Ya think ????
----- Original Message -----
From: "David Raymond" <daraymond@iowatelecom.net>
To: <topband@contesting.com>
Sent: Friday, July 13, 2012 6:48 PM
Subject: Re: Topband: Mother of all ferrite common-mode coaxial chokes
> This thread is beginning to wear thin. 73. . .Dave, W0FLS
> ----- Original Message -----
> From: "Jim Brown" <jim@audiosystemsgroup.com>
> To: <topband@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
>> _______________________________________________
>> UR RST IS ... ... ..9 QSB QSB - hw? BK
>>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
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