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Here are a few reflections on the recent RMAF show here in Denver, Colorado …

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I missed the Rocky Mountain Audio Fest for the first time this year, since I spent the month of October in Vietnam. IMHO, this is the best high-end audio show in North America, so was sad to miss it, but Lynn Olson (who lives near Denver) went and he and others have made some good observations on the show.  A good overview is given on the following diyAudio thread: Rmaf 2008. Lynn expects to post more here on ClariSonus, but to whet your appetite, here are a few tidbits.

There were quite a few new speakers, many with really good sound. For example, here is the new RAAL omnidirectional speaker with ribbon tweeters, designed by Aleksandar Radisavljević from Serbia (click on the pictures for a full-sized display):

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My friend, John Nunes, pointed me to a very interesting paper on the distortion of op-amps (more on this later).  I started poking around the site it resided on and found an excellent “Notable Site” for Clarisonus! This is the SG-Acoustics site, created by Samuel Groner, a masters student at the Zurich University of the Arts in Switzerland. As with many of the other Notable Sites, this one is a labor of love by a single creator, where he has posted information or research for the world to see, with no immediate commercial gain involved.  Samuel’s crisp, clean site gives his parts selection criteria, some mic amp and discrete op-amp circuits, test circuits, power supply designs, etc. Some Gerber files (for PC board layouts) are included.  All Samuel asks is that you read and agree to his “Terms of Use” and “Disclaimer”, basically asking permission for any commercial use of his designs or information.

The high-light of the site is Samuel Groner’s paper “Operational Amplifier Distortion”, published Sept. 1st, 2008. With excellent Swiss thoroughness, Samuel takes the op-amp measurement criteria established by Walt Jung in 1986 and extends it, both in methodology and in covering contemporary audio op-amps.  The full test methodology is given, which only starts with measurements taken by an Audio Precision System One. MATLAB is used to crunch and present the data. For anyone who has casually dropped an op-amp into an audio circuit without much thought, this paper is a wake-up call.  He presents distortion mechanisms most engineers have not thought of, including:

• Transfer Linearity
• Common-Mode Linearity
• Output Linearity (effect of loading)
• Input Impedance Linearity
• High-Frequency Linearity (inverting and non-inverting)

Future tests envisioned will include PSRR non-linearity, input impedance non-linearity, and non-linear AC input bias currents. His test methods look solid. Anyone contemplating using op-amps in audio should read this paper!  The other data on SG-Acoustics is valuable, too.

This is the second part of a series of articles on the history and application of different power line frequencies. The first part is here.

NATIONAL GRIDS AND UNIFORM STANDARDS

As more and more power plants were built, linking and synchronizing them was found to greatly increase overall reliability. If one generator went down, others on the grid could continue supplying power. At first, alternators within a single power-house were linked, then power plants within a single power company’s territory, then between power companies. Of course, all the linked power plants had to be on the same frequency. This put lots of pressure to conform to national or at least regional frequency standards.

From everything described so far, it would seem that 25Hz was ready to become the North American standard. However, by the 1910s, sentiment was swinging away from 25Hz. AC transmission theory was becoming well advanced, and it was realized that by adding capacitance to the line, inductive reactance could be cancelled-out. (In these early days, this was rarely done with actual capacitors, but rather with over-excited synchronous motors, which had the same effect of adding capacitance.) Sending 60Hz power long distances was no longer as much of a problem. Rotary converters and induction motors had become more sophisticated and the problem of commutator sparking became manageable. By 1918, when the paper “The Technical Story of the Frequencies” was published, the sentiment was to go with 60Hz as much as possible and use rotary converters and the newly-available mercury-pool rectifiers for conversion where needed.

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