John Curl's Blowtorch preamplifier part II - Page 4057 - diyAudio
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Old 23rd June 2013, 03:36 AM   #40561
jcx is offline jcx  United States
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On page 17 Walt shows that the dynamic impedance of the AD811 as an I/V is .85 ohms.
what are the meaurement conditions? - its easy to get milliOhm Z with feedback over audio frequencies - with VFA too

the AD811 I/V can't be an exception to this - its open loop -input Z is speced at 14 Ohms typ and it has to rely on loop feedback to reduce this value

milliOhms is impractical to achieve at DAC switching glitch energy frequencies - parasitic lead and trace inductance insure that

I'm not saying VFA are the best choice for audio DAC I/V - but its not clear what the errors the Golden Ears are hearing are when looking at recent VFA, FET or linearized BJT front end parts performance in I/V

and the input current noise of AD811, CFA as a class mean you may not be reaching the S/N spec of some flagship audio DACs

Last edited by jcx; 23rd June 2013 at 03:38 AM.
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Old 23rd June 2013, 03:54 AM   #40562
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Unfortunately, JCX, what I am interested in is the output stage current capability and less internal temperature change inside the IC with transients.
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Old 23rd June 2013, 04:06 AM   #40563
jcx is offline jcx  United States
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Hawksford's multiloop op amp I/V ? - seems like someone should try it someday - it does have "Guru" cred...
http://www.essex.ac.uk/csee/research...0amplifier.pdf

... multiloop accomplishes the separation of input perfomance from output load driving thermal effects - choose different, better speced op amps for each task
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Old 23rd June 2013, 05:49 AM   #40564
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A brute force approach can also be done. Drop a small resistor across the summing junction and parallel a bunch of active devices to amplify. Not really any more trouble than getting similar current levels to drive feedback resistors.

All good fortune,
Chris
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Old 23rd June 2013, 06:13 AM   #40565
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Thanks JCX. I should have looked up Dr. Hawksford's work, before I came to any conclusions of my own. And this verifies what I am concerned with, because designers of many commercial digital playback systems, apparently have not read this AES paper, since they ignore its implications.
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Old 23rd June 2013, 07:04 AM   #40566
qusp is offline qusp  Australia
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its interesting that when others, like myself criticised a fairly pedestrian VFB opamp IV in the recent oppo, you defended that it was built to a price, but well done, despite there not being any real significant budgeting constraints of doing it better...
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Old 23rd June 2013, 08:15 AM   #40567
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Well qusp, I'm glad that you know everything that there is to know. I am just coming up to speed with digital reproduction. I have a very good idea about the OPPO, and I told the company that about 6 mo ago. I'm afraid that I didn't have much proof, just my ears.
And what IC would you chose?
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Old 23rd June 2013, 08:38 AM   #40568
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Quote:
Originally Posted by bcarso View Post
It is unfortunate that few manufacturers reveal what their output impedance really is, and in particular how much it may be code-dependent.

Brad
To my consternation I recently learned that the well-regarded ESS DAC chip has a Zout of 781 ohms nominally on each phase. Not 'current output' in my book!
I remarked elsewhere that this is a schitzophrenic chip, not able to decide whether it should be a current out or a voltage out DAC. But maybe I was too harsh.

jan
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Old 23rd June 2013, 10:38 AM   #40569
bcarso is offline bcarso  United States
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Originally Posted by jan.didden View Post
To my consternation I recently learned that the well-regarded ESS DAC chip has a Zout of 781 ohms nominally on each phase. Not 'current output' in my book!
I remarked elsewhere that this is a schitzophrenic chip, not able to decide whether it should be a current out or a voltage out DAC. But maybe I was too harsh.

jan
And afaik, again no discussion/disclosure of the code dependence of that stiffish conductance. This leads to additional consternation for those who calculate, or observe, the noise gain when the ESS chips feed the I/V converter. A seemingly tractable few nV/sq rt Hz gets to be most annoying!

As well the ESS stuff is doubly consternating by its additionally schizoid and multiple personality disorder nature, as according to its makers, it is somehow more digital than analog. Rubbish of course, the continuation of the conflation of the symbol domain with the signal domain. See the writings of Bruno P. in this regard.
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Old 23rd June 2013, 11:02 AM   #40570
qusp is offline qusp  Australia
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Originally Posted by john curl View Post
Well qusp, I'm glad that you know everything that there is to know. I am just coming up to speed with digital reproduction. I have a very good idea about the OPPO, and I told the company that about 6 mo ago. I'm afraid that I didn't have much proof, just my ears.
And what IC would you chose?
For the ESS, if I was constrained to ICs, or at least a similar budget and space to what they used, I would use the SMD monolithic Dual ALD mosfet network parts, with 4 internal fets, use 2 of them for CCS for the other 2, or SMD monolithic dual bipolar and perhaps a current mirror in front of something like the lme49724 as part of a composite opamp (much cheaper than its OPA1632/THS brethren). use the large order to negotiate tighter matching criterion on the fets.

High transconductance is mandatory to get the most out of this part due to the issues being discussed here at the moment ie. its low output impedance. that being said, you can get pretty excellent results from it in 'voltage output' mode as well, just not -120dB THD+N.

Its internal speed and higher demands of clock speed mean that the layout wrt RFI and its impact on noise cannot be ignored. Band limit the input of a fairly wide bandwidth circuit, but not overly wide bandwidth. Mosfet inputs possibly being preferred for their better innate rejection of RFI, vs the innate lower noise and high gm of BJTs

Feed the AVCC regulator into a /2 divider and leverage that to get rid of DC. keep the return path to the DAC AVCC supply pins from the IV stage as unobstructed and low impedance as possible. keep decoupling as close and small as possible for lowest inductance. Pay close attention to the decoupling of the DVDD supplies, in order to better isolate them and their internal logic from the more critical analogue pins. use steep passive filtering on all regs for the analogue pins and clock, for lowest noise. Others may disagree here, but I feel lowest noise is more important than low impedance or ultra-wide bandwidth for reference and clock supplies, as long as local decoupling is up to snuff; certainly if you are budget and space constrained.

With higher ambition and budget all manner of other options exist, but if going for best measured specs and 'current mode' output, it pretty much rules out jfets for the input differential due to impedance. Another barrier is its relatively high current of nearly 32mA in mono mode.

i'm not even close to knowing all there is to know, yet another example of your exaggerating the situation in order to try and make your case stronger. You havent been speaking from a humble lack of experience about this before, more like mentioning your vast experience before talking DACs and ICs down with generic hand-waving statements like before. You use 'authoritative voice' with a wide brush, but somehow disregard decades of research and implementation by others far more knowledgeable than me; a babe in the woods. this research and implementation has been repeatedly pointed out to you and you must have been surrounded by it in your position and NOW you say you are only just getting into digital?

jan: yes its not a typical current output DAC, but 780Ω isnt that unmanageable and its not like the Ti 179X chips are ideal current sources, its when people parallel them for dual mono and fail to take into account that this lowers each mono output less than 100Ω. All this effort and expense undertaken while not getting any better performance than what you can get with a single one. These people seem not to notice that the chip is built from the ground up, with convenient pinout and internal fully isolated L/R channels, so as long as your layout and power supply is dual mono, you have a dual mono dac. of course without 2 or more chips its not fashionably dual mono

Last edited by qusp; 23rd June 2013 at 11:18 AM.
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