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Headphone Amplifiers - Part 1

by John Siau March 14, 2014 3 min read

Headphone Amplifiers - Part 1

By John Siau

March 14, 2014

Headphone Amplifiers - Part 1

Specifications are Only Part of the Story!

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 "An Examination of Headphone Amplifier Performance Specifications", a white paper by John Siau.

If you care to read the whole whitepaper, you can follow the link above. 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 $1,000 to $2,000 - 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.

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.

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).


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.

Download the full white paper:

"An Examination of Headphone Amplifier Specifications" - John Siau

Recommended reading:

More information on headphones and headphone amplifiers can be found here

If you’d like to learn more, 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.

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ChatGPT Reviews the AHB2 and Includes Listening Impressions!
ChatGPT Reviews the AHB2 and Includes Listening Impressions!

by John Siau June 02, 2023 2 min read

What happens when you ask ChatGPT to write a review?

I asked ChatGPT to review the Benchmark AHB2, and I was shocked at the results!

It turns out that ChatGPT wrote a review by gleaning information from the many publicly available reviews of the AHB2. I recognized phases and sequences of words that had appeared in prior reviews, and yet the text was new, original, and well written. The ChatGPT review seemed to summarize the collective impressions of the reviewers without directly quoting any particular reviewer.

It should be possible to ask ChatGPT to write a review on any well-known product and the results may be an AI-generated review that reflects the collective opinions of many human reviewers.

Try this the next time you a planning to make a purchase online. These AI-generated reviews may be more useful than traditional search results. In essence, the search results are combined into a single well-written review.

It is also possible that ChatGPT is biased to give glowing reviews, so use caution! Try it out and have some fun experimenting with this new technology!

Here is my dialog with ChatGPT:

John Siau:

Write a review of the Benchmark AHB2 complete with listening impressions.


Title: Unleashing Sonic Purity: A Review of the Benchmark AHB2 Amplifier

Introduction: The Benchmark AHB2 amplifier has gained a reputation for its exceptional audio performance and unwavering dedication to sonic purity. In this review, we delve into the heart of this powerhouse amplifier, exploring its features, design, and most importantly, the listening impressions it evokes...

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ChatGPT and John Siau Discuss Star-Quad Cables

by John Siau May 30, 2023 7 min read

Today, Benchmark's John Siau and OpenAI's ChatGPT had a conversation about star-quad cables. It turns out that ChatGPT got quite a few things wrong, but learned quickly when these errors were pointed out by John Siau.

ChatGPT is quick to recognize mistakes, and quick to provide detailed corrections. In each of the first few corrections, additional errors were made. As each of these were pointed out, ChatGPT began to provide accurate information.


John Siau:

How do star-quad cables reject interference?


Star-quad cables are designed to provide improved rejection of ...

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System Performance Calculations

by John Siau March 02, 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:

  • Will my audio system produce audible noise?
  • Will my audio system produce audible distortion?
  • How will my audio components work together as a system?
  • How loud will my audio system play?

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.

Speaker Sensitivity: dB SPL (2.83 V, 1 M)
Amplifier Power Per Channel into 8 Ohms: Watts
Peak Output Level (at typical listening position): dB SPL


This application note provides interactive examples that help to answer the questions listed above.

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