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In Reply to: RE: What an astonishingly insightful (and iconoclastic) assessment of some formerly revered designs.... posted by tube wrangler on March 21, 2017 at 22:44:30
Dennis, I do not believe that you are stupid.I believe you will be able to understand, after watching the video linked below, that the CMRR of a differential amplifier will only reject the noise signal that is picked up in both wires equally (the only signal that is common mode) and will not reject any of the signal that is not common mode (the music signal).
I believe that if you really think about it you will be able to see that none of the input signal (the music signal) can be common to both wires but instead the music signal is differential between the two wires.What we (meaning you and I) call music signal starts it's life at the diaphragm of a microphone.
Take a look at the circuit of a u47 microphone. The circuit is single ended. The music signal is the difference between the output of the tube and ground. The signal goes to a parafeed output transformer through C2 to the primary of the output transformer as a single ended signal. There can be nothing common mode up to that point. The other end of the primary is grounded.
The secondary of the output transformer is not connected to ground so the music signal is now the difference between the two ends of the secondary.The two conductor shielded cable, with the shield connected to ground for shielding purposes only, connects the music signal to the mic-pre in what is called a balanced connection.
The mic-pre input transformer has a primary that is not connected to ground and will only respond to the difference between the two ends (the music signal).
Any noise picked up in the wire run between the microphone and the pre-amp will be equal in amplitude and phase in both wires (common mode) and that input transformer will not respond to signal that is common and therefore rejects it (common mode rejection).
So the noise that is picked up in the wire run is common and rejected while the music signal is not common (it's differential) and is therefore not rejected.
Having said all that, please understand that I am not trying to promote push pull amplifiers.I, like you, believe that single ended amplification is the better way to go but for completely different reasons than the ones you stated above.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 03/22/17Follow Ups:
There's nothing wrong per se-- with any of the explanations
I've read here. In fact, I think this is an excellent
discussion, but there is more!Common-Mode finds its way into every part and every
wire, and every application in audio that is operating near
any source of alternating current OR musical energy..Differential circuitry and balanced connections reduce
Common-Mode, as you well know.I do not disagree with the THEORY or APPLICATION of
differential circuits or balanced connections. Fine--
we can agree here, but that is only a tiny surface
discussion-- the whole picture is much more.....What you're not addressing is the fact that splitting
a common-mode signal into two halves and then re-assembling
it is not perfect-- it cannot be.I think we can all agree that differential circuitry
reduces (attenuates) all common-mode-- signals, distortions,
and the common-mode that is music.In a differential system output, you don't just get 1/2
plus 1/2 to form ONE. Since no build or wiring is perfect,
and no device is perfect, there is also Common-Mode mixed
in with it. The common-mode always contains both distortions
and musical artifacts... There is NO way to tell which is
which-- music can and does do anything that distortion does.While the sum of the two halves of a common-mode signal
that has been split into two halves does equal a Single-Ended
signal mathematically, it does not consist ONLY of a true
differential output. The output is mixed with what Common-Mode
and differential distortions are left after the Common-Mode
attenuation of the differential circuit has taken place. The
output has also attenuated musical artifacts that are found
in the common-mode.Engineers measure some of these things as various distortions--
aberrations from the desired "perfect" output, which would be
devoid of all Common-Mode.That never occurs in real world equipment, and it never
occurs in music, either..Single-Ended operation is all Common-Mode. You can filter it,
but you can't process-out the Common-Mode because that is
your musical signals! NOTE the Plural here...The advantage of S.E. operation thus becomes obvious:
there is no Common-Mode processing/reduction of the
constantly changing musical signal's Common-Mode content.Some of that common-mode content is musical artifacts,
not distortion.What could be simpler? The S.E. signal is not
differentially processed, so there is no way to suppress
the common-mode content of the signal that is musical
artifact..-Dennis-
.
Edits: 03/22/17 03/22/17
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