High Output Impedance in Headphone Amplifiers Blurs Bass Transients by Skewing Driver Frequency Response

When a standard 32-ohm dynamic headphone is plugged into an amplifier with a 10-ohm output impedance, the driver loses nearly a quarter of its driving voltage before the signal even reaches the cable. This severe voltage drop strips the amplifier of its electromagnetic grip on the voice coil. The consequence is a physical loss of control over the diaphragm itself.

Many listeners assume that an amplifier simply pushes a uniform audio signal into a headphone. In reality, the headphone pushes back. A dynamic driver is an electromagnetic motor, and its resistance to electrical current changes drastically depending on the frequency it is asked to reproduce.

This shifting resistance is known as a reactive impedance curve. Because the driver's impedance is not static, the amplifier's own internal resistance—its output impedance—dictates how accurately the headphone can start and stop its physical movements.

How Frequency Alters Driver Resistance

A dynamic headphone driver consists of a voice coil suspended in a magnetic field, attached to a diaphragm. When an audio signal passes through the coil, the electromagnetic forces push and pull the diaphragm to create sound waves.

However, this mechanical system has a natural resonant frequency, typically located in the bass region around 70 to 100 hertz. At this specific frequency, the driver requires significantly less electrical effort to move, causing its electrical resistance to spike dramatically.

A headphone advertised with a nominal impedance of 300 ohms, such as the Sennheiser HD800, does not measure 300 ohms across the entire audio spectrum. At its 100-hertz resonance point, the impedance curve peaks at nearly 600 ohms.[4]

This massive swing in resistance creates a complex electrical load for the amplifier. If the amplifier has a high output impedance, it cannot deliver a consistent voltage across these varying resistance levels, leading to unintended shifts in the headphone's frequency response.

The Voltage Divider Effect on Bass

The interaction between an amplifier's output impedance and a headphone's reactive impedance is governed by the voltage divider formula. The amplifier and the headphone act as two resistors in a series circuit, sharing the total available voltage.[5]

When the amplifier has a near-zero output impedance, almost all the voltage reaches the headphone, regardless of the frequency being played. The amplifier acts as a perfect voltage source, maintaining a flat frequency response that honors the manufacturer's original tuning.

Conversely, an amplifier with a high output impedance, such as an output-transformerless tube amplifier, retains a significant portion of the voltage internally. Because the headphone's impedance rises at its bass resonance, it captures a larger share of the voltage exactly at that frequency.

The result is an artificial amplification of the bass frequencies. Driving a 300-ohm headphone that peaks at 600 ohms with a 120-ohm amplifier forces a 1.7-decibel boost at the resonance point, skewing the tonality and muddying the lower registers.[7]

Back EMF and the Loss of Damping

Frequency skew is only half of the problem caused by high output impedance. The other half involves the physical control of the driver's diaphragm, a concept rooted in the generation of back electromotive force, or back EMF.

Because a dynamic driver is a moving coil in a magnetic field, it acts as a microphone as well as a speaker. When the audio signal stops, the diaphragm's momentum keeps it moving, generating its own electrical current that travels backward into the amplifier.[3][6]

To stop the diaphragm from ringing, the amplifier must absorb this reverse current. An amplifier with a near-zero output impedance effectively short-circuits the back EMF, applying an electromagnetic brake that halts the driver instantly and preserves crisp transient details.

If the amplifier has a high output impedance, the back EMF faces resistance and cannot be absorbed quickly. The driver is left to coast to a stop on its own mechanical suspension, causing bass notes to overhang and blur into one another.

Calculating the Damping Factor

Audio engineers quantify this electrical braking ability using a mathematical metric called the damping factor. It is calculated simply by dividing the headphone's nominal impedance by the amplifier's output impedance, providing a clear ratio of electrical control over the moving mass.[2][6]

A higher damping factor indicates superior control over the driver's physical movement. For example, pairing a 300-ohm headphone with a 1-ohm solid-state amplifier yields a robust damping factor of 300, ensuring that back EMF is neutralized immediately.

Pairing that same 300-ohm headphone with a 120-ohm tube amplifier drops the damping factor to a mere 2.5. At this level, the amplifier has virtually no electrical grip on the voice coil, allowing the driver's natural resonance to dominate the acoustic output.

This lack of damping is often subjectively described by listeners as a lush or warm sound. In reality, it is the acoustic signature of an under-damped driver failing to accurately track the trailing edges of low-frequency transients.

Applying the One-Eighth Rule

To prevent both frequency response skew and transient blurring, the audio industry relies on a standard guideline known as the one-eighth rule. This rule dictates that an amplifier's output impedance should never exceed 12.5 percent of the headphone's nominal impedance.[1]

Following this mathematical boundary guarantees a minimum damping factor of eight across the entire frequency range. At this ratio, the voltage divider effect is minimized, keeping any unintended frequency response variations strictly below the one-decibel threshold of human audibility.

For a standard 32-ohm consumer headphone, the amplifier's output impedance must be 4 ohms or lower. For highly sensitive 16-ohm in-ear monitors, the source must measure under 2 ohms to maintain accurate tonality and tight bass control.

Ultimately, matching output impedance to the specific reactive load of a headphone is the most critical step in preserving audio fidelity. Ignoring the electrical physics guarantees that the listener is hearing the amplifier's limitations rather than the music itself.