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Old 29th December 2015, 01:59 PM   #1
xslavic is offline xslavic  Moldova
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Default Dual mono pcm1794

Hello,i bought this dac as a kit,added my own transformer ,installed everything and it sounds really well.
I needed XLR output because my diy 4x LM3886 BTL has noise through RCA line,it uses opamp for conversion to balanced.So added to the amp XLR inputs and XLR outputs for the DAC using Pure silver wire in Teflon and 15ohm metal film output resistor.
My question is? any better method?
why shouldnt i do it this way?
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Last edited by xslavic; 29th December 2015 at 02:14 PM.
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Old 4th January 2016, 12:05 AM   #2
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Quote:
Originally Posted by xslavic View Post
Hello,i bought this dac as a kit,added my own transformer ,installed everything and it sounds really well.
I needed XLR output because my diy 4x LM3886 BTL has noise through RCA line,it uses opamp for conversion to balanced.So added to the amp XLR inputs and XLR outputs for the DAC using Pure silver wire in Teflon and 15ohm metal film output resistor.
My question is? any better method?
why shouldnt i do it this way?
The balanced input wiring within your amplifer is not completely clear to me. Is the opamp you mention within the amp and configured to perform differential to single-ended conversion? Or, are you saying the opamp os within the DAC performs single-ended to balanced conversion on the DAC's output? A schematic or wiring diagram would help us to usefully answer your question.
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Last edited by Ken Newton; 4th January 2016 at 12:08 AM.
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Old 4th January 2016, 11:31 AM   #3
xslavic is offline xslavic  Moldova
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The stage that causes problems Is in the Amplifier.It uses a stereo Opamp ( ad797br) to convert to differential input because it uses 2x2 lm3886 so each lm3886 pair amplifies positive and negative side.Having balanced output to speakers(no ground involved).Lowest noise is ad797br regarding current noise, but it's still very loud.So that's why is made xlr output from dac,because it has it being a current output dac with lme49722 as converter transimpedance,and muses for single ended conversion.Wich does not cause problem,because close to source.Is that topology correct? 15ohm for cable capacitance compensation( output resistor) and at Amplifier(lm3886x2) Lpf wich blocks DC.that's for Xlr output.And I just remove opamp that does conversion from the amp because it cuts the noise.
because on RCA input noise can't be canceled,tried all the tricks,it's the opamp itself that does this.
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Last edited by xslavic; 4th January 2016 at 11:38 AM.
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Old 4th January 2016, 11:28 PM   #4
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Quote:
Originally Posted by xslavic View Post
The stage that causes problems Is in the Amplifier.It uses a stereo Opamp ( ad797br) to convert to differential input because it uses 2x2 lm3886 so each lm3886 pair amplifies positive and negative side.Having balanced output to speakers(no ground involved).Lowest noise is ad797br regarding current noise, but it's still very loud.So that's why is made xlr output from dac,because it has it being a current output dac with lme49722 as converter transimpedance,and muses for single ended conversion.Wich does not cause problem,because close to source.Is that topology correct? 15ohm for cable capacitance compensation( output resistor) and at Amplifier(lm3886x2) Lpf wich blocks DC.that's for Xlr output.And I just remove opamp that does conversion from the amp because it cuts the noise.
because on RCA input noise can't be canceled,tried all the tricks,it's the opamp itself that does this.
Your overall system configuration is still not totally clear to me, partly due to english not being your first language, I suspect. Please post a wiring diagram or schematic - from DAC output circuit through to Amp input circuit. A picture is worth a thousand words.
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Old 9th January 2016, 11:48 AM   #5
xslavic is offline xslavic  Moldova
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Default pcm1794 dual dac iv xlr

Hi,this is output stage of dac.using lme49720HA metal body.
The second image is the converter on the amplifier board,with Vref being the ground and gain is 1.But it is very noisy,tryed without resistors as voltage follower but nothing helped so i decided its time to remove it.The solution would have been a different aproach for the converter ,but dont have enough space on the board.
So i decided to avoid this stage by using XLR outputs.
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File Type: gif Current_Std_IV_Opamp.gif (9.6 KB, 119 views)
File Type: gif single_ended_to_diff_los713.gif (20.2 KB, 116 views)

Last edited by xslavic; 9th January 2016 at 12:01 PM.
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Old 12th January 2016, 05:01 PM   #6
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The following may seem a silly question, but is VREF at the amplifier side directly connected to the amplifier local ground, or remotely only to the DAC ground via the interconnect cable?
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Old 12th January 2016, 05:27 PM   #7
xslavic is offline xslavic  Moldova
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To amplifier ground and also to DAC ground via RCA cable.
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Old 12th January 2016, 07:08 PM   #8
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Originally Posted by xslavic View Post
To amplifier ground and also to DAC ground via RCA cable.
Alright, it seems that your system may then benefit from implementation of a balanced (impedance) signal interface.

Likely, the least troublesome and costly way to establish such an interface on the DAC side would be to connect the DAC's unbalanced single-ended op-amp outputs to XLR connectors via their existing 10 ohm isolation resistors, and then also connect the opposite phase XLR pins to the DAC ground via 10 ohm isolation resistors. This will create a single-ended yet balanced impedance source.

The amplifier side would then require the addition of XLR connectors feeding either a transformer-coupled balanced to unbalanced conversion, or, at a much lower cost, modify the exisiting unbalanced op-amp input recievers to true balanced impedance differential amplifier input receivers.
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Last edited by Ken Newton; 12th January 2016 at 07:18 PM.
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