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.
As an engineer I like to use "rules of thumb" to make quick estimates that help to explain the physical world around me.
These rules of thumb are easy-to-remember approximations that eliminate the need for complicated and needlessly precise calculations.
If you feel discombobulated by the complexities of high school physics, there is hope! I encourage you to step back and take a fresh approach.
If you learn a few simple rules of thumb, you can unravel mysteries of the physical world, amaze your friends, and yourself.
In this paper I will present 15 simple rules that I find useful when working with music and audio.
- John Siau
The Benchmark AHB2 power amplifier and HPA4 headphone amplifier both feature feed-forward error correction. This correction system is an important subset of the patented THX-AAA™ (Achromatic Audio Amplifier) technology. It is one of the systems that keeps these Benchmark amplifiers virtually distortion free when driving heavy loads. It is also the reason that these amplifiers can support 500 kHz bandwidths without risk of instability when driving reactive loads.
This paper explains the differences between feedback and feed-forward systems. As you read this paper, you will discover that you already understand the benefits of feed-forward correction because you use it instinctively to improve a feedback system commonly found in your automobile. If feed-forward correction can improve your driving experience, it may also improve your listening experience!
If you look at the back of any Benchmark product, you will find balanced XLR analog-audio connectors. As a convenience, we also provide unbalanced RCA connectors on many of our products. In all cases, the balanced interfaces will provide better performance.
We build our unbalanced interfaces to the same high standards as our balanced interfaces, but the laws of physics dictate that the balanced interfaces will provide better noise performance.
This application note explains the advantages of balanced interfaces.