Ultimate Headphone Guide Articles: What is a headphone amplifier, and why do I need one?
Editors note: More informational info that will appear in "The Ultimate Headphone Guide" (UHG). Please feel free to comment on important points that may have been missed, but remember they need to remain short and to the pointmuch will no doubt go unsaid. For more information see this post on the Ultimate Headphone Guide. The following is Skylab's contribution introducing the need for a headphone amplifier. Thanks Rob!
What is a headphone amp and why do I need one?
If you are a high-end headphone aficionado, you undoubtedly already know what a headphone amplifier is, and probably own several. But for most people, as popular as headphones have become, the whole idea of a headphone amplifier is a bit foreign. And given that headphone amps can range from fairly inexpensive to VERY expensive, it's important to understand what they do, and when you are most likely to need one.
Headphones are basically miniature speakers. And all speakers need an amplifier. The fact is, headphones do not work without a headphone amplifier. What is sometimes confusing is that many music playing devices already include a built in headphone amplifier. Every smartphone does. Every tablet does. Pretty much every computer does. And some CD players do as well.
So, why do I need an external, stand-alone headphone amplifier? Because the fact also is that the built in headphone amps are likely going to be a significant compromise, given the size/space and power limitations they have to work with. A stand alone headphone amp has much more space to dedicate to better circuitry, and a more robust power supply, allowing both overall better controlled sound and, importantly for some headphones, the ability to provide considerably more output power to the headphone.
The key is that amps need to deliver enough power for the headphone not to sound distorted at the musical peaknot the average volume. The purpose of a headphone amp is not to blast your ears into deafness! It's to have the sound you hear be cleaner and more controlled, especially during peaks. But you should always practice safe listening, and keep the volume at a very comfortable level.
Headphones have a number of properties with will determine how much they need a headphone amplifier, but the most important are the efficiency/sensitivity, and the impedance.
The efficiency or sensitivity of a headphone indicates how loudly it will play given a certain amount of amplifier power. This specification SHOULD be stated as a certain decibel level achieved with 1 milliwatt (mW) of power. A VERY efficient headphone will provide 100 dB (or more) of output given 1mW of power from the amp. Typically this is the range an IEM will be in. And as such, a high efficiency headphone will be less likely to require a dedicated headphone amplifier, from a purely power/volume perspective.
Many non-portable, high end home headphones are less efficient, though, and some, like the currently popular planar magnetic headphones, are really not at all efficient, and require more than a few watts of power to generate the same output level that an high-efficiency IEM can generate from one milliwatt. Headphones with sensitivity specs in the 95dB or less range (as a very loose guide) are more likely to require a headphone amp.
The other factor is the relationship between the headphone impedance and the output impedance of the amplifier. As a general rule, the headphone impedance should be 10 times the output impedance of the amplifier. This is a complex subject, but in brief, when the amplifier's output impedance is much lower than the headphone impedance you get good electrical damping of the headphones and they will sound tighter and more articulate. High-impedance cans with over 300 Ohms impedance like the Sennheiser HD 600 and HD 800 will be very happily driven by high-output impedance amps like output transformerless tube amps, which have output impedances in the few tens of Ohms. Custom In-Ear Monitors like the Jerry Harvey JH 13 with around 10 Ohms impedance are best served by a headphone amp with under 1 Ohm output impedance.
Lastly, by using higher quality components and power supplies, a dedicated headphone amp will provide better quality sound at ANY volume level. Remember, we are concerned here with having the PEAK output be clean and undistorted, and having the overall signal reproduced as cleanly as possible; it's not about the quantity of the signal, it's about the quality of the signal. A great headphone will sound good from just about any headphone jack, but it won't sound great without an equally worthy headphone amplifier.