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  • December 15, 2023 8 min read

    The complex nature of headphone impedances

    The complexity of headphones lies in the wide range of impedance and efficiency among different models. While average passive speakers have impedances mostly between 4 to 16 ohms, headphones can range from 3 to 600 ohms. This vast range makes it difficult for ordinary devices to match with different headphones.

    What are headphone impedance, sensitivity, sound pressure level (SPL)?

    First, let’s delve into the definitions of these terms.

    1. Impedance: Measured in ohms (Ω), this refers to the electrical resistance a headphone presents to the amplifier. Higher impedance means the headphones resist the electrical current more, requiring more voltage to drive them effectively. Lower impedance headphones require less voltage but may demand more current.
    2. Sound Pressure Level (SPL): SPL is the pressure level of sound, measured in decibels (dB), produced by the headphones. It's the pressure level of the sound waves that the headphones create and what we perceive as loudness.
    3. Sensitivity: Measures how efficiently the headphones convert an electrical signal into an audible sound. It is usually expressed in decibels of Sound Pressure Level per milliwatt (dB SPL/mW). Sometimes, you might see sensitivity labeled as Efficiency,

    It indicates how loud the headphones will be for a given input power. A higher sensitivity rating means that the headphones can produce a louder SPL with less power. Conversely, lower sensitivity headphones will need more power to achieve the same SPL.

    The high-impedance = hard to drive myth

    For us who consider audiophile-grade headphones, we look forward to the best sound quality achievable withing our budget. It is crucial to understand how impedance and sensitivity relates to amplifier output.

    Often, high-impedance headphones are considered by many people as “hard to drive” headphones, but is that true or just a myth? Here we aim to demystify the concept of "hard to drive" when it comes to headphones in a simple way. , especially in relation to impedance and sensitivity.

    Let's clarify first what 'Hard to Drive' means. Headphone enthusiasts use the term "hard to drive" loosely to communicate. But they may not exactly refer to the same thing in the context of the discussion. So the word can be misleading.

    If you consider a headphone "hard to drive" based on the amount of electrical power it needs to achieve adequate loudness, then sensitivity is the primary factor. The lower the sensitivity, the higher the power output(mW) the headphones need to achieve the same volume, regardless of its impedance. So headphones with lower sensitivity are indeed “hard to drive”, as more miliwatt is needed. In terms of power requirements, sensitivity is the key factor.

    Then, why are there people thinking high impedance headphones are hard to drive?

    The idea that high-impedance headphones automatically means "hard to drive" is oversimplified and not necessarily true.

    Power in headphones is calculated as Power (Watts) = Voltage (Volts) × Current (Amps), influenced by Ohm's Law (V=I×R).

    When R is higher(High-impedance headphones), maintaining the same power level (loudness) requires a higher voltage and lower current. In contrast, low-impedance headphones require less voltage but more current to achieve the same power level. (Here we assume the high-impedance and low-impedance headphones have the same sensitivity.)

    Impedance affects how the power is delivered – more specifically, the balance between voltage and current. The amplifier's design should vary accordingly: amplifiers for low-impedance headphones focus on high current output, whereas those for high-impedance headphones need to deliver high voltage.

    And here comes the real problem of headphone amps in portable devices, when paired with high-impedance headphones, they are often insufficient in voltage output, making the headphones connected not loud enough. This does not necessarily mean the headphones are more power hungry. Just the voltage requirement exceeds the capability of headphone amps inside.

    Why do these built-in headphone amps struggle in voltage output?

    Headphone amplifiers in portable devices like smartphones and laptops are often powered by lithium-ion (Li-ion) or lithium polymer batteries. These batteries are commonly used due to their favorable energy density and rechargeability and typically have a nominal voltage of around 3.3V to 3.7V per cell.

    In the context of headphone amps, working at or near this voltage level is more efficient than converting to a higher voltage. This is because voltage conversion, such as stepping up the voltage, involves additional circuitry, which introduces inefficiencies and losses. Higher voltage also means the requirements of more complex battery management and protection circuits which increases design complexity and cost. Doing so can be impractical in compact and lightweight devices. Therefore, headphone amps in portable devices are designed to operate more efficiently at the lower voltage levels and are not suitable for high-impedance headphones.

    Why are there high impedance headphones anyway?

    If low-impedance headphones are well-suited for portable devices, what is the purpose of high-impedance headphones, and what drives people to seek out these models?

    For efficient power transfer, an amplifier's output impedance should be much lower than the headphones' impedance. This principle ensures effective energy transfer and better control of the headphone drivers, reducing energy loss in the system.

    Historically, before the advent of portable quality audio devices, most people connect their headphones to home stereo receivers, record players, or reel-to-reel tape players. It was normal for consumer devices to have a high impedance output (around 47-Ohm) at the headphone jack, requiring headphones with impedance over 100-Ohm for decent damping. At that time, there was no demand for low impedance headphones.

    With higher quality portable audio devices become widely available, the demand for low impedance headphones began to grow. But high impedance headphones are in the market first and preferred in certain scenarios despite not being ideal for portable devices, mainly due to their design and sound quality benefits.

    • Lighter Voice Coil: Higher impedance in dynamic headphones usually means the voice coil is thinner and lighter. This reduced mass allows for more precise movements of the diaphragm, potentially enhancing the transient response speed, especially in the high-frequency sound performance.
    • Better Amplifier Control: High impedance allows for easier control by the amplifier. It typically results in a higher damping factor, which refers to the amplifier's ability to control the movement of the headphone's diaphragm. Better control leads to more accurate sound reproduction, especially in terms of transient response (how quickly the sound can start and stop).
    • Better signal-to-noise ratio: Every amplifier circuit has inherent background noise. This noise essentially remains constant regardless of the volume knob's position, in other words, it's a fixed value. The noise is less noticeable with high impedance headphones as they are better at blocking out background noise.
    • Durable Design: In certain extreme or accidental situations, such as amplifier malfunction or sudden increase in volume, high-impedance headphones have better tolerance and are less likely to be damaged.
    • Compatibility with Professional Equipment: High-impedance headphones are designed to work well with professional audio equipment, which often has higher voltage outputs. This makes them a standard in recording studios and for audio mixing.
    • Reduced Interference: Higher impedance can reduce susceptibility to interference, greater channel separation (crosstalk), leading to clearer audio.

    The benefits of having a headphone amplifier with different outputs that match different Impedances

    Obviously, the world of headphones is more fascinating and diverse than that of speakers. Since it lacks the common standard like the passive speakers. Most passive speakers typically fall between 4 to 8 ohms, and regardless of the size, their sensitivity is generally between 85dB to 95dB.

    But what about headphones?

    From the perspective of impedance and sensitivity, the known range of headphone impedance on the market currently varies from as low as around 12 ohms to as high as 600 ohms. As for the range in sensitivity, it's even more striking – the highest can reach near 130dB, while the lowest is only around 80dB. The difference in sensitivity can be as much as 50dB. This 50dB difference equates to a power difference of 50,000 times!

    Here's a practical example: if one set of headphones has an impedance of 65 ohms and a sensitivity of 80dB, while another has an impedance of 16 ohms and a sensitivity of 130dB, and both are used with the same amplifier with the volume set at the same level, the 80dB headphones might sound as faint as a mosquito's buzz, while the 130dB headphones could be overwhelmingly loud, potentially damages the headphones themselves.

    From a power calculation perspective, driving a set of 65-ohm, 80dB sensitivity headphones to reach 120dB requires a headphone amplifier's power output equivalent to an 80-watt power amplifier for an 8-ohm passive speaker. When the output is that high, using such output for low-impedance in-ear headphones with high sensitivity is obviously inappropriate and risky.

    Having a headphone amplifier with different outputs for different impedances and sensitivities offers a significant advantage for audiophiles to enjoy using a variety of headphones. It ensures optimal sound performance, maximizes versatility, and protects your investment in the long run.

    Take DA&T Q-J as an example. The desktop headphone amp comes with four different headphone jacks, utilizing different headphone amplifier circuits, to drive 3.5mm, 4.4mm, 6.3mm, and 4-pin balanced headphone jacks. This allows earbuds, in-ear monitors, on-ear headphones, and over-ear headphones to have different amplifier circuits and configurations corresponding to them. The gain is adjustable from 0 to 14 dB, ensuring that each type and style of headphones receive appropriate power, are safely driven, and optimized for sound quality.

    Here we list several additional benefits of using DA&T Q-J as the headphone amplifier besides optimal power matching.

    • Tailored Volume Levels and Precise Volume Control: With multiple headphone outputs, each tailored to a specific range of headphone impedances, users gain more precise control over the volume. The adjustable gain allows the user to easily set the ideal volume level. For high-sensitivity headphones, a lower gain setting prevents excessive loudness and potential hearing damage, while for less-sensitive headphones, a higher gain setting can provide the necessary volume boost. The different gain settings also give different tonal representations. It is easier for the user to get an ideal sound.
    • Optimal Damping Factor: The right impedance match between the amplifier and headphones ensures an optimal damping factor. This leads to better control over the headphone drivers, resulting in tighter bass response and a more accurate soundstage.
    • Protection for Headphones: By optimally matching the amplifier’s output to the headphone’s impedance, the risk of damaging the headphones due to overdriving is minimized.
    • Future-Proofing Investment: If you plan on expanding your headphone collection or upgrading to headphones with different impedances, a versatile amplifier with multiple outputs is a wise investment. You can enjoy the best possible sound from a wide range of headphones, from low-impedance in-ear monitors like Shure SE215, high-impedance studio-grade headphones like Beyerdynamic DT880 600 ohm, to home listening high-resolution over-ear headphones like Sennheiser HD 800 S and won't need to replace the amplifier when you switch headphones, saving you money and ensuring compatibility.
    • Reduced Noise Floor: Mismatched impedance can introduce obvious background noises and create distortions in the sound. Dedicated outputs minimize this risk, ensuring cleaner and more accurate audio reproduction. By adjusting the gain, users can also optimize the signal-to-noise ratio of the amplifier. At lower gain settings, the noise floor of the amplifier can be reduced, leading to a clearer and more pristine sound quality, especially at lower listening volumes. This is crucial for critical listening sessions where every detail in the music matters.
    • Enhanced Compatibility with Source Devices: Different audio sources output at varying signal levels. Adjustable gain allows the amplifier to match these levels effectively, ensuring that the input signal is neither too low (resulting in weak audio) nor too high (leading to distortion). This compatibility is vital when using a variety of source devices, from professional-grade audio interfaces to consumer-level smartphones.
    • Improved Dynamic Range: With adjustable gain, the dynamic range of the audio can be better preserved. At appropriate gain settings, the amplifier can accurately reproduce both the quietest and loudest parts of the music without distortion or compression. This results in a more lifelike and engaging listening experience.

    Conclusion

    By understanding the complex relationship between headphone impedance, sensitivity, and amplifier output, users can make more informed decisions and enjoy a superior audio experience.

    Dedicated headphone amplifiers for specific use are no doubt one solution. However, getting a headphone amplifier that can accommodate multiple impedances is also a good choice, especially for audio enthusiasts and professionals who use a variety of headphones. It ensures that regardless of whether one is using low-impedance, portable headphones, or high-impedance studio models, the sound quality is not compromised.

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