High Performance WM8741 Upsampling DAC New Version build thread (show 'n tell too) - Page 10 - diyAudio
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Old 20th February 2012, 12:25 PM   #91
TheGimp is offline TheGimp  United States
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Quote:
Originally Posted by randytsuch View Post
I looked at the datasheet, and agree.

The part in the link in my excel file is $6.80, unless I am missing something?

This is different from the links I posted a few weeks ago, I saw your post about it, and checked the datasheet, and changed the part used.

Randy
OK,

I didn't see a reply and thought my original post had been missed (or I missed something in the data sheet).

Steven
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Old 21st February 2012, 03:05 PM   #92
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Default Shared mouser project

Here's Randy's BOM as a shared project on mouser (without the headphone amp section.)

http://www.mouser.com:80/ProjectMana...sID=89c84f9dce

access id: 89c84f9dce

You should be able to log into mouser and import it into your own account. (it's missing component references)
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Old 21st February 2012, 03:22 PM   #93
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Thanks for that

I think I made a couple mistakes.

I'll try to check it soon.

BTW, I decided to split my order, some stuff from Digikey, and most of it from Mouser.

My reason is that Digikey sells Elna Silmic II's. You need to search for "Elan RFS". Here is the catalog page
Silmic II

I am going to end up ordering the ferrites, a few IC's, and the Vregs from DK, to hit their $25 min.

While I'm sharing, you can get Oscons from here
oscon

They also carry poscap's, which are supposed to be good.

Randy

Quote:
Originally Posted by merlin2069er View Post
Here's Randy's BOM as a shared project on mouser (without the headphone amp section.)

http://www.mouser.com:80/ProjectMana...sID=89c84f9dce

access id: 89c84f9dce

You should be able to log into mouser and import it into your own account. (it's missing component references)
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Old 22nd February 2012, 02:20 PM   #94
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I've added parts reference to the mouser bom (without the headphone amp section.) - that should help identify what goes where.

Which headphone amp parts are removed from the dac? I'd like to make sure the bom is complete before I order.
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Old 22nd February 2012, 07:38 PM   #95
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Just wondering if you bypass the headphone section (and opamps), and use the vout +/- from the wolfson dac what can we stick on the output?

I've got a dcb1 buffer I'd like to use on the output of this dac - but there is no gain with the dcb1.
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Old 22nd February 2012, 08:26 PM   #96
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OPA2365 THD is specified at 600 Ohm load, however output voltage swing is specified at 10K Ohm load.

I'd be inclined to stick to a 10K (or greater) load.

WM8741 spec is 2Vrms out, so I would expect the buffer output to be limited to slightly less than that, depending on the value of the analog supply.

If you mean drive directly from the VOUTRP/VOUTTRN and VOUTLP/VOUTLN pins, from the WM8741, that is ill-advised. If you do, use diff-input transformers for coupling.

Last edited by TheGimp; 22nd February 2012 at 08:38 PM.
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Old 22nd February 2012, 08:52 PM   #97
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So I'm planning to use a transformer on the dac output, for a bal to se conversion, so I don't need the output stage.

I'm not sure if I have created confusion by creating a BOM without the output stage? You could decide to include the opamp, and related parts, so you don't have to worry about it. Or, just build the entire board, with all parts.

So, I am have just confused people?

FYI, I am planning to build a "customized" version. I only plan to use the USB input, and not planning to use the output stage. I am not going to install the parts I don't need, based on this.

Randy
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Old 23rd February 2012, 12:13 AM   #98
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I'm planning a customized version too.

So, how do isolate the dac from the output stage?
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Old 23rd February 2012, 02:40 PM   #99
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I think the problem with driving the transformer directly from the DAC is that the DC resistance of the transformer is too low. The dc output of the analog signals is going to be one half AVDD. You would need to capacitivly couple the output(s) to the transformer to prevent excessive dc loading. One would need large (47uF min, probably 100uF) non-polarized caps for this purpose.

You need a buffer/level shifter before the transformer, and the OPA2365 with the capacitor coupling caps performs these functions. Level shifting is accomplished via the output cap C41. But now you have a single ended signal.

One possibility might be to bypass the OPA2365, and use the headphone amp position as unity gain amps to drive the transformer. It would require finding a better op-amp that was pin compatible, adding a V- supply to give more voltage swing, and capacitivly coupling between the D/A and the buffers.

I'm not sure how feasible this really is without compromising performance and s/n ratio.

Another option might be a longtail pair made of FETs (2SK170s?) with a current source in the tail, driving the transformer. (I was looking at doing this but with vacuum tubes instead of FETs.

Last edited by TheGimp; 23rd February 2012 at 02:44 PM.
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Old 9th March 2012, 06:38 AM   #100
regal is offline regal  United States
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Quote:
Originally Posted by TheGimp View Post
I think the problem with driving the transformer directly from the DAC is that the DC resistance of the transformer is too low. The dc output of the analog signals is going to be one half AVDD. You would need to capacitivly couple the output(s) to the transformer to prevent excessive dc loading. One would need large (47uF min, probably 100uF) non-polarized caps for this purpose.
.

If you use the differential signals from the WM8741 , you would have one on each end of the of the two primaries in series . So the net DC across the primary would be zero, I don't see the need for a capacitor unless the primaries are poorly matched. What am I missing?
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