For most amplifiers, a THD (total harmonic distortion) below 1% is considered good, and anything below 0.1% is excellent. At these levels, the distortion is essentially inaudible during normal listening. But that single number on a spec sheet tells you less than you might think, and understanding what’s behind it will help you make smarter buying decisions.
What THD Actually Measures
When an amplifier receives a pure audio signal, it inevitably adds small amounts of extra frequencies that weren’t in the original recording. THD expresses the total energy of those unwanted frequencies as a percentage of the original signal. A THD of 0.01% means the distortion is one ten-thousandth the strength of the music itself.
You’ll also see “THD+N” on spec sheets, which stands for total harmonic distortion plus noise. This is actually the more useful number because it captures everything the amplifier adds to your signal: not just harmonic distortion, but also hum, electrical interference, and background hiss. A low THD+N result tells you the amplifier is clean across the board. When a manufacturer lists only “THD” without the “+N,” they may be presenting the more flattering number.
Practical Thresholds by Use Case
The general rule is that THD below 1% is acceptable for any playback device under typical listening conditions. Most people cannot hear distortion at this level, especially with music playing at normal volume. For casual listening, a home theater receiver, or a Bluetooth speaker, anything under 1% is perfectly fine.
For dedicated stereo listening or hi-fi setups, you’ll want to aim lower. Quality stereo amplifiers and receivers commonly achieve 0.05% to 0.1% THD. High-end models push into the 0.002% to 0.01% range, which is well beyond what human hearing can detect as distortion. At that point, you’re paying for engineering headroom and bragging rights more than audible improvement.
Here’s a rough guide:
- Below 0.01%: Exceptional. Found in high-end and audiophile-grade amplifiers.
- 0.01% to 0.1%: Excellent. More than clean enough for critical music listening.
- 0.1% to 0.5%: Good. Suitable for most home audio and home theater systems.
- 0.5% to 1%: Acceptable. Fine for everyday use, though not ideal for detail-focused listening.
- Above 1%: Distortion may become noticeable, particularly on sustained tones and at higher volumes.
Why the Spec Sheet Number Can Be Misleading
Manufacturers often list THD at a specific power output, and that detail matters enormously. An amplifier might achieve 0.002% THD at 73 watts per channel but jump to 0.7% at 80 watts. Both numbers are technically accurate, but the amplifier behaves very differently at those two power levels. As you push closer to maximum output, distortion climbs rapidly.
This is why you’ll sometimes see a receiver rated at, say, 100 watts with less than 0.1% THD, and then at 120 watts the signal is no longer clean. The lesson: check what power level the THD spec is measured at. If a manufacturer advertises impressively low THD but only at a fraction of the amp’s rated power, the real-world performance at your listening volume could be quite different.
Also pay attention to the frequency range. A THD measurement taken at 1 kHz (a single midrange tone) will look better than one measured across the full 20 Hz to 20,000 Hz spectrum. The best spec sheets state something like “0.09% THD from 20 Hz to 20,000 Hz,” which tells you the amplifier stays clean across the entire audible range. A spec that doesn’t mention frequency range is likely cherry-picking the most favorable test point.
Tube Amplifiers Play by Different Rules
If you’re looking at vacuum tube (valve) amplifiers, throw the THD benchmarks above out the window. Tube amps routinely measure 1% or higher THD, yet many listeners and nearly all guitarists prefer their sound. The reason comes down to the type of distortion they produce.
Tube amplifiers tend to generate even-order harmonics (the 2nd, 4th, 6th, and so on), which are musically related to the original note. These harmonics sit at octave intervals and blend naturally with the source signal. Your ear perceives this as warmth and richness rather than harshness. Solid-state amplifiers, when they do distort, produce more odd-order harmonics (3rd, 5th, 7th), which sound less musical and more grating. Tubes also clip differently: when they’re overdriven, the signal rounds off gradually (soft clipping) rather than flattening abruptly (hard clipping), which produces a smoother, more compressed sound.
For a guitar amplifier, THD is essentially irrelevant as a quality metric because distortion is part of the instrument’s tone. For a tube hi-fi amplifier, a THD of 0.5% to 2% is normal and doesn’t indicate poor quality. Judging a tube amp by the same THD standards as a solid-state amp misses the point entirely.
What Matters More Than the THD Number
Once THD drops below about 0.1%, other factors have a far bigger impact on how your system sounds. Speaker quality, room acoustics, and the source material itself will shape your listening experience more than the difference between 0.05% and 0.005% distortion.
If you’re comparing two amplifiers and both have THD under 0.1%, focus instead on whether the amp delivers enough clean power for your speakers, how it handles dynamic peaks, and whether it stays cool and stable during long listening sessions. A well-designed 50-watt amplifier with 0.08% THD will sound better in practice than a poorly designed 200-watt amplifier with 0.01% THD if the latter clips or overheats when you actually push it.
For home theater receivers specifically, look for THD specs measured with all channels driven simultaneously rather than just one or two. Many receivers share a single power supply across all channels, and distortion increases when five or seven channels draw power at once. A receiver rated at 0.09% THD with two channels driven might perform significantly worse during an action movie scene that demands output from every speaker.