April 2014

Headphone Amplifiers
Specifications are only part of the story.
Article By Benchmark Media Systems

  A couple years ago, some of us at Benchmark noticed a weird discrepancy between our HPA2 headphone amp (built into our DAC1 and DAC2) and some comparably-priced headphone amps. The advertised specifications of all the amps were basically the same, but they sounded noticeably different. Benchmark launched a detailed investigation to identify the differences. The results were surprising and are detailed in a white paper by John Siau.

If you care to read the whole white paper, you can do so here. Otherwise, please continue reading the summary presented in this blog post. Either way, our findings were the same:

We tested three high-quality pro-audio headphone amplifiers with built-in D/A converters. All three had similar published specifications. All units are priced between $1000 and $2000. We verified that the manufacturer's specifications were accurate, but we have shown that these published specifications are not sufficient to tell the whole story. In this case, the published specifications were not a good representation of typical operating conditions.

The key to the riddle is that all of the published measurements were made with an ideal resistive load. Performance changed dramatically when headphones were connected in place of the resistor loads. With actual headphones loading the amplifiers, the specifications mirrored what we had experienced in listening tests. There were significant differences in the measured performance of the three units when driving headphones, but not when driving resistive loads. We found that our subjective listening tests were validated by the measurements when the tests accurately reflected real-world conditions.

It would be nice if we could build headphones that would behave like an ideal resistor. This would make a headphone amplifier's job easy.  Unfortunately, electro-mechanical transducers are far from ideal. The burden falls squarely on the headphone amplifier. As it turns out, headphone amplifiers are not created equal:

In order to make a fair test, we compared the HPA2 (again, included with our DAC1 and DAC2) to comparably spec'd headphone amps costing between $1000 to $2000 -- not inexpensive models you'd expect to be deficient. All these amplifiers had similar “ideal” specs, but how would they hold up in real-world tests? We wanted to find out.

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You can see in the graph above, the ideal test, everything is basically even. None of the three amps show much distortion. Without actually running the signal into a load, the Total Harmonic Distortion + Noise (THD+N) is comparable. While the HPA2 measure slightly better than the others, you probably wouldn't hear a difference between these amplifiers in this ideal world. These are the measurements/specifications that manufacturers show consumers.

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In our second test we added a 60-Ohm resistive load to simulate headphone loading. Notice that distortion begins to increase in the non-HPA2 amplifiers (green and magenta curves). In contrast, the performance of the HPA2 is nearly unchanged when driving a 60-Ohm resistor (blue curve).

Click here for larger image.

In our third test, we replaced the 60-Ohm resistor (ideal load) with a pair of 60-Ohm headphones.  Distortion rose significantly in the non-Benchmark headphone amplifiers. The 60-Ohm headphones do not behave like 60-Ohm resistors. Clearly the other two amplifiers were having difficulty controlling the headphone transducers. In contrast, the HPA2 measured almost the same as it did in the unloaded conditions (1st test).

This third graph shows the THD+N for the three amplifiers while driving a pair of Sony MDR-V6. The Sony headphones aren't extremely hard to drive, but they do tax the performance of the non-Benchmark amplifiers.

All three headphone amplifiers have enough power to drive the MDR-V6 headphones at their 500 mW rated power. Nevertheless, units 2 and 3 were unable to fully damp the mechanical resonances of these popular headphones. Consequently, units 2 and 3 show significant distortion at low frequencies.

Units 2 and 3 produce audible levels of distortion when driving the popular Sony MDR-V6 headphones. Clearly these two amps are not able to maintain control over the headphone drivers.  Distortion can color the voicing of the headphones and cause listener fatigue. This correlated with the original listening tests that prompted this investigation.

In conclusion, all three of these headphone amps have the same specifications under ideal loads, but they don't perform the same when driving headphones.

If you'd like to learn more, please feel free to read the white paper. Or, get in touch with someone at Benchmark, and we'd be happy to answer your questions.  We work hard to develop products that perform and measure well under all real-world conditions -- not just selected ideal conditions.

See Part 2 of this article at this link.

www.BenchMarkmedia.com