Balanced DAC stage makes no sense to me - diyAudio
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Old 25th March 2002, 01:03 AM   #1
jwb is offline jwb  United States
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Default Balanced DAC stage makes no sense to me

Somebody please give me a clue here. My
understanding of the operating principle of
for example a balanced transmission line
is that you can cancel the common mode
noise from the signal at the receiving end,
only because both the positive and the
inverted to signal are known or believed
to have been subjected to the same noise
environment. So this makes sense with
interconnects and so forth.

Now, take a look around at all the people
making "balanced" DACs by running the
digital signal into two separate DACs. How
the heck is that supposed to improve
the situation? You take a digital signal D
and convert it to analog signal A in DAC 0.
You do the same thing with DAC 1, but the
product is analog signal A', due to defects
or differences in the manufacturing process.
I don't even care if you got both DACs from
the same wafer. A and A' are going to be
different signals.

So now you've got these two definitely
different analog signals A and A'. How does
the designer expect to improve the signal
by unbalancing it or by using balanced
amplification? The signals A and A' do NOT
have common-mode noise: they were simply
produced by two slightly different DACs. I
cannot see how this balanced operation is
supposed to work.

Thoughts?
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Old 25th March 2002, 01:56 AM   #2
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Makes perfect sense to me.

Jocko
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Old 25th March 2002, 07:32 AM   #3
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Lightbulb BALANCED DAC

Hi JWB,
Please see my answer to MBK on the AudioAsylum:
http://www.audioasylum.com/audio/twe...ges/55951.html

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Old 25th March 2002, 12:43 PM   #4
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I guess there are five reasons to digitally create a balanced signal:
1) you get some averaging, both of imperfections in the individual DACs as well as of noise
2) you will get twice the analog signal, so you have less to amplify
3) junk on the power supply of the DAC will in first approximation generate a common mode signal that gets rejected
4) if the DAC's output stages or the I/V-opamp are slightly nonlinear, i.e. have a transfer function y= x + 0.0001*x^2, these nonlinearieties will cancel in first order approximation
5) if you need a balanced signal because your amp is truly balanced, it makes more sense the generate it from two dacs that to convert a single-ended signal to blanced with an op-amp

Eric
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Old 25th March 2002, 01:23 PM   #5
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JWB,
I Agree with capslock.

Quote:
Originally posted by Jocko Homo
Makes perfect sense to me.

Jocko
Jocko, comments like these really aren't welcome here. Many of us here are very knowledgeable about particular areas of diyAudio however, none of us can honestly claim to know all there is to know about all areas of it. Please treat the other members of this forum with the same level of respect and courtesy that makes this board better than the rest. Your comment above and other like minded comments you have made in other threads have failed to help anyone and serve to annoy rather than educate. Please keep your posting in line with the spirit of this forum. I am sure you probably have much to contribute to this board, but please do it in a fashion that promotes education and discussion in a friendly environment.
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Old 25th March 2002, 02:11 PM   #6
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Default Not Helpful

I thought the Austrailians were rough and ready types like Paul Hogan. Are you sure you aren't from California? Sure Jocko stirs it up sometimes but so what. I have seen Nelson Pass make a few rude comments from time to time without getting flammed for it. Jocko has put plenty of useful info on the forum to justify his slight character flaws. He can't help it if he has a sense of humor.
I'm the one that put a picture of Il Duce of the forum. God knows what you think of me..... If it will make you fell better, I will give him a dope slap on the back of the head and say " Whatsa matter with you!" the next time I see him.

H.H.
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Old 25th March 2002, 02:51 PM   #7
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HH, i've not lost one ounce of sleep over anything said on this board and nor do i have a problem with humour and tho i sometimes dislike the direction of certain comments made on this board, I usually keep my peace on such matters. I thought your pics were good for a laugh as I believe they were intended. However, comments such as the one above go way past humour.
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Old 25th March 2002, 04:27 PM   #8
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Default "Makes no sense to me........"

I merely stated a fact.

If he had asked for an explanation that I thought he would get, he would have gotten it. But he seems to miss the whole point about balanced in general, and more importantly CMRR.

So I let capslock do it for me.

Jocko
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Old 25th March 2002, 04:51 PM   #9
jam is online now jam  United States
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Audiofreak,

This being a forum, replies are sometimes short and is sometimes mistaken for rudeness and sometimes misunderstandings occur.
I am certain Jocko did not mean to be rude even if he hails from Texas.

Jam
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Old 25th March 2002, 05:05 PM   #10
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Speaking of "not getting it.........."

I have 2 close friends, both of whom run successful "high-end" audio companies.

I distinctly remember one day when one lectured the other, ad nauseum, that he didn't understand the concept of balanced. It revolved around the possibilty that the 2 signals may not be quite the same. Friend #1 ranted endlessly to Friend #2 that you could "ground" (read: make zero) one of the outputs and it would still work.

So for those of you in Rio Linda...........CMRR has nothing to do with the source, except for its drive impedance.

Balanced picks up 3 dB in signal to noise ratio. It give you 6 dB in gain. It reduces modulation on power supply rails. It reduces common-mode noise ON THE CABLE.

And it usually costs little or nothing for all that. Such a deal.

Doing it the right way......the way that may not make sense.....is much better than trying to do it with cross-coupled op-amps. like SSM-2142, et. al. Trust me......I've had to do it that way, and it is just OK.

Which reminds me...........nah........that story is way too long for now.


Jocko
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