By John Siau
March 20, 2014
The movement of headphone transducers must be well controlled in order to produce high-quality audio. It is easy to build a headphone amplifier that produces sound. It is an entirely different matter to produce an amplifier that is clear, clean, and enjoyable.
Headphone amplifiers need power and accuracy to achieve control. They also need to be protected from short circuits and overload conditions. The cheap, dirty, and common way to protect the amplifier is to add a series resistor between the amplifier and the headphone jack. This simple solution protects the amplifier from short circuits and overloads. Unfortunately, the resistor isolates the headphones from the amplifier, causing a loss of control. This series resistor is a big mistake.
While it's true that a series resistor can protect a headphone amplifier and keep it stable, there are somemassive trade-offs. The series resistor introduces high distortion, poor frequency response, and poor phase response. Relatively low amounts of these defects may be acceptable, butour tests show that these defects can increase to objectionable levels when headphones are driven through series resistors.
If the series resistor is removed, high-demands are placed on the headphone amplifier. It must survive short circuits, and overloads. It must also remain stable when driving the dynamic load of moving headphone transducers. The direct-drive solution demands a small and robust power amplifier. Benchmark's HPA2™ is a small well-protected power amplifier that is stable when directly driving the most difficult headphone loads. The HPA2™ completely eliminates the series output resistor.
Today's state-of-the-art headphone amplifiers, such as the HPA2™ , are often called "0-Ohm" headphone amplifiers. These new high-end designs are essentially miniature power amplifiers, which provide outstanding control of the moving transducers in a set of headphones. These "0-Ohm" designs eliminate the series resistor, and provide direct drive to the headphones. This important change reduces distortion and flattens the frequency response when driving a headphone. Our lab measurements confirm the effectiveness of "0-Ohm" amplifiers.
The figure above shows distortion versus frequency. The blue line is measured at the input to a set of headphones driven directly from a "0-Ohm" amplifier. The red line is measured at the input of the same set of headphones when a 30-Ohm resistor is inserted between the amplifier and the headphones (simulating a low-cost amplifier). The series resistor causes a very significant increase in distortion (as shown by the red line).
Finally, look at this figure, above. It's a common frequency response plot. Again, the blue line is the 0-Ohm amplifier from the HPA2™ and the red line is the same amplifier running through a 30-Ohm series resistor. The figures really speak for themselves. A "0-Ohm" amplifier maintains control of the headphone frequency response. In contrast, the series resistor creates a filter when loaded by headphones. This unintended filter changes the frequency response of the headphones.
If you're interested in learning more about "0-Ohm" headphone amplifiers, we've written a white paper with all the relevant information and measurements. This paper provides measurements which demonstrate the significant advantages of headphone amplifiers with very low (near 0-Ohm) output impedances. A low output-impedance increases the damping factor of the amplifier-headphone system. "The 0-Ohm Headphone Amplifier" white paper by John Siau shows that a low output-impedance reduces distortion while improving frequency and phase response.
We have added an "Audio Calculators" section to our webpage. Click "Calculators" on the top menu to see more like these:
Secrets contributor Sumit Chawla recently caught up with Benchmark’s VP and Chief Designer, John Siau to get a little more in-depth on several subjects.
Q: "Benchmark is one of the few companies that publishes an extensive set of measurements, but you also balance that with subjective testing. Can you talk about the equipment, the listening room, and the process for subjective testing?"
Q: "Was there ever a time where you learned something from a subjective test that was not captured by measurements?"
Q: "You conducted some listening tests to determine whether distortion in the “First Watt” was audible. What test material did you use for this, and what did you find?"
Q: "The AHB2 amplifier incorporates THX Audio Achromatic Amplifier technology. When and how did the partnership with THX come about?"
Q: "Linear power supplies have been and remain quite popular in high-end devices. You favor switch-mode power supplies. When and why did you make this switch?"
... and more!
At Benchmark, listening is the final exam that determines if a design passes from engineering to production. When all of the measurements show that a product is working flawlessly, we spend time listening for issues that may not have shown up on the test station. If we hear something, we go back and figure out how to measure what we heard. We then add this test to our arsenal of measurements.
Benchmark's listening room is equipped with a variety of signal sources, amplifiers and loudspeakers, including the selection of nearfield monitors shown in the photo. It is also equipped with ABX switch boxes that can be used to switch sources while the music is playing.
Benchmark's lab is equipped with Audio Precision test stations that include the top-of-the-line APx555 and the older AP2722 and AP2522. We don't just use these test stations for R&D - every product must pass a full set of tests on one of our Audio Precision test stations before it ships from our factory in Syracuse, NY.