In my opinion, most power amplifiers have far too much gain. The AHB2 is designed to accept studio-level input levels and this can significantly improve noise performance between connected devices. This means that the AHB2 clips with a 22 dBu input instead of an 8.2 dBu (2 V RMS) input. Gain is only 9 dB instead of the more typical 20 to 30 dB gain found in most power amplifiers. For example, the similarly sized Bryston 3B consumes 120W idle.īenchmark chose to build the amplifier with very low gain. A traditional class-AB of equivalent power would consume 100 to 120 W idle. The AHB2 is significantly more efficient than a traditional class AB, and has much lower distortion. The THX topology also allowed us to utilize two power supply rails in a class-H (or G) configuration without any measureable distortion penalty. Distortion of the AHB2 is at the measurement limits of our AP 27 test stations.
The optimum solution uses some bias current, but much less than a traditional class AB. We were willing to increase the power dissipation as much as necessary to optimize the distortion performance. Our goal was to achieve very low distortion, with a focus on low crossover distortion. The same feed forward error correction can be used to remove crossover distortion from a class-B amplifier can be used with class AB biasing to create an amplifier with extraordinarily low distortion. The THX topology was interesting to Benchmark for an entirely different reason: THX has demonstrated a design that rivals the efficiency of class D amplifiers. This also allows class-H or class-G tracking rails without the usual distortion problems associated with these designs. The technique is so effective that the output stage can be run in class-B operation while achieving very low distortion. The patented topology uses feed-forward error correction to virtually eliminate crossover distortion. By John Siau,chief engineer at Benchmark at DIY audio forum
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