Posted by
Jim Brown-10 on
Jul 02, 2004; 3:26pm
URL: http://elecraft.85.s1.nabble.com/OT-Line-level-tp368401p368414.html
On Fri, 02 Jul 2004 06:29:33 -0400, Mike S wrote:
>At 11:32 PM 7/1/2004, Jim Brown wrote...
>>BTW -- forget all that ancient stuff about 600 ohms. Pro audio hasn't used a 600 ohm
reference for at
>>least four decades.
>
>That may perhaps be true for a limited range of "pro audio" applications,
>but 600 ohm audio transmission lines are still VERY common both in
>telephone and PA applications. If you forget that stuff, you WILL end up
>with real problems.
I cannot speak with authority for telco practice, but I am very much up to date on pro audio
practice -- I am a member of the Standards Committee of the Audio Engineering Society, and
I have recently been an invited speaker on EMC issues to AES, IEEE, and SBE (the Society
of Broadcast Engineers).
It is not been common practice (or good engineering practice) to drive from a 600 ohm source
or terminate with a 600 ohm load for at least 40 years. Yes, broadcast facilities and equipment
were built that way 40 years ago, but they have not been built that way for many years. Richard
Hess, then at ABC, published an excellent paper on that topic around 1970. Some very old
tape recorders and long obsolete passively matched telco interfaces (for example, an
equalized dry pair) required 600 ohm (or 150 ohm) terminations to work properly, or for their
level matching to be correct. Both practices have been obsolete for many years. In fact, as
long ago as 1975, it was hard to find anyone at our local telco who knew what an equalized
line was!
Some manufacturers of equipment who "don't get it" continue to talk about their equipment as
having 600 ohm outputs, but if you measure their output impedance, your instrumentation will
tell you they are 50-100 ohms. A quick look at the schematic will tell you the same thing.
Further, the output stages of most pro equipment are NOT designed to drive a load. 600 ohms
is more of a worst case design spec -- that is, the most current that the output stage can
provide -- and the performance of the output stage will be better (a dB or two more headroom,
less distortion on peaks) if it looks into the 10K load that is the IEC standard.
Jim Brown K9YC
http://audiosystemsgroup.com
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