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Total Harmonic Distortion meter

THD Meters – how they work

Many years back the must haves for anyone building audio equipment would have been the multimeter (DVM), oscilloscope and a function generator. With specialised audio test equipment falling in price over the last twenty years two other pieces of equipment are now also found on the test bench, the THD meter and spectrum analyser.  Indeed your laptop or desktop could be used for many of these functions but it can be cumbersome and/or inaccurate because of sound-card bandwidth limitations.

Hantek DSO
Hantek DSO – 1202B

The THD Meter is definitely a must have, even something for impressing one’s audiophile friends.  Consisting of an oscillator, internal or external, a notch filter to attenuate the fundamental to as little as possible and a voltage / instrumentation amplifier to drive the resultant combination of harmonics and noise to the output meter or video graphics display.

Block Diagram of THD Meter using Wien Bridge Notch Filter
Block Diagram of THD Meter using Wien Bridge Notch Filter

Interesting DIY angle to THD metering from “Wensan” – DIY Audio – Simple THD Meter

Robert Cordell,  American (as in USA) electrical engineer and expert in the audio field kindly publishes his high end THD Analyzer circuit on his website, Cordell Audio.

Analysing the Waveform – skirting the FFT

The analysis of signal waveforms is rather mystical to most of us and becomes a highly complicated mathematical subject when breaking a signal or rather batch of frequencies down to a function of time. FT, or Fourier Transform converts the waveform data in the time domain into the frequency domain each containing individual signals of phase, amplitude (magnitude) and frequency.

Modern computer software makes this previously time consuming and highly mathematical task accessible to the home experimenter using what is now commonly known as FFT or Fast Fourier Transform.

Digital Signal Processing

Digital signal processing in the modern computer has made decomposition of signals in audio a common discussion point on the forums, possibly not always in our best interests because of the complexity and hence confusion.

FFT and Nyquist are of interest to the engineer, especially now in the DSP domain where bit rates and sampling frequencies have become an integral part of our lives.

Our modern (and even older) hardware can run software for analysing and changing the audio signal – and sometimes it’s free. See Audacity and it’s features.

Shit in Shit Out

Technically speaking it is not probable that an ill designed amplifier using inferior components will sound good. 🙂

It is probable that with the assistance of application notes, spec sheets and white papers from most manufacturers to build a quality product with the minimum of fuss.

Hantek  – USB oscilloscope, multi-function

I have four oscilloscopes, three of them analogue. I like the ease of triggering and set up. Coming from the analogue era certainly assists – the digital scopes don’t appeal from a GUI perspective until one needs to record an event or compare functionality to price. They become indispensible. Just look at the hand-helds.

The digital scope I use is the Hantek 3062. It has a logic and spectrum analyser, frequency counter and of course it’s a scope.  It cost about R4K five years back from Kmeasure, based in Pretoria. Very good service. I mention this as an aside, there are always sensible, cost effective solutions in our pinched South African market.

Lastly, of course we need a good quality function generator.  I find Instek to be of exceptional quality and reliability. Available at Mantech Electronics. (Cape Town, Johannesburg, Durban).

Further Reading:

Cordell Audio:  Robert Cordell Build your own THD Meter

KMeasure – Professional measurement systems (based in Pretoria)

Signal Lab – SigView Signal Analysis software

The VCF or Voltage Controlled Filter



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