I/V after amplification using current amp?

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#22
Re: Help please Mr. Pass.

khundude said:
[snip]Why not use FETs instead? I may very well do just that but I would still be interested in if there are any benefits of current gain and then I/V. The reason I think this would be better is clear in my head and I need someone with know how to tell me not just yeah or no, but why? [snip]
Click to expand...


Is this diyaudio or PuzzleMania?
How can anybody say whether your reasons are good or bad if you don't tell us what they are?

Jan Didden
 
#23
Re: Help please Mr. Pass.

khundude said:
[snip]Why not use FETs instead? I may very well do just that but I would still be interested in if there are any benefits of current gain and then I/V. The reason I think this would be better is clear in my head and I need someone with know how to tell me not just yeah or no, but why? I am trying to learn! It makes sense to me that a DAC's current output that is very weak will convert passively to a very weak voltage signal that needs near 0 ohm impedance to work best. I understand that it is a balancing act between the amount of voltage vs. roll-off of bandwidth. The lower the voltage, the wider the bandwith, right? [snip]
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You're confusing voltage and gain. Often, higher gain goes with lower bandwidth (but this is a VERY general statement). It stands to reason that if you amplify (= more gain) the DAC current before I/V conversion you might lower the bandwidth.

Anyway, I'm still not clear what you actually want. You want more output voltage from the I/V? You want more bandwidth (but remember you need to filter the signal anyway to get rid of DAC switching artifacts so higher bandwidth is not necessarily better).
Or you just want a tube circuit and to heck with whether it is better or worse?

Jan Didden
 
#24
Something I forgot to mention earlier. Carefully review the DAC datasheets. Some of the newer devices actually warn against using passive I/V's.

Older R2R DACs had much smaller current outputs as compared to today's delta-sigma products. The new products product tons of current, sometimes as much as 4x or more than older designs.

-David
 
#25
amplify current?

Maybe i am getting this wrong but why would one want to amplify the output current from the dac? There is more than enough current out from all dac chips i know to get usually more voltage then needed. In my opinion an active I/V has far greater potential then just a resistor. What a well designed stage does is that it just isolates the dac output from the I/V resistor so that it does not see any voltage swing and it does that by simply "passing" the current on to the resistor. Nelsons I/V is a nice design even though i personally like bipolars better forn this specific task.

BR,
Anders
 
#26
The D1 I/V converter does not amplify the output current of the
DAC, it simply "cascodes" it with a Common Gate device without
feedback so that a relatively high output voltage can be
developed without the output of the DAC seeing that voltage.

It works like glue - the distortion and noise are extremely low,
the bandwidth is very high, and it is insensitive to digital noise.

😎
 
#27
Nuff said. Thanks to all, I just wanted to raise the question to see if there could be any benefit of something new. I will likely build a NP D1 I/V or just buy the Twisted Pear IVY which seems very similar in design, if I am correct.

So it would suffice to say that my little idea of running a current DAC to a current amp and then I/V at the headphone would yield no performance gains?
 
#29
I think he referring to the "super symmetry" arrangement that TI has licensed from Pass Labs. In this application there is no similarity as Mr. Pass clearly stated a few posts ago that the D1 stage uses no feedback, where the THS4131 used in the IVY does.
 
#30
Re: Re: Re: Re: current amp

analog_sa said:
Your case is slightly different 🙂

It is almost impossible to find a device less well suited for this task than tubes. Even in grounded grid connection a tube stage will not have a low enough input impedance.
Click to expand...
The secret is in impedance matching. A transformer can be used to match the input impedance of a common grid tube amplifier to the DAC, this way:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=100297
 
#34
I will be constructing the simplest incarnation of the D1 with JFETs. I have been reading for days and can find quite a few different version. However, if I am going to build the D1 JFET I/V stage I would find it much easier decide which version by asking the man. Mr. Pass, do you have a schematic and pretty pictures of your favorite D1 I/V? I have searched and read for quite some time and have found many schematics. Second, as I am not an EE, do PCBs sound better or are they just easier to build with? Finally, are there any PCBs available for the D1 JFET I/V(not entire DAC) stage or would others be interested in getting some made? I am good with my hands and soldering(I hope good enough) but I lack the experience in electronic design. If boards were to be made, would it make sense to do the I/V stage seperate from the balanced to SE conversion so that one could actually go balanced for less money for once and if SE is required, an additional board could be made for that? Or would it make sense to do it all on one board and just populate only what you need?

Nelson, have you played with or heard the ESS Sabre DAC yet?
 
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