It’s been a while since the last installment in our Know Audio series, in which we investigated distortion as it applies to Hi-Fi audio. Now it’s time to return with part two of our look at distortion, and attempt some real-world distortion measurements on the bench.

Last time, we examined distortion from a theoretical perspective, as the introduction of unwanted harmonics as a result of non-linearities in the signal path. Sometimes that’s a desired result, as with a guitar pedal, but in a Hi-Fi system where the intention is to reproduce as faithfully as possible a piece of music from a recording, the aim is to make any signal path components as linear as possible. When we measure the distortion, usually expressed as THD, for Total Harmonic Distortion, of a piece of equipment we are measuring the ratio of those unwanted harmonics in the output to the frequencies we want,  and the resulting figure is commonly expressed in dB, or as a percentage.

The Cheapest Of Audio Kits, Analysed

Having explained what we are trying to do, it’s on to the device in question and the instruments required. On the bench in front of me I have my tube headphone amplifier project, a Chinese 6J1 preamp kit modified with transformers on its output for impedance matching. I’ve investigated the unmodified version of this kit here in the past, and measured a THD of 0.03% when it’s not driven into distortion, quite an acceptable figure.

To measure the distortion I’m using my audio signal generator, a Levell TG200DMP that I was lucky enough to obtain through a friend. It’s not the youngest of devices, but it’s generally reckoned to be a pretty low distortion oscillator. It’s set to 1 KHz and a 1 V peak-to-peak line level audio output, which feeds the headphone amplifier input. The output from the amplifier is feeding a set of headphones, and my trusty HP334A distortion analyser is monitoring the result.

How Does A Distortion Analyser Work Then?

A distortion analyser is two instruments in one, a sensitive audio level meter, and an extremely high quality notch filter. In an instrument as old as this one everything is analogue, while in a modern audio analyser everything including the signal source is computer controlled.

The idea is that the analyser is first calibrated against the incoming audio using the voltmeter, and then the filter is switched into the circuit. The filter is then adjusted to reject the fundamental frequency, in this case 1 kHz, leaving behind only the harmonic distortion. The audio level meter can then be used to read the distortion. If you’re interested in how these work in greater detail I made one a few years ago in GNU Radio for an April Fool post about gold cables.

Using the HP offers an experience that’s all too rare in 2025, that of tuning an analogue circuit. It settles down over time, so when you first tune it for minimum 1kHz level it will retune to a lower level after a while. So mine has been running but idle for the last few hours, in order to reach maximum stability. I’m measuring 0.2% THD for the headphone amplifier, which is entirely expected given that the transformers it uses are not of high quality at all.

An Instrument Too Expensive For A Hackaday Expense Claim

It’s important to state that I’ve measured the THD at only one frequency, namely 1 kHz. This is the frequency at which most THD figures are measured, so it’s an easy comparison, but a high-end audio lab will demand measurements across a range of frequencies. That’s entirely possible with the Levell and the HP, but it becomes a tedious manual process of repetitive calibration and measurement.

As you might expect, a modern audio analyser has all these steps computerised, having in place of the oscillator and meter a super-high-quality DAC and ADC, and instead of the 334A’s filter tuning dial, a computer controlled switched filter array. Unsurprisingly these instruments can be eye-wateringly expensive.

So there in a nutshell is a basic set-up to measure audio distortion. It’s extremely out of date, but in its simplicity I hope you find an understanding of the topic. Keep an eye out for a 334A and snap it up if you see one for not a lot. I did, and it’s by far the most beautifully-made piece of test equipment I own.