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.
At the 2023 AXPONA show in Chicago, I had the opportunity to see and hear the Hill Plasmatronics tweeter. I also had the great pleasure of meeting Dr. Alan Hill, the physicist who invented this unique device.
The plasma driver has no moving parts and no diaphragm. Sound is emitted directly from the thermal expansion and contraction of an electrically sustained plasma. The plasma is generated within a stream of helium gas. In the demonstration, there was a large helium tank on the floor with a sufficient supply for several hours of listening.
While a tank of helium, tubing, high voltage power supplies, and the smell of smoke may not be appropriate for every living room, this was absolutely the best thing I experienced at the show!
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.
This application note provides interactive examples that help to answer the questions listed above.