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In Reply to: RE: The balanced line system was created for the sole purpose posted by Ralph on July 25, 2012 at 08:44:57
"So in short, we are talking about 'balanced line theory and operation'."
OK, thanks.
Guess I don't think of the geometry as an artifact but rather as a design attribute. 'Artifact' brings to mind secondary issues like finite resistance, dielectric absorption, triboelectric effects, shielding percentage and all that.
I appreciate the clarification.
Rick
Follow Ups:
But the whole point of balanced line operation is that the geometry is now made a lot less important, although there are certain things you can do with the cable that can help.
In most cases this simply means that there is a twisted pair that travels together within a single shield.
With single-ended cables you will see all sorts of geometries that all sound different. The main issue with single-ended cables is that there is no termination standard so terminations might be anywhere from 10K to 500K or even one megohm, and there has to be signal current carried in the shield or at least the ground circuit. These two aspects pretty well guarantee that the geometry will be audible.
So my clarification is that geometry and artifact are not the same, but quite often are directly related. With the balanced line system, the two become disconnected- that is to say that the effect of the geometry is really reduced by several orders of magnitude.
"The main issue with single-ended cables is that there is no termination standard so terminations might be anywhere from 10K to 500K"
The cable itself surely isn't matched is it? I used to work with Mic. cables and theater stuff in general in high school to get funds for important things like ham radio and at a guess their 'surge impedance' was in the neighborhood of 100 ohms so there was no real matching going on. I think they relied solely on the transformers and shield to reject RF a little, but not always enough... Of course in-band that hardly counted so the system ignoring the cable was around 600 ohms.
On another topic, you posted the other day something along the line of balanced amplifiers having lower NF's than single ended and I don't think that's necessarily the case. Using two amplifiers for a given amount of power gain can have lower noise to the extent that their intrinsic noise is uncorrelated however you can achieve the same thing single ended by using parallel stages. But if you were talking about overall noise SUSCEPTIBILITY that's a different matter, at least for in-band noise energy...
It takes a lot of qualifiers to clearly communicate by writing, doesn't it?
Rick
No, the cables aren't matched. Microphones are usually set up to drive about 150 ohms at the input of the mixer, line level is usually 600 ohms.
With regards to parallel sections, you can of course do that with balanced differential as well. That is why I was careful to use the word 'topology' in the post you referred to. We use paralleled tube sections in the front end of our MP-1 preamp to help get lower noise, plus we have a 2-stage vacuum-tube CCS. I have found in differential circuits that you must have a 2-stage CCS or you just will not get the performance out of the resulting voltage amplifier. So as a result, it takes a total 8 tube sections (4 twin triodes!) to execute the single stage of gain at the input of our phono section in the MP-1.
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