AHB2 Crossover Distortion Measurements

AHB2 Measurements

The following measurements and scope photos demonstrate the effectiveness of the feedforward system in the AHB2. From the first Watt to the last Watt, the AHB2 shows no evidence of crossover distortion. In contrast, all conventional class-AB amplifiers have crossover-distortion artifacts.

Dick Olsher once said that "the first Watt is the most important Watt". We agree!

In most listening situations, an amplifier spends most of its time near the 1-Watt output level. At these low levels, push-pull crossover distortion can be the dominant type of distortion. This non-musical distortion must be minimized or eliminated. Until the AHB2 was introduced, the only solution was class-A operation.

Now everything has changed. The patented feedforward error correction system in the AHB2 offers a better alternative to class-A amplifiers. The AHB2 exceeds the performance of all class-A power amplifiers, and it does so without generating the massive amount of heat produced by class-A amplifiers. The AHB2 is nearly as efficient as a class-D amplifier, but it outperforms all class-A amplifiers. This is why we say that the AHB2 is "the cleanest, quietest amplifier on the planet".

THD+N

The 1-Watt THD+N measurements demonstrate that the AHB2 is free from any crossover distortion. Note that the AHB2 also stays clean at 1/3 power and at its full rated output power. Also note that there is almost no measurable difference between 8-Ohm and 4-Ohm loading. The same holds true for bridged-mono operation.

THD+N at 1 kHz into 8-Ohms, 22 kHz BW:

-108dB (0.0004%) at 1 W (1 W is -20 dB relative to rated power)

-114 dB (0.0002%) at 33 W (1/3 rated power)

-113 dB (0.0002%) at 100 W (full rated power)

THD+N at 1 kHz into 4-Ohms, 22 kHz BW:

-106 dB (0.0005%) at 1 W

-107 dB (0.0004%) at 1.9 W (1.9 W is -20 dB relative to rated power)

-113 dB (0.0002%) at 63 W (1/3 rated power)

-112 dB (0.0003%) at 190 W (full rated power)

THD+N at 1 kHz bridged mono into 8-Ohms, 22 kHz BW:

-105 dB (0.0005%) at 1 W

-109 dB (0.0003%) at 3.8 W (3.8 W is -20 dB relative to rated power)

-114 dB (0.0002%) at 33 W (1/3 rated power)

-113 dB (0.0002%) at 380 W (full rated power)

Scope Screenshots:

Pictures can help to demonstrate what these numbers mean, so we captured some scope photos from our Audio Precision 2722 test station.

The following oscilloscope screenshots demonstrate the differences between the Benchmark AHB2 and a typical competing class-AB amplifier (with a similar power rating).

All scale factors are identical between the two amplifiers, and they can be compared directly. In all screenshots, the distortion waveforms are boosted by 60 dB relative to the output waveforms.

Note the difference in noise, and note the differences in the distortion waveforms. The competing amplifier clearly demonstrates the crossover distortion artifact that is common to all conventional class-AB amplifiers.

Notice that the performance of the AHB2 does not change as the power level increases. Also notice that there is no hint of crossover distortion.

In contrast, the typical class-AB amplifier shows a pronounced crossover distortion artifact. The relative sizes of the crossover distortion artifacts show that this distortion is a higher percentage of the signal when the competing amplifier is operating at low power (and especially the first Watt).

All scale factors are identical.

AHB2 at 1 W into 8 Ohms

  • Top trace is the output waveform.
  • Bottom trace is distortion + noise

Note that there is no indication of crossover distortion.

Competing Class-AB amplifier at 1 W into 8 Ohms

  • Top trace is the output waveform.
  • Bottom trace is distortion + noise

Note that crossover distortion is clearly visible as sharp transients in the distortion waveform. The transient shows a fast attack time followed by the recovery time of the feedback loop. This distortion waveform is typical of virtually all class-AB amplifiers.

AHB2 at 33 W into 8 Ohms

  • Top trace is the output waveform.
  • Bottom trace is distortion + noise

Competing Class-AB amplifier at 33 W into 8 Ohms

  • Top trace is the output waveform.
  • Bottom trace is distortion + noise

AHB2 at 100 W into 8 Ohms

  • Top trace is the output waveform.
  • Bottom trace is distortion + noise

Competing Class-AB amplifier at 100 W into 8 Ohms

  • Top trace is the output waveform.
  • Bottom trace is distortion + noise

 




John Siau
John Siau

Author

John Siau is VP and Director of Engineering at Benchmark Media Systems, Inc.