On Thu, 18 Sep 2008 13:31:37 -0400, Robert Close wrote:
>Jim, no mention of differential amplifiers?
See much ham gear with a diff amp or a balanced out? See many computer sound
cards with diff amps or a balanced out? More to the point -- the tutorial and
the Power Point presentation show clearly why a diff amp is not needed!
>No mention of the difference
>between balanced and unbalanced methods of signal distribution?
Sure. Browse the other tutorials on my website. This one is about HAM RADIO
interfacing.
"600ohm"
>audio is still very much used, albeit under different names.
No, it is not, and has not been for nearly 40 years. I've worked in pro audio
and broadcasting since 1959. At least since the '70s, pro audio is
transported from a low Z source (typically 100 ohms) and received by a high Z
load (typically 10K).
>The 600 ohm is
>a somewhat arbitrary number, and when devices were inefficient it was more
>efficient to match the impedance for maximum transfer of energy without
>introducing phase distortions.
Maximum power transfer has nothing to do with it. 600 ohms goes WAY back to
the days of long range transmission of broadcast quality audio across the
country on parallel wires (the '30s). There is documentation of this in the
Bell Systems Technical Journal. It also goes back to the days when all the
electronics used tubes, and a "console" was a bunch of passive 600 ohm ladder
pots that attenuated the outputs of line drivers in an equipment rack. Those
circuits WERE 600 ohm circuits, but they were used so the pots would work!
Since the '70s, the pots are simply controlling the gain of solid state
amplifiers.
> Your "solution" works , in that it reduces
>the problem to inaudible levels, which is probably "good enough"
Yes. This is HAM RADIO, not high futility audio or broadcasting or audio
studio or high level sound reinforcement. I do all of that work (except for
high futility) too.
>but it
>doesn't address the root cause of un-balancing a balanced feed by using a
>"coaxial" connecting wire. (one conductor in a shield), and how the common
>mode problem rears its ugly head, as well as the ground-loop problems.
>Isolation of grounds is always effective when we are talking about
>ground-loop issues cause by unbalanced connections.
The root cause is NOT the coax, it is fact that one signal terminal is tied
to the chassis and one is not, and there is usually noise voltage between the
two chassis.
>Stick in a transformer,
>balance the audio feed, make the ground independent of the signal, and poof,
>"no problem".
Except for the fact that unshielded transformers also couple magnetic fields
into the circuit (like the leakage flux from the power transformer in the
power supply sitting under your operating bench). Ask the owners of Elecraft
K3s who have this problem with hum coupling on their line input, which has
unshielded telephone transformers for "isolation." My neighbor has so much
coupling from an adjacent Alpha power amp that the HUM regenerates to 500
watts!
And transformers cost a lot more than WIRE. Apparently you like to spend more
money on your solutions. We could get you a job in government!
>Nice website you have, but since you opened the door to
>discussion of audio, be a bit more specific in your statements, or nuts like
>me are going to send you email.
What I wrote was QUITE specific. It simply differs from the old wives' tales
that you believe. My website has several dozen tutorials and research papers
on these issues, and they are QUITE specific and detailed. Apparently nothing
is going to stop nuts like you from sending me email. :)
>I am not as cool as you, since I don't
>"consult" in the audio field, I just design and "do" for the last 25 years.
>One thing I do know, is once you add RF into the equation a lot of the audio
>"solutions" fall apart if the root cause of the problem is not addressed.
Apparently you missed http://audiosystemsgroup.com/RFI-Ham.pdf It addresses,
in considerable detail, the REAL causes of RFI. You will also see that I have
done considerable disciplined research on how RF couples into audio circuits
and systems, and published it in peer reviewed presentations. The AES
Technical Committee on EMC and the AES Standards Committee Working Group on
EMC include more than a few Engineers who are VERY sharp and experienced with
RF issues.
73,
Jim Brown K9YC
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