On Wed, 17 Dec 2008 19:38:34 -0000, David Cutter wrote: >600ohms still seems to be the practice for ship and aircraft intercoms that >I've come across recently.
Maybe, maybe not. Often the people the write the data sheets are marketing dweebs, and have no technical background. I don't know anything about aircraft electronics practices. My observations are based on actual measurements of input and output impedances, and/or studying the schematics. Often, a data sheet says 600 ohms and all that they mean is that the output stage can be loaded with 600 ohms and meet its specs. When you measure it, you find a 100 ohm output impedance. Often, a data sheet will say 600 ohms because the marketing dweeb (or even the engineer) thinks that pro stuff is 600 ohms. It is not, and has not been since the days when tubes ruled. It's also common for engineers in industries unrelated to audio (like RF and vdieo) to apply transmission line methods to audio. That, of course, is completely inappropriate. The only audio circuits that are long enough to behave as transmission lines are VERY long telephone lines. At audio frequencies, the characteristic impedance of ANY practical transmission line is complex (that is, R + jX), and varies widely with frequency. There's a tutorial about this on my website. 73, Jim K9YC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
