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December 15, 2023 8 min read
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.
First, let’s delve into the definitions of these terms.
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.
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.
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.
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.
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.
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.
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.
October 13, 2024 5 min read
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