.
This application note provides interactive examples that help to answer the questions listed above.