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The HPA2™ Headphone Power Amplifier

Benchmark DAC with dual HPA2 Headphone Outputs

Many Benchmark products include our HPA2™ headphone power amplifier. Unlike most headphone amplifiers, the HPA2™ is designed to behave like a small but very clean power amplifier. What makes the HPA2™ different, and what do we mean when we say that the HPA2™ is a "power amplifier"?

What is a Power Amplifier?

A power amplifier provides two things; power and control. Power is determined by the ability to deliver a combination of current and voltage. Control is the ability to maintain a precise distortion-free output into a dynamic load.

Speakers and headphones are both electromechanical devices. The only real differences are size and power of the transducers. Both place a continually varying load on the amplifier. Both have moving diaphragms that must be started and stopped in response to the audio signal. Both have input impedances that change with frequency. The complex load produced by an electromechanical transducer is much different than the load produced by a simple resistor.

A good power amplifier can initiate and damp transducer motions quickly, and it can do so without introducing distortion. A good power amplifier can also provide an output voltage that does not change when the load impedance changes. These characteristics provide control.

Damping Factor

Damping factor is one measurement of an amplifier's ability to provide control. An amplifier with a high damping factor will provide good acceleration and damping of moving diaphragms. By definition, the damping factor is the load impedance divided by the amplifier output impedance. This means that the damping factor is highest when the amplifier output impedance is designed to be as low as possible. The importance of a high damping factor is well understood for amplifiers that are designed to drive speakers, but it is often overlooked in amplifiers that are designed to drive headphones.

Like speakers, headphones perform best when driven by an amplifier with a high damping factor. A high damping factor can improve the transient response, and reduce some of the distortion caused by the transducers. It also prevents variations in frequency response that are caused by the impedance variations of the transducers.


Headphones and speakers present the amplifier with a load that varies with frequency. These transducers can act just like capacitors at certain frequencies. Amplifiers can become unstable when driving a capacitive load. A good power amplifier will include circuitry that is designed to keep the amplifier stable while driving capacitive loads. But, very few headphone amplifiers are built like power amplifiers.

Most headphone amplifiers use a simple and cheap shortcut to provide stability without the need for special circuitry. This cheap solution is a series resistor between the amplifier and the headphone. This shortcut provides stability but it completely destroys the damping factor of the headphone amplifier. The series resistor increases the output impedance and this decreases the damping factor. It is very common to find a resistor of at least 30 Ohms wired in series with the headphone output. In this configuration the headphone amplifier can still provide power, but it cannot provide good control over the transducers.

In contrast, the HPA2™ is designed like a true power amplifier. It provides stability without the need for a series resistor. The HPA2™ does not sacrifice the damping factor in order to achieve stability.

Short-Circuit Protection

Every amplifier needs protection against short circuits. Headphone amplifiers are routinely subjected to short circuits. The 1/8" and 1/4" phone plugs used for headphone connections create short circuits while the plugs are being inserted or removed. They can even create a persistent short if the plug is left in a partially inserted position. Worse yet, a mono phone plug will short the right channel to ground. For these reasons, headphone amplifiers need robust protection against short circuits. Again the cheap solution dominates. The same series resistor that was inserted to provide stability also provides protection against short circuits. In some cases, the short circuit protection requires a higher resistor value than that which is required to maintain stability. Again, when the resistor is inserted between the amplifier and the headphones, the amplifier no longer has good control over the drivers.

The HPA2™ has over-current detection and over-temperature detection to prevent damage to the amplifier. These circuits are more complicated, but they provide robust protection without sacrificing the damping factor.

Output Impedance

As stated above, a high damping factor can only be achieved when the amplifier output impedance is low relative to the load impedance. A power amplifier that is driving 8-Ohm speakers will need a lower output impedance than a headphone amplifier that is driving 30-Ohm headphones. A damping factor of 20 is sufficient to provide improved performance. Beyond a damping factor of about 20, the DC resistance of the transducers becomes the limiting factor and the transducer control does not continue to improve. If we agree that a damping factor of 20 is a reasonable goal, the amplifier driving 8-Ohms speakers will need a maximum output impedance of 8/20, or 0.4 Ohms. Likewise a headphone amplifier driving a 30-Ohm load will need a maximum output impedance of 30/20 or 1.5 Ohms. This rules out the use of a series resistor between the amplifier and the headphone. The conclusion is that a headphone amplifier needs to be a small power amplifier.

'0-Ohm' Output Impedance

We call the HPA2™ a "0-Ohm" headphone amplifier because it has no series resistor between the amplifier and the headphone jack. But no headphone amplifier has an output impedance of exactly 0-Ohms. The actual output impedance is approximately 0.1 Ohms. This is a very low output impedance, and it gives the HPA2™ a very high damping factor. This allows precise control of the headphone transducers. This control provides a predictable frequency response, a reduction in distortion, and improved damping of the transducer diaphragms inside the headphones. It offers a significant improvement relative to the typical headphone amplifier.

Published Specifications

Be careful when comparing published specifications. Headphone amplifiers are often measured with no load or with a simple resistor as a load. Under these ideal conditions many amplifiers will perform well. The differences don't show up until real headphones are connected to the amplifier. In "Headphone Amplifiers - Part 1" we showed the measured performance of several headphone amplifiers. We made two sets of measurements, one using a resistive load, and one using headphones as a load. All three amplifiers measured reasonably well while driving a resistor, but the "0-Ohm" HPA2™ was the only amplifier that measured well while driving headphones. The other two amplifiers were traditional designs that included performance-robbing series output resistors.

Unfortunately, most manufacturers do not specify headphone amplifier performance with anything other than ideal resistive loads. Benchmark publishes both, and we have measured the performance differences between "0-Ohm" and conventional headphone designs. Our measurements clearly show the importance of a near 0-Ohm amplifier output impedance. Headphones do not behave like resistors!

Our tests show that substantial distortion can be developed across resistors that are wired in series with headphones.  We conducted these measurements with a variety of headphones.  In general, the distortion due to the series resistor increases as headphone impedance decreases.  This distortion can be eliminated with a properly designed 0-Ohm headphone amplifier.

Frequency Response

All headphones have an input impedance that varies with frequency. If headphones are driven through a series resistor, the impedance variations in the headphones will create variations in the frequency response. In contrast, these load-induced frequency response variations will not occur if the headphone amplifier is a "0-Ohm" design.

For example, in "The 0-Ohm Headphone Amplifier" whitepaper, we showed that the impedance variations in the 60-Ohm Sony MDR-V6 headphones caused 0.7 dB variations in the frequency response when these headphones were driven through a 30-Ohm resistor.

THD Under Load

The performance of the HPA2™ does not change when headphones are driven. THD+N measurements on the HPA2™ show a virtually identical performance under a variety of load conditions. These include measurements with no-load, 30-Ohm resistive loads, 30-Ohm headphone loads, and 600-Ohm headphone loads.


The Benchmark HPA2™ is one of the finest headphone amplifiers available. The HPA2™ will substantially improve the sound of 30 and 60-Ohm headphones. It will also make noticeable improvements when driving the lighter loads imposed by 600-Ohm headphones.

The reference-quality performance of the HPA2™ has been carefully replicated whenever we have included it in a new Benchmark product. On Benchmark products, the headphone amplifier is never just added as a convenience. We firmly believe that the headphone amplifier should be able to deliver the full rated performance of the product. In many ways, our products are built around the HPA2™. Our HPA2™ headphone power amplifier often draws more power than all of the other circuits in the product combined. Our products have power supplies that are specifically sized to meet the needs of the headphone power amplifier.

Product List

The HPA2™ headphone power amplifier is included in the following products:

 The HPA2™ headphone power amplifier was also included in the following discontinued products:

John Siau

Recommended Reading:

More Headphone Application Notes - John Siau, Benchmark

Stabilizing Difference Amplifiers for Headphone Applications - Texas Instruments Application Note

Damping Factor: Effects on System Response - Dick Pierce, Audioholics


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