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Old 9th December 2011, 12:07 AM   #301
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No, they are 'realists' who have to compete with other companies. Never stick your head up too far! '-)
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Old 9th December 2011, 12:20 AM   #302
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Quote:
Originally Posted by Ken Newton View Post
Just a comment about what DAC vendors believe constitutes sufficient performance. The AD1865 DAC chip is usually used in current output mode, but it also contains an on-board I/V op-amp. The Analog Devices datasheet takes the time to specifically mention that this on-board op-amp was designed to handle the high slew rates it would be subjected to, even at an x16 oversampling rate. They even go as far as to mention that this internal op-amp features an all NPN output stage.

Now, AD knows as much about high-performance op-amp design as anyone. So, after experimenting with the AD1865, non-oversampled, in both it's current (simple resistor i/v), and voltage output modes (internal op-amp), I can report that, sonically, there's no comparison. The current output mode via simple resistor i/v is so much more transparent, you'd think that you were listening to two entirely different converter chips.
That is nice, so there are ways to do things just better without opamps. So why are some trying very hard to persevere that opamps are the best for this purpose ?
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Old 9th December 2011, 01:37 AM   #303
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Quote:
Originally Posted by SoNic_real_one View Post
To me, newer OpAmps like the AD8099 or LM4562 are plenty sufficient for the today DAC's I/V stages. DAC manufacturers agree also with that.
Which vendors offer I/V with LM4562? I'd like to have a laugh...
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Old 9th December 2011, 08:54 AM   #304
qusp is offline qusp  Australia
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well thats good Ken, you can report there is no comparison (which is good, because i dont see one)..and after so much experimentation as well?....well we all better throw in the towel and concede that worse is actually better (re transparency)

yeah its kinda funny, i've often wondered why the dac chips (themselves integrated circuits that use feedback) are usually good enough for the pundits as a dac, but when it comes to making them output the analogue portion of the DAC, all of a sudden the designers are a bunch of penny pinching fools who wouldnt know what to do with their own design when all along they could just use a pair of resistors for more transparent results

Last edited by qusp; 9th December 2011 at 08:59 AM.
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Old 9th December 2011, 12:45 PM   #305
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Originally Posted by SoNic_real_one View Post
Yep, AD engineers are a bunch of monkeys. They got the plans for their 1990 DAC from martians and they just slapped a stinky OpAmp inside.
That by the way was absolutelly appropiate for the 16 bit signals of the era.

The use external OpAmps AD797 in the datasheet of their today flagship AD1955 is just another conspiration...

I do believe that you've missed my point.
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Old 9th December 2011, 01:39 PM   #306
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Originally Posted by jean-paul View Post
That is nice, so there are ways to do things just better without opamps. So why are some trying very hard to persevere that opamps are the best for this purpose ?
Only they can say for sure, but my guess is that it's for the following reasons:

1. Op-amp I/V is easier to implement with DACs that require something very close to a virtual A.C. ground for best performance, which is to say nearly all of them. The output protection diodes of many DAC chips forces this constraint. Implementing such a virtual ground without op-amps (and their requisite feedback loop) pretty much requires a discrete solid-state grounded base/gate I/V circuit, or a very high-gain (probably two stage) open-loop tube circuit which could be relatively quite costly.

2. Objective measurements say that op-amp I//V has the lowest static THD. By that measure, however, we would all be purchasing only high-feedback solid-state amplification components. The trouble with this view, of course, is that our ears often disagree with what spectrum analyzers say. If only we listened to music through a spectrum analyzer it would be simple to identify the best components or circuit implementations. Music appreciation is a human quality. Audio egineering should be in service of that appreciation, not the dictator of it.

3. Commodity DAC boxes are simply too easy to design, what with the few VLSI integrated circuits required anybody can "design" a DAC box. Which isn't to say that they necessarily know what they are doing. Even if they do hear deficiencies in their finished design do they know what to do to minimize those deficiencies? The great thing about op-amps is that they are so accessible for audio electronics neophytes, and convenient for the experts. The problem with op-amps is that their use is seldom optimized by such neophytes, let alone the question of whether some discrete implementation would provide better perceived performance, let further alone the question of how to deign such an discrete circuit. Even many expert engineers, IMO, get seduced by the convenience leaving the DAc analog stage implementation appearing to be almost an afterthought income cases, something not surprising given the time and cost pressures of producing a commercial product.

4. The cost. Most other implementation of an I/V plus output buffer will be more costly to design and produce than with op-amps.
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Old 9th December 2011, 02:13 PM   #307
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Quote:
Originally Posted by qusp View Post
well thats good Ken, you can report there is no comparison (which is good, because i dont see one)..and after so much experimentation as well?....well we all better throw in the towel and concede that worse is actually better (re transparency)

yeah its kinda funny, i've often wondered why the dac chips (themselves integrated circuits that use feedback) are usually good enough for the pundits as a dac, but when it comes to making them output the analogue portion of the DAC, all of a sudden the designers are a bunch of penny pinching fools who wouldnt know what to do with their own design when all along they could just use a pair of resistors for more transparent results
Firstly, please spare me your silly condecensions.
Secondly, hyperbole much?

You sound like the kind of person who let's spectrum analyzes tell them to what degree their ears should be enjoying the music being reproduced some given component. Better yet, just visit eBay and order whatever are the cheapest components claiming the lowest static THD, yes?

Regarding DAC chip design, the resistor class of DACs (R-2R, ladder, etc.) don't use signal feedback. They typically consist of a quiet precision buffered voltage reference feeding resistors, or switch precision current sources through those resistors. You will see feedback employed to maintain D.C. levels, or to minimize the output impedance of the reference buffer. If, however, they employed signal feedback at the quantizer block you'd see much better THD figures listed for the resistor class of DACs, which greatly depend on the precision of their resistor networks (or on averaging the lack of precision to noise via DEM) for low distortion (particularly obvious problem at low signal levels), just as you see low THD figures for the feedback based sigma-delta types. The sigma-delta class of DACs inherently use discrete-time digital feedback, which is not the thing thing as the actual quantizer block which can and often is a small multibit resistor class converter.
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Last edited by Ken Newton; 9th December 2011 at 02:43 PM.
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Old 9th December 2011, 06:12 PM   #308
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Quote:
Originally Posted by SoNic_real_one View Post
Yep, AD engineers are a bunch of monkeys. They got the plans for their 1990 DAC from martians and they just slapped a stinky OpAmp inside.
That by the way was absolutelly appropiate for the 16 bit signals of the era.

The use external OpAmps AD797 in the datasheet of their today flagship AD1955 is just another conspiration...
"They got the plans for their 1990 DAC from martians and they just slapped a stinky OpAmp inside."
I don't understand this statement exactly.
In the case, that you think, this topology is bad for good acoustical sonic performance - what would be a better solution for the AD1955 output filter (I have heard, that at the output of AD1955 isn't a R2R network like by PCM1704/1702/63) ??

From a death link from France - main page go to
Vincent Brient
I have save this text and attachments:

A Power DAC (simple two-way I/U converter for AD1955)
This is a hifi design using a minimum of components in the signal path. A DAC AD1955 is used and the current to voltage converter will be the power part. So I modified a CD player and made a DAC board equipped with the AD1955:
and used and external case for the power part. It uses a class A current to voltage converter able to deliver 1W per channel, enough for horns. A PIC decodes the the infra-red commands and ajustes the digital volume in the AD1955:
Analog Device recommands a 2.5V voltage at each AD1955 current output. Two 1N4148 diodes and two transistor bases make this voltage. The feedback is ajustable and set the bias current and the gain. The high frequencies are naturally filtered by the limited bandwith. A PI filter has been necessary to get a perfectly silent amplifier.
Use: I am using this amplifier from 120Hz only, a conventional active crossover and amplifier are used to get a higher power output for bass.


What about this solution?
Attached Images
File Type: gif I-V for AD1955 schema.gif (3.7 KB, 832 views)
File Type: jpg I-V for AD1955 Power DAC.jpg (19.0 KB, 799 views)
File Type: jpg I-V for AD1955 PCB top view.jpg (34.7 KB, 788 views)

Last edited by tiefbassuebertr; 9th December 2011 at 06:25 PM.
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Old 9th December 2011, 06:58 PM   #309
sonnya is offline sonnya  Denmark
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It is fun to read this thread. Discussion about using transformers, which topology to use in opamps, feedback versus no feedback, using only resistors..

All this let mé think where does the logical thinking goes!? Before advocating for a design type or material use, maybe it would in place to analyse how the current ouput is designed and it does not differ a lot from manufacture to manufacture.

Look at things Like why does the chip bias at +2.5v?

What is the worst condition the output Will swing?

How do we get maximum linearity?

I try not to be biased regarding circuit thinking as i Think let you get the Best solution in the end...
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Old 9th December 2011, 07:06 PM   #310
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Quote:
Originally Posted by SoNic_real_one View Post

The fact it is that a transformer is NOT a LF filter. Is a pass-band complex filter, imposible to adjust and on top of that with neliniarity issues. The worse is that it has an input resistance/impedance that is higher than the one necessary to be seen at the DAC current outputs.
The DC resistance of a transformer is extremely small, depending on the number of turns and the thickness of the wire. The impedance is a function of turns, turns ratio, and loading resistor. The performance of high quality transformers is extremely good in my opinion.
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