Topband: Mother of all ferrite common-mode coaxial chokes

[David Raymond]

This thread is beginning to wear thin.      73. . .Dave, W0FLS
----- Original Message ----- 
From: "Jim Brown" <jim at audiosystemsgroup.com>
To: <topband at contesting.com>
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
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