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Old 15th May 2007, 03:36 PM   #1
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Default CMRR, what is it?

I can read the textbook definition, but I can't imagine how the term CMRR works in audio amplifier.

What is CMRR? How can it influence good/bad sound reproduction of power amplifier?

It is usually concerning first stage/differential, quality of CCS.
The term CMRR also mentioned in Transformers. Are these 2 the same CMRR?
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Old 6th June 2007, 02:01 PM   #2
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In a balanced line, pin1=ground, pin2=hot, pin3=cold.

What happens if magnitude of pin2 (Vrms) is not equal to pin3 (Vrms)? Bad CMRR?
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Old 6th June 2007, 02:06 PM   #3
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In laymans terms, CMRR is the ability of a stage to not be influenced by a signal that is common to both the signal and ground* inputs to it. So something with poor CMRR, if the signal and ground both have say 1 V RMS sine on them, the output of the stage wil 'bleed through' a small amount of that signal. Which is obviously unwanted.

*Yes I know it may not strictly be ground, but I'm trying to keep it simple
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Old 6th June 2007, 02:26 PM   #4
SY is offline SY  United States
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Think of the amplifier as an opamp with an inverting and noninverting input. The master equation (ideal) is Vo = A (V+ - V-). So ideally, if the inverting and noninverting input were tied together and driven by a signal, the output would be zero. CMRR is a measure of how close to zero the actual amplifier is with this connection.

For a transformer, the CMRR is pretty much the same thing- connect the primary ends together and drive them with a signal. Whatever appears on the secondary is an error voltage, and reflects the imperfect common-mode rejection.
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Old 6th June 2007, 02:52 PM   #5
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Hi,
take any normal two tone input signal. Apply the big signal to the front end and then add the small signal on top of it.
The output will not be an exact replica of the input. Some of this diistortion is due to common mode error at the input and some from later stages in the amplifier.

If one can reduce the common mode error (by increasing the CMRR) then there is a good chance that the output distortion will also be lower.

Leach and Self have a good explanation on solutions for the front end reduction of Common Mode errors.
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Old 6th June 2007, 03:21 PM   #6
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Hi, everybody,

Thanks. SY's explenation is the simplest (for me) to understand CMRR

I'm still trying to corelate CMRR and good sounding.

LM4562. It has good CMRR and good PSRR. And good sounding.

How do CMRR affect audible sonics? By what mechanism?
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Old 6th June 2007, 03:50 PM   #7
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In balanced cable. Ideally pin2 and pin3 has the same magnitude signals with 180deg phase difference.

What happens if, lets say, pin2 has 1Vrms and pin3 has 2Vrms.

Due to unbalanced magnitude of pin2 and pin3, will the receiving unit (balanced receiver) generate harmonic distortion?
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Old 6th June 2007, 04:14 PM   #8
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I have a few amps with a pot to adjust CMRR. What is actually done to zero it out? I've read one method for setting CMRR by connecting the amp to an FM radio tuned between stations.
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Old 6th June 2007, 04:32 PM   #9
Hartono is offline Hartono  Indonesia
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David, you do have plenty of hard question
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Old 10th June 2007, 07:49 PM   #10
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Quote:
Originally posted by lumanauw


How do CMRR affect audible sonics? By what mechanism?

Such questions are never easy, but maybe this one is more profound than it appears at first sight. It is easy to just parrot the textbook answer that high CMRR is desirable since it is the differential voltage we want to amplify, not the common voltage. However, that is the general answer, that goes for diff pairs in general, with no particular application in mind. I do not claim to answer the question below, but just elaborate a little bit on what theory seems to tell us. And please pardon me for the amount of text. I have deliberately tried to avoid math in order not to scare people away.

I often find it useful to introduce two new variables, Vc and Vd, and rewrite the two input voltages to the diff pair as:

Vi1 = Vc + (Vd/2)
Vi2 = Vc - (Vd/2)

This equation system always has a solution. Obviously, Vc is the common voltage and Vd the differential voltage. (How fortunate I happened to use subscripts c and d. ) We typically want to amplify the Vd/2 components, but not the Vc components.

However, if we consider the application where the diff pair is used as the input of an amplifier with a single, non-differential input connected as Vi1 and the feedback signal connected as Vi2, the we note something interesting. Vc and Vd are very strongly correlated. If we assume an ideal amplifier with no noise or distorsion, then Vd will be a scaled (actually much smaller) and possibly phase shifted version of Vc. And Vc will mimic the input signal at just a slightly lower amplitude and possibly a iota of phase shift. In principle, it seems that amplifying Vc too wouldn't cause too much harm, since both Vc and Vd are basically the signal we want to amplify. However, if CMRR is too low, the Vc component will no longer be a negligible part of the output voltage of the amp, and will affect the feedback since only the differential voltage contributes to correcting errors in the amp. Usually, even a bad CMRR should be sufficient for this not to be a problem, I think.

What else might happen? Vd is amplified differentially and the two halves of the diff pair cancel each others errors to some extent. That manifests itself in the well-known S-shaped transfer curve of diff pairs, which is very linear for small differential voltages (at the expense of quickly becoming very nonlinear for higher voltages). So for small voltages, a diff pair is much more linear than a single-transistor stage. Now consider the Vc components of the inputs. Clearly the diff pair is identical to two separate CE stages for common mode voltages. Hence, the Vc components are amplified according to the usual transfer curve of the device (eg. exponential for BJTs). So one thing that seems to happen is that even if amplfying the Vc components seem not to cause much harm, there will be much more distorsion from amplifying Vc than from amplifying Vd (in relative terms). On the other hand, the CMRR is usually high enough to make the fundamental caused by amplifying Vc start out at a much lower level than the fundamental caused by amplifying Vd, so in absolute terms, we can probably expect more distorsion from Vd. Maybe the distorsion from Vc is not negligible, however, and maybe it has a more intrusive spectrum? Maybe this could be a starting point for understanding if and why CMRR is important?
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