Audio notch filters are not just an old analog trick; they are still a practical way to make distortion measurements more readable when the fundamental test tone is much larger than the harmonics you are trying to inspect. In this Elektor TV clip, longtime contributor and audio expert Alfred Rosenkränzer explains the idea in the context of low-distortion audio measurements, following related Elektor coverage of bench audio measurements and practical analyzer setups such as the compact balanced-input instrument used by many serious hobbyists and small labs.

What Audio Notch Filters Actually Do

In a basic distortion test, an audio generator feeds a sine wave into the device under test. The analyzer then looks at the output spectrum and tries to determine what else is present besides the original tone. The problem is scale. A clean 1 kHz test signal may be enormously larger than the second, third, or higher harmonics produced by the amplifier, filter, DAC, preamp, or other circuit being measured.

A notch filter attacks that problem by strongly reducing the fundamental frequency before the spectrum analyzer or measurement input has to process the signal. Once the main tone has been suppressed, the remaining distortion products and noise become easier to see. This is especially useful when the measurement system itself has limited dynamic range, or when the harmonics are so far below the test tone that they would otherwise be obscured.

Audio Notch Filters and Real Bench Work

Rosenkränzer has written extensively for Elektor on practical audio and test equipment, including analog audio generators, filters, USB measurement tools, and the QuantAsylum QA403. His earlier QA403 review compared the unit with an Audio Precision APx555 and discussed the realities of using semi-professional instruments rather than relying only on PC sound cards.

The point is not that every bench needs a rack full of high-end audio hardware. The more useful lesson is that the measurement chain matters. A good signal source, sensible input levels, isolation from PC noise, attenuation where needed, and a well-designed notch filter can make the difference between a clean measurement and a graph that mostly shows the limits of the test setup.

A Useful Lesson for Audio DIY

For anyone building or repairing audio gear, audio notch filters are a reminder that measurement is not just about owning an analyzer. It is about understanding what the analyzer is being asked to measure. When the fundamental tone dominates the screen, the interesting faults may be hiding underneath it. Remove or reduce that tone, and the smaller errors start to tell their story.

That makes this a worthwhile short watch for audio DIYers, amplifier builders, and engineers who want to improve their distortion-measurement workflow without treating the spectrum display as magic. The magic is mostly in the setup. And, occasionally, in not letting a perfectly good 1 kHz sine wave shout over everything else in the room.