Anticipating how circuits behave, doing our own measurements, and understanding how to evaluate our audio efforts are key aspects of any audio product developer's journey. And the March 2025 issue of audioXpress magazine will certainly help and inspire us to do that. Starting with a great conversation we had with physicist, analog engineer, and entrepreneur, Mike Engelhardt. The creator of LTspice has now created a new and much improved simulation tool suite called QSPICE (the Q is for Qorvo, the company that supported the new effort). We asked Tim McCune, former president of Linear Systems, currently consulting for Qorvo, to discuss how QSPICE can help solve many issues that audio designers simulating their circuits might confront.And do you know that Texas Instruments offers a PC-based calculator designed to speed up and support circuit design efforts? This calculator pairs well with the concepts described in the valuable Analog Engineer's Pocket Reference guide, which our own Jan Didden explored. His article is a valuable review of this combination of a reference text available as an extensive PDF design guide, and the accompanying design software, designed to speed up many of the repetitive calculations that analog circuit design engineers use on a regular basis. This PC-based tool provides a graphical interface with a list of various common calculations ranging from setting operational-amplifier gain with feedback resistors to selecting appropriate circuit design elements for stabilizing an analog-to-digital converter (ADC) drive buffer circuit. Both the Analog Engineer Calculator software and the Pocket Guide can be downloaded free of charge from the TI website.
And diving into the world of Practical Test and Measurement, Steve Temme (Listen, Inc.) explains how we can measure HDMI Audio. From its origins in 2002 as a new, smaller connector for HDTV, improving upon DVI and adding audio capability and consumer electronics control functions, High-Definition Multimedia Interface (HDMI) continuously evolved over the years to become one of the most widely adopted interfaces. Not surprisingly, testing audio over HDMI, given its multiple options and configurations, still induces many questions when it comes to testing. This article provides practical answers to those questions and explains how to perform HDMI audio measurements using SoundCheck and a commercially available HDMI audio extractor. This method can be extended to a wide range of HDMI-connected devices, including multichannel surround sound systems.
And Adam Liberman writes a comprehensive checklist on what is needed for testing power amplifiers. As Liberman explains, power amplifiers present a unique set of testing challenges that are usually not encountered with digital or low-level analog circuits, as they must supply significant current as well as voltage to their loads. Factors such as power supply current and voltage limitations, thermal characteristics of the output devices and associated heatsinks, and the resistive and reactive components of the load must be considered to produce meaningful results. Whether it’s high-powered amplifiers for venues, headphone amplifiers, or small chips in hand-held devices, many of the same considerations and test procedures described in this article may be applied. This article covers the fundamental measurements, with specifics on how to best accomplish the tests using Audio Precision's APx series audio analyzers and APx500 measurement software.
In his Sound Control column, Richard Honeycutt writes about the Early Days of Speaker Acoustics. Much of the progress made in speaker cabinet construction, bracing, lining, and stuffing, originated with the evolution of consumer electronics that gradually brought audio reproduction into homes. From the early radios and TVs to the first high-fidelity speakers in the 1950s, precious lessons remain valid such as building cabinet walls that helped to damp panel resonances and its effects. Richard Honeycutt revisits this important topic from his personal perspective.
And transitioning to a DIY project, keeping with the measurements angle, George Ntanavaras explains how he designed and built the SL-100 Sound Level Adjuster. The SL-100 is a portable device that emits 1kHz sine tone at two specific sound pressure levels, 94dBSPL and 114dBSPL and produces a reference output voltage of 150mVrms at the same frequency. Once calibrated, it can be used to adjust the sound pressure level of a complete loudspeaker measurement system consisting of the microphone, the sound card, and the acoustic measurement software (e.g., ARTA or REW). Its electrical output with the reference voltage level is intended for the calibration of the sound card inputs before any measurement is taken. This useful piece of test equipment is easy to build, and low cost but can be particularly helpful, especially if you must calibrate a loudspeaker measurement system very often.
Another DIY project for this month's issue of audioXpress comes from our prolific contributor Tom Perazella. In his latest project, he describes an upgraded speaker protection circuit, designed to complement his original Egg speakers and distributed subwoofer system, now using back-to-back power MOSFETs in place of relays to make or break the speaker connections. His system is now powered by two VTV Purifi Eigentakt amplifiers and Crown Macro Reference amplifiers, respectively, with the total power available to all drivers now at 6,400W, a level that allows unrestricted dynamics without clipping. Obviously, with this level of power, either amplifier or source problems plus operator indiscretions can cause major problems with the speakers. You can learn all the details and understand how he sorted the challenges.
In this issue, you will certainly enjoy learning about Zoran M. Dukic's High End Tube Amplifiers Designer Tool Box. Jan Didden couldn't help diving into this extensive, 428-page, self-published hardcover book, which is much more than another publication on tube amplifiers. As the title says, this book really is a tool box. As Jan Didden discovered, this book contains all the knowledge needed to check, for example, how an input low-pass filter must be dimensioned, how to investigate the different resistor types and their properties, capacitor types, which tubes to use, how to bias them, and construct the load lines, and even how to design low-noise power supplies. A one-stop resource for everything you need to design and build a tube-based audio power amplifier, and clearly a work of love!
And this issue includes the conclusion of Brian Lowe's (Belleson LLC) op-amp project for phono preamps. As described in the first part, published in the February 2025 issue of audioXpress, Lowe developed a unique custom-designed transconductance amplifier (TopAmp) with fully differential (balanced) low-noise JFET inputs and current output. Combined with a transimpedance amplifier, it offers 50dB of current gain prior to any voltage excursion, virtually eliminating distortion caused by voltage-dependent node capacitance exhibited by more conventional approaches. His innovation is now fully described with the conclusion in this issue, also containing simulations and measurements.
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