Often the question comes up whether an external amplifier is required to drive a pair of headphones properly. And if so, what is the best amplifier to buy? As you can imagine, the answer isn’t always easy, and often leads to more questions. Can some headphones really not function without investing in an amplifier? What does that mean? How do I compare amplifiers without listening to them?

Pretty generally, you can plug any pair of headphones into any source and hear sound. If there is a headphone jack, rest assured, the device has some sort of internal amplification to drive headphones. Of course, that sound may be far from optimal. 

The quality of the reproduced music is a function of a few things.

• Headphone sensitivity and impedance.
• Amplifier impedance.
• Amplifier type.
• Amplifier current and voltage.

Some headphones are designed to be used with virtually any source, regardless of amplifier power. And some, typically more high-end, audiophile-focused headphones, simply will not sound their best without the proper pairing. To achieve the highest fidelity sound, it is crucial to match the amplifier to the headphones.

Headphone Specifications

For the purposes of this article, I’m going to assume that you have a pair of headphones (or two, or three, or…), and you are looking for an amplifier to best match what you have. A great place to start is to take a look at those technical specifications that came with your headphones. Two items are the key to figuring out what sort of amplification is required.

Headphone sensitivity and impedance are related but not directly linked. Even from a single manufacturer, there are headphone models with low impedance and high sensitivity, vice-versa, and every sort of in-between.

Headphone Sensitivity and Amplifier Power

Headphone sensitivity is synonymous with efficiency and is a measurement of volume  (measured in decibels) at a specific power rating. Sensitivity and volume are directly related. When two headphones receive the same amplifier power, the more sensitive pair will be louder.

Headphone sensitivity measurements often appear as xx dB (1kHz @ 1mW). That is to say, the headphones will produce xx dB of sound when fed a 1kHz tone with 1mW of power. Extremely insensitive headphones are in the range of 90dB, while sensitive headphones are around 105dB. Since comfortable (and safe) listening levels are between 60-80dB, it would appear that 1mW of amplifier power should suffice.

Unfortunately, it isn’t quite that simple. Various music styles differ in dynamic range (the difference in intensity between the quietest to loudest sounds in the music). Classical music typically has a much larger range than pop music (up to 20db) which translates to a 100x greater power requirement. 

That’s how the initial 1mW amplifier power requirement quickly becomes 100mW. Now factor in that an amplifier typically produces more distortion at the top of its power range. So, to produce 100mW at low distortion levels, the amplifier may have to be rated to deliver 1000mW (1 Watt) or more.

Headphone Impedance

The headphones’ impedance can be considered the “load” impedance in an electronic circuit (measured in Ohms). While similar, resistance is a DC (direct current) function, while impedance has to do with AC (alternating current). Headphone amplification circuits use AC.

The general impedance standard for headphones is around 32 Ohms but can vary between 4-600 Ohms. Compare that range to the 2, 4, 8, and 16 Ohms standards of home and car audio. The impedance rating is not an indicator of overall sound quality (contrary to popular thinking, higher is not always better). 

A 32 Ohms (or less) pair of headphones with decent sensitivity (±100dB) should be easily driven by almost any source without additional amplification. However, various driver types (planar vs dynamic for example) can have vastly different amplification requirements that aren’t adequately captured by the sensitivity and impedance ratings.

Planar magnetic driver headphones have a single impedance across the entire audible frequency spectrum. Conversely, headphones with dynamic drivers vary in impedance depending on the frequency being reproduced. As a result, the single frequency impedance measurement stated in the technical specifications can be misleading.

Amplifier Impedance

Amplifier (or “source”) impedance interacts with headphone impedance. Amp impedance ratings can vary from near 0 up to around 120Ohm (for specialty OTL tube amplifiers). As a general rule of thumb, lower is better for amplifier impedance. 

Headphone and amplifier impedance must be matched appropriately (not equal) for optimal sound reproduction.

There is a general ‘rule of eighths’ (headphone impedance should be at least eight times higher than the amplifier output impedance), although pairings around 2-3x higher can work well in some instances. Equal or lower headphone impedance ratings (compared to the amplifier) will result in poor driver control (low damping factor), loose bass, and increased distortion.

Types of Headphone Amplifiers

A headphone amplifier increases the power of the audio signal. It is measured by its gain, which is defined as the ratio of the output voltage, current, or power to the input. A gain of 1 is unity and considered no amplification.

Amplifiers may generally be divided into several categories, vacuum tube (like the xDuoo TA-03S), solid-state (like the SMSL VMV P1), or hybrid (a combination of both, like the xDuoo TA-10R). All types have benefits and drawbacks, although in general, solid-state amplification is considered cleaner (lower measured distortion – either desired or unwanted) than tube-based. 

Both vacuum and solid-state categories have many subcategories (tube – OTL, triode, etc., solid-state – THX, negative feedback, etc.), but that is beyond the scope of this article. 

Amps are also sub-divided by usage. Battery or source-powered mobile amplifiers are considered portable and are designed to connect to phones, tablets, or DAPs (like the Hidizs DH1000). Desktop amplifiers are typically powered by household mains, and physically larger than portable options (like the SOUNDAWARE P1). Amplifiers may also be integrated (including an internal DAC) or have inputs and outputs to support multiple connections to external devices.

You will commonly see amplifiers referred to as balanced or unbalanced (single-ended). This is a bit of a misnomer, as balanced and single-ended refer to the interconnections between devices (DAC to amp, or amp to headphones), rather than the topology of the amplifier itself. 

Usually, an amplifier is referred to as balanced when it is a differential amplifier without shared grounds between channels, and it has balanced input and output connections. A fully balanced (discrete) amplifier should have separate, unique, and duplicated circuitry from input to output for each channel (like the DAART Yulong Aquila II).

Output impedance is effectively doubled (with a resulting 2-fold decrease in damping factor) from a balanced amplifier. However, the balanced power output is often greater than from the single-ended socket. If the amplifier’s rated power is equal between single-ended and balanced outputs, the amplifier is likely not discretely balanced inside and only features balanced inputs and outputs (like the .

Common balanced inputs are dual 3-pin XLR sockets, while single-ended inputs typically use a pair of RCA jacks. The key benefit of using balanced device interconnections is the removal of ground loop noise through common-mode noise rejection. 

Balanced headphone connections may be 4-pin XLR, dual 3-pin XLR, 2.5mm TRRS, or 4.4mm Pentaconn. Single-ended headphone connections are commonly 3.5mm or 6.35mm TRS. 

Amplifier Current and Voltage

An amplifier must be able to produce enough power to drive a pair of headphones sufficiently. Obviously, this is easier and less impactful with efficient and low impedance headphones. Various amplifier designs have different specifications and abilities, and some may struggle to drive more demanding loads.

In general, lower impedance headphones require more amplifier current, while higher impedance headphones require more source voltage. Smaller, portable amplifiers often struggle with producing high output voltage, although individual specifications vary greatly between models. 

Conclusion

Headphones and amplifiers diverge wildly in sensitivity, impedance, and power ratings. This means that not every amplifier is a good match for every pair of headphones. Determining a proper match, especially for difficult-to-drive headphones, is the key to achieving the highest quality sound reproduction.

You should get comfortable in reading and interpreting the technical specifications for both headphones and amplifiers, and comparing them to determine suitable theoretical matches. However, nothing beats hands-on experience, and sometimes a great-sounding headphone and amp pairing can be made that seems to break all the rules.

Author Trav Wilson:  Audiophile or Audio-Phool? I don’t claim to have golden ears with magical properties, nor any ability to create music. But I do have a deep appreciation for music, founded at a young age, and curated over the years. I’m also unapologetically a gear-head and love lights, buttons, meters, switches, and especially things made from traditional wood, leather, metal, and glass materials. As with everything, take it for what it is: this is just one person’s opinion. Runs noteworthy.audio.