Many studies show that high-end amplifiers, regardless of their type or class, deliver a level of sound quality where no audible differences can be detected in blind tests.

Scientific Basis of Blind Testing

• Established Methodology: Blind and double-blind testing are well-established methodologies in audio research and consumer testing. These tests eliminate variables and provide objective, measurable results. Their use in comparing amplifiers adds credibility to the claim that sound quality is not distinguishable when distortion is minimized.
• Examples of Blind Test Studies: Many audio organizations, such as the Audio Engineering Society (AES), have published papers and articles where high-end amplifiers were tested in blind tests, and the results supported the conclusion that the differences in sound quality were negligible when distortion and frequency response were consistent.

Importance of Accurate Audio Measurement

• Objective Performance Metrics: Objective measurements of THD, IMD, frequency response, and signal-to-noise ratio (SNR) are key indicators of an amplifier’s performance. When these measurements are within high-quality thresholds, the sound quality should be indistinguishable to the human ear.
• Correlation Between Measurements and Perception: High-end amplifiers, by design, prioritize these objective measurements to ensure accurate reproduction of sound without introducing audible flaws. This technical precision translates to a clear, uncolored sound, which is why listeners in blind tests often cannot discern one amplifier from another when they have similar technical specs.

There are two main types of blind tests: blind tests and double-blind tests. In a blind test, the listener is unaware of which amplifier is playing, but the operator who switches the amplifiers knows. In a double-blind test, neither the listener nor the operator knows which amplifier is being used.

The maximum level of distortion that cannot be reliably distinguished in blind tests largely depends on the type of distortion, the listener's sensitivity, and the listening environment. However, research and blind listening tests have shown some general thresholds for distortion levels that are generally undetectable to most listeners.

1. Total Harmonic Distortion (THD):

• Threshold of Perception: Most listeners, including audiophiles, typically cannot detect THD levels below 0.1% during blind tests, especially when the audio is being played through high-quality, high-fidelity systems.
• Real-World Devices: High-quality amplifiers, even from Class AB and Class D designs, often achieve THD values much lower than 0.1%, often around 0.001% to 0.01%. These levels of THD are considered inaudible to most listeners.

2. Intermodulation Distortion (IMD):

• Threshold of Perception: IMD, which occurs when two or more frequencies interact and produce unwanted harmonic content, becomes detectable to most listeners at around 0.1% to 0.3% distortion. Below this level, IMD is generally not noticeable unless listeners are particularly sensitive to it.
• Importance: IMD is considered more problematic than THD because it can distort the perception of the spatial qualities of the sound (e.g., imaging and separation). However, with proper design, IMD in high-end amplifiers can be kept well below perceptible thresholds.

3. Crossover Distortion (in Class AB):

• Threshold of Perception: In Class AB amplifiers, crossover distortion (which occurs during the transition between the positive and negative halves of the waveform) can be audible if it is significant, usually around 0.05% to 0.1%. Modern Class AB designs, however, are engineered to minimize this distortion to the point where it is inaudible in blind tests.

4. Audible Artifacts from Higher Distortion Levels:

• Above 0.1% THD or IMD: Once distortion exceeds 0.1%, most listeners can start to hear degradation in the sound quality. This includes loss of clarity, reduced separation of instruments, and a sense of "harshness" or "muddy" sound. However, the type of distortion (THD, IMD, or clipping) also plays a significant role in how easily it is detected.
• 0.3% or higher: Distortion levels above 0.3% are often immediately noticeable and would be considered unacceptable in high-fidelity (hi-fi) systems, especially during blind tests. For most listeners, distortion at this level introduces audible changes in tonality, imaging, and perceived soundstage.

5. Dynamic Range and Clipping:

• Clipping Distortion: Clipping (when the amplifier cannot handle peak signals and they become flat-topped) is a form of distortion that is often immediately audible. Even at lower distortion levels, clipping introduces harshness and unnatural sound that can easily be detected.
• Dynamic Range: Amplifiers with high dynamic range (typically above 100 dB) exhibit less noticeable distortion during normal listening, as there is plenty of headroom to avoid clipping. Low dynamic range amplifiers tend to exhibit more audible distortion as they approach their limits.

Conclusion:

In blind tests, most listeners cannot distinguish distortion levels below 0.1% for THD and IMD. When distortion exceeds 0.1%, particularly above 0.3%, it becomes more noticeable and starts to affect the perceived quality of the audio. However, high-end amplifiers—whether Class A, Class AB, or Class D—can typically achieve distortion levels well below these thresholds, making them virtually indistinguishable in blind tests as long as the design is optimized for low distortion and high fidelity.

One of the best articles on double-blind tests of high-quality amplifiers can be found here:

https://audioxpress.com/article/differences-in-amp-sound-how-do-we-find-the-truth

https://audioxpress.com/article/differences-in-amp-sound-whats-the-truth

While distortion is often the most prominent factor in determining the perceived quality of an amplifier, other technical characteristics can also influence the listening experience, though they are generally less significant in high-end amplifiers that are designed to minimize these factors. Here's a breakdown of some other technical characteristics that could theoretically impact sound quality and whether they are likely to be audible:

1. Frequency Response

• Description: The frequency response of an amplifier defines how accurately it reproduces all frequencies within the audible range (typically 20 Hz to 20 kHz).
• Impact on Sound Quality: A flat frequency response ensures that no frequencies are artificially boosted or attenuated. An amplifier with significant deviations from a flat frequency response can color the sound, leading to certain frequencies being emphasized or suppressed. However, high-end amplifiers are typically designed with minimal deviations in frequency response, so any differences are likely to be inaudible in blind tests.
• Audibility: In practice, any noticeable differences due to frequency response would likely be due to poor quality designs. High-end amplifiers with flat, accurate frequency response are usually indistinguishable to listeners in blind tests.

The frequency response of an amplifier refers to how accurately it reproduces audio signals across the full range of human hearing (typically 20 Hz to 20 kHz). Deviations in this frequency response can affect the tonal balance of the sound, making it sound too bright, dull, or colored. However, certain deviations can be inaudible, depending on their magnitude and the listener's sensitivity.

Below are key guidelines regarding the acceptable limits for frequency response deviations that typically remain inaudible to most listeners:

Flat Frequency Response ±0.1 dB:

• Most audiophiles and critical listeners will not detect any differences if the frequency response is flat with deviations of ±0.1 dB over the 20 Hz - 20 kHz range. This level of deviation is extremely low and is typically inaudible to the human ear.
• Deviation within ±0.2 dB is still considered excellent for most high-end audio systems, and differences in sound would be hard to detect in a blind test.

Flat Frequency Response ±0.5 dB:

• A deviation of ±0.5 dB across the audible range is often acceptable in high-quality consumer audio equipment. It might introduce subtle tonal coloration, but these differences would be hard to perceive unless the listener is very sensitive or the equipment is in an ideal listening environment.
• For example, a slight boost or dip in a specific frequency range (e.g., a small bump in the midrange or treble) might be audible, but the change would likely be subtle and not immediately obvious during typical listening.

Flat Frequency Response ±1 dB:

• Deviations of ±1 dB are still acceptable for most consumer amplifiers and might be found in lower to mid-range equipment. In many cases, this level of deviation may result in slightly altered tonal balance (e.g., slightly boomy bass or slightly bright treble), but it would not be drastic enough to be easily identified in a blind test, unless a listener is highly sensitive or using very high-quality headphones or speakers.
• At this level, differences might only become noticeable during very critical listening sessions, with extreme attention to detail.

Deviations Greater Than ±1 dB:

• When deviations become larger than ±1 dB (e.g., 2 dB, 3 dB, or more), audible tonal shifts become more noticeable. For example:
  • A bass boost of 3 dB at low frequencies can make the sound overly boomy or muddy.
  • A treble boost can result in a harsh or sibilant sound.
  • A significant dip in midrange frequencies can make vocals or instruments sound distant or lacking presence.
• These deviations can be noticeable, especially on high-resolution systems, and will likely be heard by most listeners during blind tests, particularly if they are experienced or using high-fidelity equipment.

Subtle Deviations and Listener Sensitivity:

• Some listeners, especially audiophiles with highly trained ears or those using very high-quality systems, may be able to detect even small deviations (e.g., ±0.5 dB or less). However, for the average listener, deviations of this magnitude are typically not noticeable during normal listening conditions.
• Real-world listening environments (e.g., room acoustics, speaker placement, and background noise) often mask slight frequency response deviations, making them harder to detect.

Real-World Considerations:

• It's important to remember that in real-world listening scenarios, the room, speaker placement, and acoustics play a significant role in shaping the sound. Even if an amplifier's frequency response is perfectly flat, the room interaction with speakers can introduce much larger deviations in frequency response, which will be more noticeable than small deviations in the amplifier itself.
• Thus, amplifier frequency response should be considered alongside other factors like room correction and speaker characteristics, especially in home audio systems.

Conclusion:

• For most listeners, frequency response deviations of up to ±0.5 dB are inaudible in a blind test, and even deviations of ±1 dB are unlikely to be easily detectable under normal conditions.
• Deviations greater than ±1 dB become increasingly audible and would likely impact the tonal quality of the sound, making them distinguishable to most listeners, especially those with trained ears or when using high-end equipment.

2. Signal-to-Noise Ratio (SNR)

• Description: The SNR refers to the level of the desired audio signal compared to the level of background noise or unwanted signals. A higher SNR indicates a cleaner signal with less noise.
• Impact on Sound Quality: A low SNR can result in audible noise, such as a hiss or hum, especially at higher gain levels. However, high-end amplifiers typically have SNRs of 100 dB or greater, meaning that any potential noise is far below the threshold of audibility. It is generally not something that would be noticed unless the noise level is high, which would indicate a poor amplifier design.
• Audibility: For high-quality amplifiers, differences in SNR are unlikely to be heard, as noise levels are too low to be noticeable.

3. Crosstalk

• Description: Crosstalk occurs when signals from one channel (left or right) interfere with signals from the other channel. It's an issue that arises in multi-channel systems (stereo, surround sound) when the channels are not sufficiently isolated.
• Impact on Sound Quality: When crosstalk occurs, you may hear the sound from one channel bleeding into the other, leading to a reduction in stereo separation. High-end amplifiers are engineered to have minimal crosstalk (typically around -100 dB or better).
• Audibility: In a high-end amplifier, any crosstalk would be at such low levels that it is not noticeable during normal listening. Poorly designed amplifiers, however, might introduce crosstalk at higher levels, which could be audible.

4. Power Output

• Description: The power output of an amplifier determines how much power it can deliver to the speakers, affecting the maximum volume and headroom (ability to handle peaks in the audio signal).
• Impact on Sound Quality: While power output doesn't directly affect the tonality or quality of sound, an amplifier that cannot supply enough power for your speakers may distort at higher volumes or during dynamic passages. A well-designed high-power amplifier will have more headroom and avoid clipping or distortion at high output levels.
• Audibility: Differences in power output are usually only noticeable if the amplifier is pushed to its limits. For most listening situations, the quality of the sound isn't directly tied to the power output, assuming the amplifier has enough headroom for the speakers.

5. Impedance Matching

• Description: Impedance matching refers to how well the amplifier’s output impedance matches the impedance of the speakers. Mismatched impedance can lead to inefficient power transfer, reduced performance, and potential distortion.
• Impact on Sound Quality: High-end amplifiers are designed to handle a wide range of speaker impedances and provide efficient power transfer. Poor impedance matching can lead to losses in sound quality, but this is typically not an issue with modern, high-quality amplifiers.
• Audibility: For most high-end systems, impedance matching should not be a noticeable factor unless there is a severe mismatch or the amplifier is driving low-impedance speakers beyond its capability.

6. Damping Factor

• Description: The damping factor refers to the ratio of the load impedance (typically the speaker impedance) to the amplifier’s output impedance. It influences the amplifier’s control over the speaker’s motion, particularly at lower frequencies.
• Impact on Sound Quality: A high damping factor results in better control of the speaker diaphragm, which can improve transient response and reduce unwanted resonances. Some believe that a lower damping factor can result in a "looser" bass response, but high-end amplifiers usually maintain a sufficiently high damping factor to ensure tight, controlled bass.
• Audibility: In well-designed amplifiers, differences in damping factor are generally not audible to the listener unless the amplifier is very poorly designed or the speaker is particularly difficult to drive.

In a well-conducted blind test, most audiophiles are unable to reliably hear the difference between top-quality amplifiers of different classes—Class A, Class AB, and Class D—when those amplifiers are designed to minimize distortion, noise, and other factors that affect sound quality. This conclusion is supported by numerous studies and blind listening tests in the audio industry, and here's why:

1. Class A Amplifiers:

• Characteristics: Class A amplifiers are known for their simplicity and low distortion. They deliver a high level of sound quality due to their continuous operation and high linearity, meaning they don’t switch off during signal processing. However, they are inefficient and generate a lot of heat.
• Perceived Differences: Class A amplifiers are often revered for their "pure" sound, but in blind tests, their advantages in sound quality are often indistinguishable from Class AB or D amplifiers that are well-designed.

2. Class AB Amplifiers:

• Characteristics: Class AB amplifiers combine the best of Class A (low distortion) with Class B (efficiency). They are more power-efficient than Class A amplifiers, making them more practical for driving larger speakers or operating in environments where heat generation is a concern.
• Perceived Differences: Class AB amplifiers can offer similar performance to Class A, especially if they are designed with low THD (Total Harmonic Distortion) and IMD (Intermodulation Distortion). In blind tests, many listeners cannot reliably distinguish between high-end Class AB and Class A amplifiers, especially at normal listening volumes.

3. Class D Amplifiers:

• Characteristics: Class D amplifiers use pulse-width modulation (PWM) to amplify signals, making them very efficient and generating less heat than Class A or AB amplifiers. Early Class D amplifiers suffered from issues like higher distortion and poor sound quality, but modern Class D designs have overcome many of these problems.
• Perceived Differences: Modern Class D amplifiers can perform at a very high level, with low distortion and high efficiency. With recent advances in technology, many audiophiles find it difficult to distinguish Class D amplifiers from Class A or AB amplifiers in blind tests. While some might claim that Class D amplifiers sound "different," the differences are usually subtle and not consistent across tests.

Blind Test Results:

• Improvements in Design: As amplification technology has evolved, high-end amplifiers from all classes (A, AB, D) are now designed to minimize distortions, noise, and other unwanted artifacts that affect sound quality. This means that when these amplifiers are well-designed, there is very little audible difference in normal listening conditions.
• Listener Experience: While some audiophiles might perceive differences due to their preferences or expectations, blind tests show that most listeners (including trained listeners) cannot consistently tell the difference between top amplifiers from different classes.

Conclusion:

In summary, in a properly controlled blind test, high-end amplifiers of different classes (A, AB, D)—when designed to meet the same high standards of performance—are likely to sound indistinguishable from each other to most listeners, including audiophiles. The differences that are audible in blind tests are generally very subtle and not related to the amplifier class but rather to other factors like distortion, SNR, and power output.

That said, audiophiles may prefer one class over another for reasons beyond sound quality, such as personal taste, perceived warmth, or efficiency. But from a purely sonic perspective, high-end amplifiers across these classes perform very similarly.

The ability to hear a difference between a high-end tube amplifier and a high-end transistor (solid-state) amplifier in a blind test is a topic of ongoing debate in the audiophile community. Blind tests have shown that the vast majority of listeners, even those with highly trained ears, cannot reliably distinguish between the two when distortion levels are minimized, and the amplifiers are well-designed.

Key Points to Consider:

1. Distortion Characteristics:

   • Tube Amplifiers: Tube amplifiers tend to introduce higher levels of second-order harmonic distortion, which some listeners perceive as more "musical" or "warm" due to its harmonic relationship with the fundamental frequencies. This type of distortion may contribute to the characteristic sound often associated with tube amps.
   • Solid-State Amplifiers: Solid-state (transistor) amplifiers typically have lower distortion, especially when it comes to second-order harmonics, and can be more linear and accurate. They also tend to have better control over bass frequencies and faster transient response.

2. Human Perception of Distortion:

   • Human hearing is quite sensitive to certain types of distortion, especially when it involves clipping or high-frequency noise. However, the types of distortion produced by well-designed tube and solid-state amplifiers are often below the threshold of detection in typical listening environments.
   • Some listeners believe they can hear a "warmer" sound with tube amplifiers, which could be attributed to the harmonic distortion, but this is more subjective and often depends on the context (e.g., speaker pairing, room acoustics).

3. Blind Test Results:

   • Blind tests have been conducted on various amplifiers, including tube and solid-state models, where listeners were unable to consistently distinguish the differences between high-end tube amps and high-end solid-state amps. This includes both objective tests (measuring distortion, frequency response, etc.) and subjective listening tests.
   • Many listeners, when subjected to double-blind tests, were unable to reliably identify whether the amplifier was a tube or transistor design when distortion and other factors were controlled for.

4. The Subjective Preference for Tube Sound:

   • Even if there is no measurable difference in terms of distortion, many audiophiles report a subjective preference for the sound of tube amplifiers due to the unique coloration they introduce. This can be a psychological or emotional response to the sound, often associated with the “romantic” sound of tubes.
   • The perceived warmth of a tube amplifier can also be influenced by biases and expectations. In a blind test scenario, when these biases are removed, listeners often cannot tell the difference.

5. Listener Expectations:

   • Psychological factors play a role in how people perceive the sound. If listeners are expecting a difference based on their knowledge of tube amplifiers versus solid-state amps, they might be more inclined to hear differences, even if they don't exist in a controlled test.

Conclusion:

In blind tests, where expectations and biases are removed, most listeners cannot reliably distinguish between high-end tube and solid-state amplifiers, especially when both are well-engineered and designed to produce low distortion. While tube amplifiers do have a distinctive sound due to their harmonic characteristics, this difference is often subjective and more about personal preference than an objectively audible improvement in sound quality. When distortion is kept to a minimum, any perceived differences may be too small for listeners to detect consistently. For both tube and solid-state amplifiers, distortion levels below 0.1% (THD and IMD) generally ensure that there is no audible difference in a blind test, even for experienced audiophiles.

Thus, while some audiophiles swear by the warmth and musicality of tube amplifiers, objective testing shows that for most listeners, there is no significant audible difference in blind tests between high-quality tube and solid-state amplifiers.

Tihomir Haralović, M.Sc. in Physics