What is a Headphone Amplifier, and Why Would I Need One?
In an audiophile's quest to getting the best audio from his pair of headphones, he often encounters a time when a headphone amplifier is necessary. For the regular headphone user, a headphone amp is still a consideration when choosing the best headphones for the most desirable listening experience.
In this article, we'll describe headphone amplifiers in greater detail and provide you with the information to decide for yourself if a headphone amp is worth it to you.
What is a headphone amplifier?
A headphone amp is a relatively low-powered amplifier that balances the impedance and boosts the voltage of an audio signal to best match the connected headphone in which it sends the audio signal. Headphone amps allow many professional high-impedance headphones to reach their full potential.
As their name suggests, headphone amplifiers are designed to amplify audio signals to drive headphones properly.
A headphone amp connected to a pair of headphones is analogous to a power amplifier connected to a passive loudspeaker.
Headphone amps are most commonly found installed near headphone jacks in consumer and professional audio devices. These amplifiers can be completely analog or part of a digital device's DAC (digital-to-analog converter). Note that headphones are inherently analog and require analog audio signals to convert into sound. Active headphones, which include wireless and active noise-cancelling (ANC) headphones, have built-in amplifiers in their designs.
Wireless headphones have receivers to accept the wireless signal and decode the audio signal from it. An amplifier then boosts the audio signal so that it drives the headphone drivers properly.
Active noise-cancelling headphones have internal summing amplifiers to adjust the anti-noise signal and combine it with the intended audio signal. These amps are different than what we are typically referring to when discussing headphone amps.
Finally, there are standalone headphone amplifier units, which are generally the topic of discussion when the term “headphone amplifier” is being used. Standalone headphone amps are common in audiophile and professional studio markets. There are consumer-grade models used by hi-fi enthusiasts and audiophiles and professional audio models, which are typically used in recording studios.
Adding an amp between the audio device and a pair of high-end headphones may cause notable improvement in audio clarity, detail and dynamics. On the flip side, failure to do so with certain high-end headphones can lead to poor results in the listener's experience.
Headphone amplifiers boost the amplitude (voltage) of the audio signal to drive the connector headphones with greater accuracy and power. Amps will also alter the impedance of signals to better match with the connected headphones. To put it simply, headphone amps alter the audio signal in such a way that will benefit the performance of the headphones.
The digital-analog converters (DACs) found in some headphone amplifiers work to convert digital audio (the 1's and 0's) into the analog audio (alternating electrical current) required to actually move the drivers and allow them to act as transducers. Headphones need analog audio signals to convert into sound waves, and DACs ensure a digital source can properly interact with the headphones.
Are Headphone Amplifiers Worth It?
Your pair of high-impedance, professional-grade or HiFi headphones may require a headphone amp to reach their full potential. Similarly, the audio quality may suffer greatly if there is no headphone amplifier to adjust the audio signal properly. Headphone amplifiers are nearly always worth it in these situations. However, they can be very expensive. A related question, therefore, is: are expensive headphone amplifiers worth it?
For example, there's a big difference between the inexpensive ~$20 USD Pyle Pro PHA 40 and the ~$32,000 HIFIMAN Shangri-La SR Electrostatic Amp.
So what makes a headphone amp worth it? Well, first and foremost, I'd say the price point must fit within your budget. Next, I'd suggest researching product descriptions, online forums and product reviews to see what amplifier model best suits your particular headphones.
For instance, the aforementioned HIFIMAN Shangri-La SR is wickedly expensive but is meant specifically for ultra-high-end HIFIMAN's Shangri-La SR electrostatic headphones. It is not meant to drive high-end dynamic headphones, for instance. To add to that point, the Pyle Pro PHA 40 mentioned above is by no means a high-end amplifier made for HiFi applications. Rather its primary function is to provide gain and impedance conversion to a line level signal and split it into 4 different headphone sends.
Plenty of quality headphone amplifiers are available for a few hundred or a few thousand dollars that may suit your specific headphones perfectly. As discussed at the beginning of this section, a headphone amp is worth investing in if your headphones require one. The question, then, becomes how much money you're willing to spend.
Built-In Amps in Headphone Output
As we've discussed, headphone amplifiers are not only found the standalone units that are obviously titled headphone amplifiers. They are also found inside each and every headphone jack. These jacks are part of many common devices, including:
These amps often have DACs (digital-to-audio converters) to convert the digital audio to analog audio in order to drive the headphone drivers. Headphone jack amps range in quality but are nearly all of poorer quality than the standalone amplifiers we're focusing on in this article. Headphone jacks are often only capable of driving low-impedance headphones. Many professional and HiFi headphones are designed with higher impedances and would not perform at their potential if connected to a regular headphone jack. This is due to the size/space and power limitations of the headphone jack amplifiers. They must fit within the device and be in close proximity to the jack while also working on the device's power source (often batteries). That being said, there is certainly a market for high-end low-impedance headphones that will work effectively with our portable devices and their headphone jacks.
Reaching the Full Potential of the Headphones
Speaking of reaching the full potential of our headphones, let's discuss how headphone amplifiers help us get the most out of our headphones. As mentioned, the built-in headphone jack amplifiers are often not powerful enough to drive high-end headphones. Amplifiers need to deliver enough power to properly drive the headphones with clarity and precision without causing distortion at the peaks of the amplified audio signal. Let's first look at the headphones that would likely require amplification. There are plenty of factors that would allow headphone audio quality to be improved by an amp. The two most important factors are efficiency/sensitivity and impedance.
What is Headphone Efficiency/Sensitivity?
Headphone sensitivity (sometimes called efficiency) is the relationship between the power of the audio signal sent to the headphones and the loudness of the resulting sound produced by the headphones. Ideally, this rating should be presented as a sound pressure level (in decibels) per 1 milliwatt of signal power (dB SPL/mW). The more sensitive a pair of headphones, the more sound volume they'll produce at a certain signal strength.
Generally speaking, headphones with a sensitivity rating of 95dB or less are more likely to require a headphone amp. That being said, the sound of headphones with higher sensitivity ratings may very well be improved with an amplifier as well. Note that the amount of power a headphone amplifier is capable of providing its connected headphones is a product of the headphones' impedance. Higher impedance headphones tend to require more power to produce the same amount of sound. The benefit that comes with higher impedance and requiring more power is that the headphones tend to sound more accurate.
What is Headphone Impedance?
Headphone impedance is essentially the measure of the headphone drivers' resistance to the audio signal driving them. Impedance values are given in ohms (Ω). All else being equal, higher impedance headphones ought to be less sensitive than lower impedance headphones since their drivers require more power to be driver effectively. However, headphone design plays a big role in determining these stats, and there is no correlation between impedance and sensitivity across different headphone designs.
Generally speaking, headphones with higher impedances require stronger signals to drive their drivers. Though any headphones could be hooked up to an amplifier (connection types permitting), those with higher impedances would likely benefit more from an amplifier than those with lower impedances. Although there are no set rules, headphones with impedances over 80 Ω will generally benefit from a headphone amplifier. The increase in voltage provided by the amplifier gain will allow the audio signals to drive the high-impedance driver with more power, precision and volume. As another general rule, the headphone (load) impedance should be at least 8 times the amplifier's output impedance (source).
Balanced Headphone Signals
Some headphone amps also provide balancing in case the high-end headphones are balanced as well. Balanced headphones have the following advantages over unbalanced headphones:
No crosstalk since there is no common ground/signal return wire.
Doubling of voltage applied to the driver which improves clarity and amplitude.
That being said, if the headphones are unbalanced, there is really no advantage here.
Electrostatic Headphone Amplifiers/Energizers
Electrostatic headphones are a special breed. These headphones, as their name suggests, work on electrostatic principles. The electrostatic headphone driver has a positively charged diaphragm placed between two stator plates that act as a sort of parallel plate capacitor. The audio signal is applied to the stator plates, which effectively complete the circuit. As one stator plate receives a positive voltage from the signal, the other stator plate receives an equal but negative voltage. The audio signal effectively charges the stator plates, which causes the diaphragm to move back and forth due to attraction and repulsion.
For the driver to work properly, it has a wildly high impedance. This helps to mitigate any stray voltage from the plates. Thus, a very high voltage is required at the stator plates to drive the electrostatic driver properly. These high-voltage signals have very low currents and are achievable via specialized electrostatic headphone amplifiers/energizers. On top of that, the electrostatic amp/energizer may also need to provide the bias voltage required to charge the diaphragm.
The STAX SMR-007TII is a well-known electrostatic headphone amp that works well with the famed STAX SR-007 headphones. This tube amplifier has 3 inputs (1x XLR and 2x RCA) with 1x RCA parallel output and two headphone outputs with Molded 5-pin connectors for use with STAX headphones.
The SMR-007TII has an incredibly high input impedance of 50 KΩ (100 kΩ when balanced). Its rated input level is 200mV with a maximum input level of 30V rms at minimum volume. This model provides 54 dB (x500) of gain to the signal and has a maximum output voltage of 340V rms at 1 kHz. The SMR-007TII also provides 580V (x2) as a biasing voltage for the electrostatic headphone drivers.
Active Noise-Cancelling Headphones Have Built-In Headphone Amps
Active noise-cancelling (ANC) headphones need amplifiers to function properly though these amps are not the same as the amps we've been discussing thus far. Rather, the built-in amplifiers in ANC circuits are tasked with the following:
Amplifying/processing the noise-cancelling microphone signal in order to produce the anti-noise signal.
Summing the intended audio together with the anti-noise signal.
Note that, generally speaking, the ANC headphones, when wired, rely on the amplifier of the headphone jack or the standalone amplifier to boost the “intended audio” signal to the appropriate level.
Wireless Headphones Have Built-In Headphone Amps
Wireless headphones receive their audio signals wirelessly. Let's briefly discuss how they work.
The intended audio signal, whether analog or digital, will be encoded into a wireless format (either a radio frequency signal, including Bluetooth or an infrared signal) by a transmitter. The transmitter could be a standalone device, or it could be built into the audio device. Bluetooth wireless, for example, is a standard protocol that many digital devices have to transmit digital audio wirelessly via radio frequencies in the range of 2.400 to 2.485 GHz. The wireless receiver, which is built into the headphones themselves, decodes the audio signal from the wireless signal for the headphone drivers to convert to sound. Before the audio signal is sent to the drivers, it passes through a DAC and/or amplifier. These devices will convert digital signals to analog if need be. They will also provide the appropriate amount of gain to the signal and be designed for optimal impedance matching with the headphone drivers.
A Few More Headphone Amplifier Examples:
Behringer Microamp HA-400
Rupert Neve Designs RNHP
It is our hope that this article has helped clear up some things about when/why headphone amplifiers would be a worthwhile investment. If you have any questions, or want to learn more about how to improve the sound quality from your headphones, reach out to us here and set up a free consultation!