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Old 5th July 2010, 09:01 PM   #1
Ryssen is offline Ryssen  Sweden
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Default PCM1794-->Resistor I/V-->LL1571...

I ordered a PCM1794 chip and a 28pin adapter to use in this DAC Experience with this DIY DAC ?
and I will use a resistor as I/V converter,and after that LL1571.
I have googled the net for examples of the resistor I/V,and I found 2 different topologies.First this one that uses resistors from the output to ground.

Click the image to open in full size.

And this one that uses a resistor between the balanced output.

Passive output

Wich one to use?
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Old 6th July 2010, 01:42 AM   #2
Ryssen is offline Ryssen  Sweden
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And how do I calculate a LP filter and wich cutoff frekvency should it be?
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Old 6th July 2010, 09:25 AM   #3
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The two topologies are the same.
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Old 6th July 2010, 08:52 PM   #4
grael is offline grael  France
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The two topologies are about the same because on the passive output from the Rakk dac, they use "center tap" to ground, so the resistor across the secondary is reflected on the primary as "resistors to ground".

You want to use LL1571 as 1:3.5 to lower the resistor presented to the chip output?
Because you can't wire it like that with a center tap... Only when used as 1:1.75

In your position, i'd use resistor to ground on the primary, then LL1571 as 1:3.5
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Old 6th July 2010, 08:56 PM   #5
Ryssen is offline Ryssen  Sweden
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Yes,they both put a load on the DACīs output..
But as the LL1571 doesnīt have a center tap connected as 1:3,5 Iīll have to use the 4 resistors from the DACīs output to ground.
http://www.lundahl.se/pdfs/datash/1571.pdf

But how about the filter at output,or should it be on the secondary?
I donīt wanna set my F5 amp into oschillation...

Edit:grael posted before me.

Last edited by Ryssen; 6th July 2010 at 09:05 PM.
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Old 6th July 2010, 09:01 PM   #6
grael is offline grael  France
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I don't know anything about LPF on the output...

The rakk dac works without it, so does my pcm1798 with LL1636.
( New Small DIY Gigawork Dac? )

You could try without first.

edit : we are writing at the same time!
I'd say :
Code:
DAC + ----- R--------R----- DAC -
                 |                   
                 |
               Ground
Then Dac+ and Dac- on the primary

With four resistors R = 135 Ohms, you get the same behaviour as the Rakk Dac (load on the dac chip is a bit higher than Rakk dac because your step up is a bit lower)

Last edited by grael; 6th July 2010 at 09:23 PM.
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Old 6th July 2010, 09:59 PM   #7
grael is offline grael  France
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R=120 Ohm would do the trick, to keep with standard resistor values
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Old 6th July 2010, 10:29 PM   #8
Ryssen is offline Ryssen  Sweden
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Yes,I will try without it,but if anyone has any suggestions,they are welcome..
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Old 6th July 2010, 11:21 PM   #9
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Yes,at the same time!The above I wrote is about the filter.

I will try with 120 ohms load on the DAC.
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Old 7th July 2010, 10:35 AM   #10
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That's the ezdac schema you posted. Not a fan of that "design": why include upsampling (a mixed-result, controversial approach at best, IMO) but compromise on much more important I/V with a freakin' R-only I/V "topology"?
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