|
Welcome (back) to Linear Audio, your tech audio resource! Volume 3 is already our fourth issue! At 188 pages, Vol 3 packs another smorgasbord of stimulating, interesting and thought-provoking articles. Why 'tweaks that cannot work' can still sound different, digital interpolation, class-A thermal case design, mic preamps. Headphone preamps with in-ear equalization, correcting cartridge (and speaker) resonances, passive I/V conversion, active subwoofer design, a simulated inductor and a low noise preamp for your test bench. Then an essay on the state of analog and digital audio, and closing with two book reviews. Another issue you'll want to have at your side so you can grab it again and again. Enjoy!
Articles in this Volume:
|
Many class-A amplifiers are placed in a largish enclosure with heatsinks that ‘look about right’. But, just as with the electronics involved, you can calculate the requirements for such an enclosure and specifically the heat sink size and geometry. Patrick K goes to the whole process with an example project. He shows that with some simple calculations you can quickly home in on a good solution to make sure that the amp is not overheating and that you don’t spend more than necessary on the hardware. As a bonus, he provides many tips and tricks for enclosure design and construction to ensure that the final amplifier not only performs well but also has a very professional look-and-feel.
Steven van Raalte set out to combat the resonance in phono cartridges. Manufacturers often rely on this resonance to compensate for the hf roll off caused by cartridge induction and load impedance. Although the frequency response can be made to look quite flat, the result is often severe phase shifts and resultant time smearing. Steven develops a Sallen-Key based topology that ‘undoes’ the time smearing, and shows how it makes the square wave response of the combination cartridge- filter impeccable. He even shows how to dimension the filter such that the RIAA 2120 Hz turnover point can be deleted and become part of the filter! At the end of the article he also shows how to use the technique to compensate for the resonance at the low frequency side of a speaker.
With all our focus on individual designs and equipment and trying to improve our amps and speakers, it is easy to lose sight of the big picture. Stan Curtis, who has literally designed 100’s of audio components but also founded several well-regarded audio companies, steps back and looks at that big picture. He concludes that for analog audio, we’re pretty much at the point where possible improvements are small and incremental, but that with digital audio there’s still a lot of work to be done.
Digital Interpolation is a way to reduce complexity and requirements for post-DAC analog filtering. Richard Lyons shows not only how it is done, but also discusses the D-A conversion process itself, what aliasing is and how to handle it, the Nyquist criterion and of course analog filtering. Truly a Tutorial for anyone who wants a clearly to follow explanation of the issues involved.
If you are looking for that ultimate headphone replay amplifier, you couldn’t do any better than with this discrete circuit. Richard Marsh develops his circuits from the well-known diamond topology but with a twist. His headphone amp is a very clean and linear discrete design that’s equally at home driving low-impedance or high impedance ‘phones. But Richard doesn’t stop there; he realizes that many headphones do not produce flat frequency response at your ears. Therefore, he borrows from ‘room equalization’ technology and develops a simple way to use that to equalize the headphone response at your ears. 
Part I of this article appeared in Linear Audio Vol 1 in Spring 2011 and readers have been asking for Part II ever since. This is a very comprehensive treatment of all factors involved in designing condenser mic preamps. Scott discusses mic capsules, how to select jFETs, the issues of battery versus phantom power. Each discussion is accompanied by illustrative circuit diagrams that in themselves are gems. Balanced or unbalanced outputs, bipolar output versions and biasing are thoroughly addressed. Combinations of discrete and opamps in mic preamps are also shown and the merits discussed, as is capacitive feedback as well as techniques to get the most out of the battery lifetime. Finally, an example implementation is shown which was used to record live performances, and the resulting recording is available at the Linear Audio website and illustrates the incredible realism that can be obtained when addressing all important design factors.
Sigurd Ruschkowski got inspired by his friend Erno Borbely (who should not need any introduction) to design a discrete I/V converter for his newest DAC. This converter uses a low-value resistor to convert the DAC output current to a low level signal. The signal is then amplified with an open loop discrete low noise amplifier to regular source levels. Unbalanced and balanced output implementations are shown with performance measurements. As part of the setup, a high performance shunt regulator is also presented.
Professor Robert Munnig Schmidt teaches Mechatronics at one of Netherlands prestigious universities, but his hobby is in audio. Since a (sub)woofer is nothing else than an electrically controlled mechanical system, he decided to apply mechatronic techniques to the design of a high performance subwoofer. After a thorough investigation into issues like efficiency, dynamic speaker properties and motional feedback limitations Robert proceeds to design a subwoofer that follows the THX© specification for subwoofers in a large listening room. The result is a 2-driver, less than 0.1m3 sized box providing up to 96dB intensity at 20Hz and 106db above 40 Hz at 5m distance.
Dr. Ramaswamy is back with another usefull piece of lab equipment. Imagine a box with two connections where you can 'dial in' any inductance between 10mH and 10H for your filter and cross-over trials. Fully floating of course and able to handle up to 1A signal current. 
No lab or hobby shack should be without a preamplifier to measure very low signals with very low noise, defined gain and wide bandwidth. Samuel Groner’s latest creation has 0.4nV/RtHz and a bandwidth extending to 1Mhz with low impedance sources. Gain is 60dB (1000x). This article documents the full design path, from the design brief, through topology considerations, device selection and design tradeoffs, and concludes with a final design complete with PCB layout. 
Our resident tube book reviewer Stuart Yaniger wonders whether a 4th edition of a well-established book is worth purchasing. He is pleasantly surprised to find that this 4th edition of Morgan Jones’ major work does indeed contain many new and improved sections making it a worthy addition to his bookshelf.
Ovidiu Popa’s expertise on low noise design makes him well qualified to review Burkhard Volgels 2nd edition of TSOS. His conclusion is that this edition is a major overhaul, with much added material, making it an indispensable reference for the serious hobbyist as well as the design professional.