4th April 2010, 11:06 PM  #11 
diyAudio Member
Join Date: Apr 2003
Location: Katowice

hi,
Connecting LL1674 trafo directly to output of DAC puts it automatically to curent mode because od low DCR of primary windings. In the model DAC is treated as an ideal current source which has infinite output impedance. Real DAC isn't ideal current source and has finite impedance and this value should be mentioned in datasheet. For example PCM1704 has 1kOhm. LL1674 should be fine with 1:4 because the higher turns ratio, the lower impedance is seen by DAC  lower THD. If you got too high output level  great!  decrease load resistance on secondary  you let the DAC to see even lower impedance. If you use trafo with 1:4 ratio you do not have to use any active buffers. BTW What resistor you use on secondary? 
5th April 2010, 08:51 AM  #12 
diyAudio Member

AN_912.pdf, right hand paragraph, top of the page: "Equation 4 through Equation 7 can be used to predict the impedance seen by each DAC output (ZNORM and ZCOMP)".
Znorm=Zcomp=R0*RL/(2RL+4R0*N^2). If no R0 is used it should be comparable to a resistor with infinite resistance? RL<<R0 > Znorm = RL/4N^2 (Very approximate) Given the LL1674, N=4 > Znorm = RL/64 Vs= (sqrt(2)*Imax/2)*(N*R0*RL/(RL+2R0*N^2)) Approximate R0 with infinity and we get Vs=(sqrt(2)*Imax/2)*(RL/2N) We want Vs=2 and Imax is 4mA I think? RL=5657ohm Plug this in and Znorm=88ohm seen by the dac. Do we just add the DC resistance to this, making Znorm=121ohm? I'm waiting for the guys over at TPA to start taking orders for the BII. Now, either I've done the equations wrong or else it doesn't seam like such a perfect match any more? In the thread discussing the BII it's said the load seen by the dac is very important. It needs to be very low if the advantage of current out mode is to be gained. 121ohm isn't very low in this regard. Last edited by markusA; 5th April 2010 at 08:55 AM. 
5th April 2010, 09:23 AM  #13 
diyAudio Member

I made a ****, Imax should be 16mA.
I was just thinking of my last post. If you don't have R0 the equations behave somewhat odd. The LL1674 has a 33ohm dc resistance on the primaries. 33//R0 = 33 if the R0 appoaching infinity approximation is used. This way we would get very different figures. For the models purposes we would have R0=33 in this case. Vs approaches 1.6V when RL becomes very large. Not good but manageble I guess? However, Vs drops when RL decreases... So if these equations is to be of any use I need to know what to use as R0. And what to to with the real life values? R dacoutput=195ohm & R dcll1674=33ohm. Last edited by markusA; 5th April 2010 at 09:45 AM. 
5th April 2010, 11:36 AM  #14 
diyAudio Member
Join Date: Apr 2003
Location: Katowice

markusG,
I think you should do it this way: take your ll1674, put on secondary some fixed resistor (eg. 2,2k) then calculate R0 with equation (9) for Rl=2,2k it will be 68ohm and use this value as termination resistor. In this case impedance seen by DAC will be 55ohm (including DCRs of pri and sec ofcourse) and output level about 2V. It is quite managable. You must strike a happy medium between acceptable output level end lowest possible impedance seen by DAC. I don't know how we should treat real output impedance od DAC and situation when R0 is infinite.. 
5th April 2010, 12:02 PM  #15 
diyAudio Member

I don't know if I'll even be needing a trafo?
I would like to understand the math though so I can make an educated decision. Although I don't quite understand how you got 55ohm? I'll accept it at face value and make the following comment. Russ claims (see link in one of myprevious posts) that THD will be affected by such a large value. The 12dB THD difference between current vs voltage output will be much less using the calculated values. The question becomes whether one should really care if the dac is working in current mode or voltage mode? (Since we cant get the Znorm below 10ohm) If the dfferences are small enough it becomes more important get a good impedance match to the amp and the right gain/voltage level. 
5th April 2010, 12:15 PM  #16 
diyAudio Member

If I were going to use a transformer with the BUF II it would be a 1:1 600:600 type. Because your *always* using the DAC as a voltage source when you use a transformer. This is because the DAC is truly a voltage source with an output impedance of 195R. 195R is not a lot of impedance. So it is nothing close to an ideal current source. To compensate you need a load with *extremely* low impedance to make the "nonidealness" have as little impact as possible . Much lower than you could achieve with any transformer I know of. I have said before even a 10 ohm load modulates the outputs enough to cause 1012db THD gain.
You also have to look at the transformer as a reactive component and your not likely going to get the impedance down low enough where THD will be stellar across the audio band. Good, yes. But, not as good as possible. I don't mean to make that sound too negative. It is not meant so. It just to set expectations. Also the distortion (not just harmonic) of the transformer itself will be much worse than 108db. So I would not even let that number come into play. So if I were using a transformer I would not think of the DAC as a current source. As for it to be an effective current source requires an input impedance at AC and DC which is simply too low for any transformer (any many active stages) to achieve. Now it is perfectly acceptable to add resistors to GND at the outputs of the DAC which then really just become voltage dividers. This is a very effective way to manage the output swing. Cheers! Russ
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Less pulp more juice Twisted Pear Audio. Last edited by Russ White; 5th April 2010 at 12:30 PM. 
5th April 2010, 12:26 PM  #17 
diyAudio Member

Thanks for chiming in Russ, you kind of make my point.
Current vs Voltage mode becomes a moot point and when you don't have to focus on superlow load impedance it's possible to focus on other aspects of the build. 
5th April 2010, 01:01 PM  #18  
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Join Date: Apr 2003
Location: Katowice

Quote:
marcusG, about 55ohm. to the Rl value you should add DCR of secondary winding (605ohm) so with Rl=2,2k you will get 2,8k (> R0=87ohm), then it is reflected to primary and with equation (10) you get Znorm=22ohm, but there is DCR of primary winding 33ohm so 33+22=55ohm 

5th April 2010, 04:17 PM  #19  
diyAudio Member

Quote:
Your missing the whole point. It is both a voltage and a current source. it does not switch to either it is always both. Its just that for it to appear to be an ideal current source (infinitely high relative output impedance) the load it is driving needs to be close to zero ohms. For it to appear to be any ideal voltage source(infinitely low relatively output output impedance.) the load need to be close to infinite impedance. So you see using it as a voltage or a current source if purely dependent on the relative impedance ratio between the source and the load. At 195R 55R is not a very low impedance at all. The ratio is what is important and it needs to be orders of magnitude higher. I think I have explained this as well as I can. Cheers! Russ
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6th April 2010, 07:30 AM  #20 
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Join Date: Apr 2003
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It makes more sense to me now. I thought that there are 2 separate modes switched automagically  two diffrent modes like in other DACs like AD1865, but it usually it has separated pins.
Now it we want to use BufII in voltage mode, imput impedance of transformer should be many times higher than 195ohm so maybe trafo 10k:10k will be better choice than 600:600? One thing I am thinking about is the lowest THD do not guarantee best sound. I have DAC based on AD1853 (current balanced output) orginally designed to operate with opamp output stage (one of the best OPA627BP and AD727) so DAC sees near zero impedance but sound was worse then this when using transformers. So what is the point? NFB > IMD distorsion? 
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