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EduDAC - single ended out, Amanero in
EduDAC - single ended out, Amanero in
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Old 14th July 2017, 06:58 PM   #1
SomekPoland is offline SomekPoland  Poland
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Default EduDAC - single ended out, Amanero in

Hi

I decided to design a DAC (my first) to use it with W. Colburnes headamp. I've made some starting choices.

-DAC chip: WM8741 - great DAC, a bit forgotten now

-LT1763 power regs, CLC at DACs power inputs, preregulation on capacitance multipliers.

-Amanero input (with isolator?)

Disrette output stage, mainly for buffering and filtering. (LTP, one gainstage and buffer, FETs runned hot).

What do you think? I wanted to get any opinions before drawing a schematics
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Old 16th July 2017, 12:27 AM   #2
SomekPoland is offline SomekPoland  Poland
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Default First steps

Okay, so I did a few things.

WM8741 works in HW mode. Filters are switchable, no deemphasis, nor attenuation. Configured for 176.4/192kHz inputs.

LT1763s in SOIC8 for power supply (yes I know bypass cap should be 10nF). No inductors yet, they will be there, no worries.

Zero Flag tied to the GND (I don't see any use for it).

If somebody could check the HW settings
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Old 16th July 2017, 01:00 AM   #3
abraxalito is online now abraxalito  United Kingdom
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As you don't really need any gain from the discrete OPS, why the LTP? Why not just a source follower with filter components around it? Or is the LTP for doing differential-SE conversion?
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Old 16th July 2017, 12:50 PM   #4
SomekPoland is offline SomekPoland  Poland
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Exactly the second reason. I don't want to leave anything hanging. For now I'm keen to adapt Pass discrette opamp (LSK389 LTP, ZVP33XX gainstage and BF862 follower, all with CCS).
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Old 16th July 2017, 05:03 PM   #5
SomekPoland is offline SomekPoland  Poland
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Default First layout

So I did some routing for digital part. Thats what I camed with.
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Old 26th July 2017, 10:17 PM   #6
SomekPoland is offline SomekPoland  Poland
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For now I decided to stick to recommended analog stage from datasheet.
Here is a early idea of placement. I want to use quad opamp, but it looks kinda messy, to be honest. What are your ideas?
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File Type: png wm8741 SCHEMATIC v2.png (36.6 KB, 102 views)
File Type: png WM8741 plus analog.png (156.3 KB, 101 views)
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Old 27th July 2017, 02:10 AM   #7
abraxalito is online now abraxalito  United Kingdom
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The resistor values in the filter are extremely low in order to get lowest noise (see note 1 on p63 of the datasheet). However you're using FET opamps which contribute significantly more noise than these low resistor values.

I suggest giving up the aim to get the very lowest noise as recording noise is going to dominate over DAC noise in the majority of cases. After all the vast bulk of digital recordings are still 16/44.1.
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Old 27th July 2017, 11:20 AM   #8
SomekPoland is offline SomekPoland  Poland
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Those are only stock models. I'll use other, low noise opamps, probably from TI.

"I suggest giving up the aim to get the very lowest noise as recording noise is going to dominate over DAC noise in the majority of cases. After all the vast bulk of digital recordings are still 16/44.1."

So what do you propose Abraxalito? Maybe go for normal, low noise 0805 resistors, instead of MELFs?
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Old 27th July 2017, 12:17 PM   #9
abraxalito is online now abraxalito  United Kingdom
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I upgraded a WM874x DAC with a 100X rise in the impedance of the filter networks following the chip itself. This gave a very large improvement in bass dynamics (the treble is still less than squeaky clean though). So I suggest scale up the given resistor values by a factor of at least 20, up to 100. Scale down the caps by the same factor. I haven't done a comparison of MELFs vs thick film so have no suggestions on that front.
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