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Old 21st February 2003, 03:27 PM   #11
Henrik is offline Henrik  Denmark
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Output can be made as simple as this:
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Old 21st February 2003, 04:52 PM   #12
Chris is offline Chris  Norway
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Default Output

Hi Henrik
The sm 5872 is what Thorstein call a "Timeslicing (or Delta Sigma) DAC" an futher he sais it; "require lowpass filtering and (if we insist on being boring and conservative) a balanced to SE converter in case of most high performance devices".
AND more:
"The two outputs (from the dac) however need to be combined into a single-ended output and then further lowpass filtering is needed as the signal is overlaid with large quantities of ultrasonic noise. This is actually much more problematic than the already problematic situation with Multibit DAC’s."

The picture you have attached is from the datasheet is the manufacturers (npc) suggestion for the needed lowpass filter And its also the same type of filter the marantz cd63 uses.
Burr Brown dacs like the PCM 1710,1716,1728 and 1732 has integrated this in the dac chip so that you can do anything about this.
Also in the datasheet you can find fig 19 output interface 1 wich is a balanced to SE config for this dac. It all explained at this website:

http://thunderstoneaudio.nav.to/
Under: Thermionic Valve Analogue Stages for Digital Audio.

So what i have realized is that you need this filter at the output of the dac and that you cannot remove it.
A friend of mine bought the zapfilter for his Marantz 63KI and to make it work he had to connect the zapfilter after the lowpass filter. He could not simply connect it to the dac ouputs like its intended.

I did not know this when i started this thread and I`m sorry for the confusion that may have caused. My inital idea was to make a simpler output with fewer components in the signal path.I realize now that it will be bit more difficult.

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Old 21st February 2003, 05:10 PM   #13
Chris is offline Chris  Norway
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Quote:
Originally posted by Henrik

My Onkyo sound much better when I take the output just after the first opamps.
Thats what I suspected too (Less parts, more sound !!??!). Do you think its possible reduce the output stage to just one opamp?
Using only what npc suggests for output?
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Old 22nd February 2003, 11:07 AM   #14
Henrik is offline Henrik  Denmark
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Hi Chris!
Quote:
Thats what I suspected too (Less parts, more sound !!??!). Do you think its possible reduce the output stage to just one opamp?
At the moment I use the red Left+ as SE output as shown in the attached scematic, and this is so so much better, hard to believe. I also tried the red Left+ and Left- as balanced output, but it turnd out awaful, no soundstage, everything bumped into the center. So my answer is yes to your question.
Quote:
Using only what npc suggests for output?
LO and LON.
At the moment I am building the analog stage as shown in the lower scematic, which is based on Nelson Pass supersymmetry or X. I don´t think I need the lowpassfilter at the input, since the input capacitance of the mosfets will do the job. I hope that I can run this thing balanced, if not I will remove the X feedback and let it convert the LO and LON into a singleended output.
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Old 22nd February 2003, 11:08 AM   #15
Henrik is offline Henrik  Denmark
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Forgot the picture.
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Old 22nd February 2003, 11:55 AM   #16
Henrik is offline Henrik  Denmark
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Chris
Thanks for the link to the "Thermionic Valve Analogue Stages for Digital Audio", It might be helpfull to me.
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Old 24th February 2003, 05:21 AM   #17
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Hi Henrik,
You must have gone to the link and seen
"Thermionic Valve Analogue Stages for Digital Audio".
The second circuit is probably what you are looking for.
The DAC will see a balanced load because of the 'balanced' filter. The tube will hardly load the filter and so the 'balance' will not be disturbed. In addition you get enough voltage to drive almost any amp.
I have used a tube and OPA2134 interchangeably after a passive filter on a DAC. The tube sounds much better , though both sound good.
If you were a tube purist you may want to implement the circuit without a solid state device. In any case let us know how your implementation worked out.
Cheers.
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Old 24th February 2003, 06:05 PM   #18
Henrik is offline Henrik  Denmark
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ashok
Yes I am goning with the second scematic.
I got the missing information on how to deal with this modification from Thorsten Loesch´s article "Thermionic Valve Analogue Stages for Digital Audio".
The good thing about his design, besides from the tube, is that he uses the inductors, wich only reduces the voltage level very little, so one only need a follower.
I have red the recommended filtering from the datasheed of the SM5864 (which is close to te one in my CD) without inductors, but the cost is you miss half the output voltage swing, but I will play a little with this idea, and see what happens, I have attached the test scematic.
And I will be back with some comments on the test.
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Old 26th February 2003, 02:17 AM   #19
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Hi Henrik,
Did you consider using small value inductors ( 1mH and less) in the passive filter? You will get steeper roll off than RC filters. The inductors are freely available in small values and look like fat 1/4 watt resistors. I have used some successfully.
Cheers.
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Old 26th February 2003, 07:08 AM   #20
Henrik is offline Henrik  Denmark
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Ashok
I will try some variations between the one posted just before and this one to hear the difference with and without the inductor:
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