Book Review: Learning Audio Testing and Measurements

For those who need to refresh their knowledge or learn about audio test and measurements, Jan Didden reviews a great - and big! - book on the topic. Audio Tests & Measurements by Igor S. Popovitch is one of the many self-published works by this proficient writer based in Australia, also the author of several popular books on vacuum tube amplifiers. This particular book runs about 180 pages, but that is deceiving as the book’s format (A4 size) is about twice the size of most soft cover books.

This article expands the conversation around feedback theory for electronic circuits and audio amplifiers. Do we "really" need wideband open loop in audio amplifiers? The subject came up at the 154th Audio Engineering Society (AES) convention in Helsinki, Finland. Looking at the frequency domain, not necessarily, Jan Didden argued. In this article he looks at the question in the Time Domain. Narrow Bandwidth, High Open Loop Gain Amplifier versus a Wide Bandwidth, Low Open Loop Gain Amplifier in the Time Domain.

Although feedback theory for electronic circuits has been established for over a century, it is still a subject for discussion. This short essay is an attempt to shed some light on one of those recurring controversies. The subject came up at the 154th Audio Engineering Society (AES) convention in Helsinki, Finland. Audio amplifiers are basically integrators or, if you will, low-pass filters. That means that at some frequency, their open-loop gain starts to fall off. With that, the loop gain available for feedback also starts to fall off, leading to a diminishing of the curative effects of feedback: distortion starts to rise, gain accuracy starts to drop, output impedance starts to go up, etcetera.

Linear Audio, the bookzine created in 2010 by Jan Didden, is now available on a USB with The Complete Linear Audio Library. We asked three Linear Audio and audioXpress authors - Gary Galo, Bob Cordell, and Dimitri Danyuk - to review it and share their thoughts. Important to know, the USB drive doesn't contain only the PDFs of published articles. It also ads additional articles by Jan Didden and others, together with materials of historical interest.

In this second article, Brian Lowe (Belleson LLC) continues his search for the Unicorn of "Measuring Voltage Regulator Ripple Rejection," this time focusing on the test fixtures. Testing a voltage regulator with exceptionally good performance requires an even more exceptional signal source and measurement setup to measure sub-microVolt ripple across a wide frequency range. Complementing the final article, Jan Didden, offers an informative sidebar about super regulators. This article was originally published in audioXpress, June 2019.

A few years ago, Jan Didden designed a high-voltage delay unit and shared its details in a 2019 audioXpress article. Many people built it and like it, but others said that it would be totally unnecessary. As so often happens in audio, arguments pro and con raged without a clear resolution. So Didden thought this was something he needed to get his head around. He found several sources, which he thought were credible, and this article is the result of his investigations, fed by those sources. This article was originally published in audioXpress, August 2020.

From audio development and product design to acoustics! Read the Acoustic Solutions Round-Up and expert articles. Check out the NAD M10 Masters Series Streaming Amplifier review, then learn about Sound Liaison, a recording label translating live acoustical recordings in the best possible quality. Jan Didden reflects on "The Internal Life of Vacuum Tubes," and George Ntanavaras introduces his APR-17 Aperiodic Loudspeaker design. audioXpress August 2020, Focus on Acoustics, is now available in print and digital.

Purifi Audio, the Danish company founded by Bruno Putzeys, Lars Risbo, and Peter Lyngdorf together with a remarkable engineering team, promises Class D amplifiers with record-breaking performance (as well as speakers). Now, Bruno Putzeys made available a detailed report of his investigation on hysteresis distortion, which motivated the development of the company's products. As Putzeys describes, hysteresis in the ferromagnetic material of the amplifier output filter coil is what causes "a recognizable grainy texture in the sound."

Stuart Yaniger explores new possibilities in audio measurement systems, expanding on the basics of using sound cards combined with new accessible-to-all software tools, such as REW, and a recently improved measurement interface option. That option is the Linear Audio Autoranger Mk II Interface, which audioXpress’ Technical Editor Jan Didden designed a few years ago and which is now available as an improved Mk II version. This article was originally published in audioxpress, April 2020.

The April 2020 edition of audioXpress is now available and features two essential Market Updates, one focusing on the industry transition to active headphone and earphone designs, and another on how to leverage the latest voice capture and processing technologies. The issue also explains how to evaluate listening effort, and Stuart Yaniger puts the Linear Audio Autoranger Mk II Interface to the test, while Ed Simon writes about AC Power Rectification. Another issue to read and use as reference.