CMRR and "Fully balanced designs"

[philmagnotta]

Dear forum:

Due to the ever increasing number of power amp's, integrated amps and preamp's advertised as "balanced" and or "fully balanced" designs, which have gained popularity primarily due to home theater, I'd like your opinions on the following please:

1. The majority of those designs, as in pro-audio also, feature some type of circuit at the XLR inputs, but from there, continue as only single-ended types.
They vary considerably regarding their CMRR performance.
Also, most of them, offer RCA inputs as well, except in pro-audio where the alternate input types are 1/4" jacks (instead of RCA) and often, Tip-Ring-Sleeve types instead of XLR.
What is interesting is that the common types of specifications given for each design usually show better specifications when using the RCA (or unbalanced) ins rather than the so-called balanced inputs.
In pro-audio, the quasi balanced ins usually offer (as the only advantage) a higher operating level at the inputs.

2. My main interest concerns the actual design of fully balanced, mirror imaged circuitry types. That is to say: separate circuitry for the positive and negative signals for each channel from input to output.
Since there are many manufacturers designs available, there will be-of course-differences in excellence in those designs.
When a manufacturer offers the common basic specifications, one can not usually figure out the quality of the unit.
Regarding CMRR, I assume that it refers to the ability of the circuitry to reject as much of the unwanted signal as possible across a given bandwidth and at various levels, etc.
What should one look or inquire about regarding these designs?
What sort of "Hype" or marketing language do you usually come across?

3. Besides CMRR, what other types of specifications should show as an example of the advantages of fully balanced designs compared to single ended types?

4. Will there always be an advantage to balanced versus unbalanced in properly designed systems of each type?

Thank you,

[MatchASM]

Originally Posted by philmagnotta

... "balanced" and or "fully balanced" designs, which have gained popularity primarily due to home theater...

I don't think so. I think this popularity mainly comes from professional audio.

What should one look or inquire about regarding these designs?

What do you want to do with them?

What sort of "Hype" or marketing language do you usually come across?

"balanced makes your home theatre system sound better"

Besides CMRR, what other types of specifications should show as an example of the advantages of fully balanced designs compared to single ended types?

Linearity performance might be slightly better, since the signal amplitude is divided over two phases, meaning that single-ended amplitude is 2x smaller when compared to the supply voltage. But in a well-designed system, this really shouldn't matter.

Will there always be an advantage to balanced versus unbalanced in properly designed systems of each type?

Yes, you can run miles of cable without worrying about picking up interference from other electric appliances. That's about it in terms of real-world advantages. The disadvantage is that at some point inside the signal source, an amplifier stage sees twice the load, meaning performance might be degraded compared to an unbalanced system. If you don't risk picking up interferers and you're not turned on by XLR connectors, I recommend you go with unbalanced (in whatever form, RCA or jack or pretty much whatever).

--
Greetz,
MatchASM

[tomchr]

The point of differential signalling is to get high rejection of interference on the signal pair. This is useful if the cable between the source and the amp is very long (maybe 100's of feet or more). However, note that the rejection only works if the interference is actually common mode -- i.e. the same amount of signal in injected on both signal wires. For a tightly twisted pair, this is generally the case.

An ideal differential amp will only "see" the voltage difference between the two inputs and amplify just that. It will ignore any common-mode voltage present on the two inputs. In other words, the two inputs could be 1 kV above amp ground and (assuming the amp had enough common mode range to handle this) the amp would still only see the differential voltage between the two inputs.
In the real world, the amp does not completely reject the common-mode signal. Its ability to reject the common-mode signal is the CMRR - Common-Mode Rejection Ratio. That's the money spec.

Consumer audio being consumer audio is really more about marketing than about specs. Most people do not have the test equipment needed to verify the specs. So it's easy for manufacturers to claim that their designs are "fully differential" or "fully balanced" (what does balanced really mean in this context?!) and few people would be able to challenge this.

~Tom

[cbdb]

One thing to watch out for is the degradation of the CMMR of an input due to mismatches in the impedance of the circuit feeding it, including the wire. Bill Whitlock of Jensen has done a lot of work on this. He has shown that some circuits can lose 20 db CMMR from a small imballance (which is typical) in the capacitance of the interconect cable.

[454Casull]

Originally Posted by cbdb

One thing to watch out for is the degradation of the CMMR of an input due to mismatches in the impedance of the circuit feeding it, including the wire. Bill Whitlock of Jensen has done a lot of work on this. He has shown that some circuits can lose 20 db CMMR from a small imballance (which is typical) in the capacitance of the interconect cable.

What is an example of a small imbalance in the capacitance of a cable?

[cbdb]

For a Zin of 20k a 2ohm imbalance will degrade CMMR by 20db.

Heres more
info:Balanced Interfaces

[WuYit]

tomchr,

So it's easy for manufacturers to claim that their designs are "fully differential" or "fully balanced" (what does balanced really mean in this context?!) and few people would be able to challenge this.

Would "no reference to ground" be an accurate definition of "fully balanced"?

[CBS240]

Hi

There are advantages to using a balanced topology, but there are some disadvantages as well. The design of the circuit is important in minimizing common mode distortions. If the intent is to have a balanced output, the IPS and VAS should be designed as a single balanced circuit not two seperate single end amplifers simply driven with oposite phase signals. Using a single balanced topology instead of two SE circuits will help reduce common mode distortions as a difference between the two outputs. Common mode PS distortions are not fed to the speaker either. Ground return current of the speaker is eliminated as well as there is no reference to GND.
On the flip side a balanced amp usually requires a balanced input so this limits the program sources available. For a single end input to balanced output many folks use OP-amps, one inverting the other non-inverting, to create a balanced signal as transformers have thier own issues. Unfourtunately this usually leads to phase shifting at higher frequencies because an inverting OP-amp has different properties than a non-inverting circuit. Phase differences at the balanced output create distortion. The circuit and PCB layout is more complicated because you now have two output stages along with all the other extra parts.

But why use balanced output anyway? A lower voltage power supply is an advantage even if the current taken from it is two fold and more uf's of filter caps are required. But then, lower voltage filter caps are cheaper and smaller per uf. Lower voltage transistors can be used which opens the door to better device selection.

The balanced amp I created can crank ~120Wrms into 8R using +/- 24V. The output transistors are 60V devices. The VAS is a balanced bridge topology so one side works off of the other. The pre-amp circuit can accept either XLR or SE and requires a simple jumper to select the input source. In SE mode the pre-amp circuit converts the signal to balanced. If you feed the + input to the bridge, the two outputs will be balanced in 180 degree phase but the - output voltage will be smaller. The circuit uses a deliberately misbalanced bridge topology and a common mode error correction scheme to balanced the signal voltage, esentially it converts the difference in voltage amplitude at the two outputs to a difference of output Z but with equal output voltages. At 2MHz, of course gain drops a bit but phase alignment is perfect. (circuit's pretty fast) The input Z of the next amplifier stage is very high so the difference in the two signals become eliminated from the transfer function resulting in a much better SE to BAL conversion than one could achieve from using op-amps. I once posted a discription of the circuit here but no-one was really interested. Oh well. Perhaps I will do a write up after the final design is complete, which could be a while because of lack of funding.

Anyway, IMHO the pros out weigh the cons and balanced topology is the way to go, if you have a good solid balanced signal and proper PCB layout.

[Steve Eddy]

Originally Posted by philmagnotta

2. My main interest concerns the actual design of fully balanced, mirror imaged circuitry types. That is to say: separate circuitry for the positive and negative signals for each channel from input to output.

Such designs, while technically "balanced" in that the impedances of each line is balanced with respect to ground, which is what "balanced" refers to, has no common-mode rejection.

To reject common-mode noise, the input needs to be differential. And having separate circuits on each side like that is not differential. It will pass common-mode noise as if it were signal.

se

[philmagnotta]

Hi:

Thanks everyone for replying.
As usual, here at diY, I new that I would get in-depth answers.
I definitely want to continue with this thread, but have to be absent from my computer for a few hours...just wanted to let everyone know that I did not just fade away from the topic.
I'll return in a little while as this is a topic of great interest.
Thank you.