This thread is beginning to wear thin. 73. . .Dave, W0FLS ----- Original Message ----- From: "Jim Brown" <[email protected]> To: <[email protected]> Sent: Friday, July 13, 2012 6:31 PM Subject: Re: Topband: Mother of all ferrite common-mode coaxial chokes
> On 7/13/2012 2:04 PM, Paul Christensen wrote: >>> One of the dumbest things I've seen recently from a very good company is >>> RF chokes in series with the shield connections on analog and RS232 I/O >>> boards for the Elecraft K3. >> Right. I think Joe, W4TV, was one of the first to identify that problem >> back around 2008. > > I think I was the first. Joe and I discussed the K3 a lot. > >>> Modern pro output stages have a very low source Z (typically >>> 100 ohms for line level) and high input Z (typically 10K for line >>> level), and consumer stuff is roughly 3-5X those values. >> Somewhere in my files from the late '80s is a white paper authored by >> Richard Cabot. I believe Richard was the chief designer of the Audio >> Precision brand of audio test equipment. The focus of the article was on >> the standarding of all audio output stages, balanced or unbalanced to a >> value of 40-50 ohms. He created models showing the effect of changing >> drive >> Z from 1-ohm through 600-ohms into long audio cables (and independent of >> terminating Z) that start to take on transmission line qualities. His >> conclusion was that a target of just under 50 ohms was optimum. > > Dick is a sharp guy. The best work I've seen on this topic was by the > late Deane Jensen, c.a. 1980, showing that the capacitive loading of a > long cable could make an output stage unstable, and that 100 ohms was a > good value to prevent that from happening. That's been pretty much > accepted as definitive, which is why most pro output stages are in that > range. This can, of course, be device-dependent, and the lower value > Dick suggested would be fine if the output device was unconditionally > stable for all loads. It's VERY common in large installations for there > to be 500-1,000 ft of cable bridging an output stage, and commonly used > balanced audio cables range from as much as 40 pF/Ft (older stuff like > Belden 8451) to as little as 13 pF/Ft for cable rated for AES3. > > That said, a cable would have to be VERY VERY VERY long to require > transmission line analysis at audio frequencies, and applying > transmission line analysis is VERY complex, because Zo varies over two > orders of magnitude through the audio spectrum, converging to 75-100 > ohms at HF for practical audio cables. See > > http://audiosystemsgroup.com/TransLines-LowFreq.pdf > > 73, Jim K9YC > _______________________________________________ > UR RST IS ... ... ..9 QSB QSB - hw? BK > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
