WM8741 dual differential, passive, transformer output stage - diyAudio
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Old 9th December 2014, 10:51 PM   #1
wutzke is offline wutzke  Germany
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Default WM8741 dual differential, passive, transformer output stage

Hello,

I have two WM8741 DACs (in mono mode) in my player. I am planning to design a passive transformer output stage for these DACs.

The differential outputs of the DACs are biased at 2.5V. Possible transformer wiring options that come to my mind are the following:

Option 1:
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Option 2:
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Option 3:
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Option 4:
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I have a few questions regarding this approach. Any feedback would be greatly appreciated.

Can you please recommend an option? Are the other options?
What would be a suitable transformer?

Do I need resistors in series with the coils?
Should I add high or low pass filters?

Thanks a lot!
Wutzke
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Old 10th December 2014, 11:16 AM   #2
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This is a pair of suitable and very good transformers: Jensen JT-11-YMPC DAC output transformers pair

I used some LL1690 as you did in Option 2, but my in-line resitors are 1k and the tranformer's ground pin tied to OUT - and no resistor across Out + and Out -.
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Old 10th December 2014, 12:24 PM   #3
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I used the Jensen JT-11-DMPC with amazing results. Series resistors are not needed and discouraged. The specifications in the data sheet apply to AC impedance, of which the transformer is huge. As long as you keep your connected load impedance within spec you will have no issues. There is no DC coming out of the differential DAC, so you don't need to meet a DC resistance minimum load spec.

Your connection options really depend on the gain structure of your system, not so much which one would sound better than the other. For a decent transformer they will (barring that third diagram) be the same but with potentially different output voltages.

Regarding transformers try and make sure you are purchasing an output transformer, not an input transformer or something else not specifically designed for output service. Jensen's website has some nice articles on the differences between input and output transformers.
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Old 11th December 2014, 07:32 PM   #4
wutzke is offline wutzke  Germany
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Quote:
I used some LL1690 as you did in Option 2, ...
@Groundloops, is your primary center tap connected to GND?
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Old 11th December 2014, 07:48 PM   #5
wutzke is offline wutzke  Germany
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Quote:
I used the Jensen JT-11-DMPC with amazing results. ...
@zigzagflux, did you used the JT-11-DMPC with a WM8741 DAC? If yes, could you please shortly describe your wiring.
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Old 12th December 2014, 04:05 AM   #6
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No, I use the AK4393, which is very similar to the WM8741. They are both 24 bit Delta Sigma and use differential voltage outputs. The WM8741 has a higher maximum sampling rate.

Wiring is easy as can be- tie the two DAC voltage output pins to pins 2 and 5 of the Jensen. Pins 8 and 11 are the transformer output, which you can use either balanced or unbalanced.

Distortion is nearly identical to that published by AKM, indicating the transformer added almost nothing to the signal. I am sure my sound card setup was not perfect, either. The 20Hz FFT in particular I am pleased with. Not easy to do with a transformer.
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File Type: jpg 20Hz 0dB.jpg (162.4 KB, 66 views)
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Old 12th December 2014, 06:27 AM   #7
Zoran is offline Zoran  Serbia
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dont connect CT tap of primary at the Ground potential. You vill have DC from 2.5V to 0v across Rdc of windings. SAnd may damage the dac chip.
Connect to Vmid pin (there is the same potential as one at the analog outputs...
.
I am not sure that chip has driving capabilities to meet the transformer reactive impedance values. So it is not bad idea to use some low impedance output buffer.
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Old 12th December 2014, 07:17 AM   #8
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Quote:
@Groundloops, is your primary center tap connected to GND?
No. Zoran gives the explanation why. It's a frequent misconception to connect the center tap to the DAC's GND.
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Old 13th December 2014, 07:41 AM   #9
wutzke is offline wutzke  Germany
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@All, what do think about option 1?
The DAC has two differential outputs, the LL1527 (for example) has two primary coils. I could connect OUT1+/OUT1- to the first coil and OUT2+/OUT2- to the second coil.
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Old 13th December 2014, 08:39 AM   #10
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Seems like a good option. Since ALL DAC outputs are at 2.5V there is no DC current through the coils. No need to put anything on the center taps - in fact grounding the center tap would be a recipe for disaster. Putting 2.5V at the center tap is pretty useless - it would do nothing good but requires a 2.5V source exactly the same as the DAC DC output, and also for some pretty critical decoupling.
Option 1 is the most sensible.

Jan
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