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Noise in AK4490 DAC - please help?
Noise in AK4490 DAC - please help?
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Old 10th August 2018, 09:07 AM   #1
GTrenchev is offline GTrenchev  Bulgaria
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Default Noise in AK4490 DAC - please help?

Hi all,

I haven't done diy audio in a while. I wanted a nice project, so I picked up an AK4490 board from DIYINHK (the red one). I have a headphone amp that I built before, which is LM4562 driving BUF634. I built a power supply with LM317, LM337 and LT1086, which outputs +/- 12V, +5V and +3.3V. This is the power supply:

Click the image to open in full size.
IMG_20180804_104110 — Postimage.org

The USB interface is also DIYINHK, this one:
Click the image to open in full size.
xmos-dsd-dxd-384khz-high-quality-usb-to-i2s-pcb — Postimage.org

And this is the DAC board:

Click the image to open in full size.
https://postimg.cc/image/qsixzshlz

Everything goes together in this case (the XMOS is under the DAC):

Click the image to open in full size.
https://postimg.cc/image/9s01r6wvr

The headphone amp is installed, and the DAC board is detached for debugging:

Click the image to open in full size.
https://postimg.cc/image/perb46sjr

The thing works, however, I have noise at the analog outputs of the DAC. The noise resembles typical ground loop noise, with about 30mV pk-pk amplitude. It can be heard clearly with headphones. It looks like this:

Click the image to open in full size.
https://postimg.cc/image/9t9zkfqxz

Noise would come from the DAC board, no matter if it is powered or not. I tried to unplug digital power only, then analog power, then opamp (+/-12) power, nothing helps. It is as if the analog outputs are floating and just picking up noise.

I've tried a number of things.

1. The headphone amp itself is dead silent. I have detached it from the DAC and it has no noise. But I've used it before, and I know there is no problem with it.

2. I measured all the outputs of the power supply, with and without load - the noise is below the oscilloscope sensitivity, i.e. below 2mV. Checked up to 100MHz - no oscillations or noise.

3. I have tried multiple grounding schemes, whereas nothing works - star grounding from all ground points of the DAC, one wire to one point, thick wires, short wires, RC filters on the power supply lines. Nothing works.

4. Grounding the XMOS to the DAC directly. Grounding it to the star point. No change.

5. I've tried using a different XMOS. The old version of the one above, which is USB powered and has a separate ground. Does not help.

6. Grounding the case. Not grounding the case.

7. Putting everything out of the case does not help either.

I know this is a too complex problem to give solutions right away, but at this point any suggestion is welcome. I really want to make it work right, as it sound really nice
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Perboards are fun
XMOS->PCM1794->LME49720->BUF634(x2)->KRK KNS08400

Last edited by GTrenchev; 10th August 2018 at 09:11 AM.
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Old 10th August 2018, 09:53 AM   #2
wealas is offline wealas
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You do not seem to have connected the ground pins on the DAC and XMOS board connectors?
For each data line you should make sure the closest ground pin is also connected between the two boards.
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Old 10th August 2018, 10:06 AM   #3
GTrenchev is offline GTrenchev  Bulgaria
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Join Date: Feb 2011
Thanks for the reply! The photos are just illustrative; I've disconnected everything now, as I'm hunting for the noise. The XMOS has nothing to do with it actually, the noise is there even with the XMOS disconnected and out of the case. I kept it in so that it draws power from +3.3V, i.e. keep the power supply loaded as it should be. Yes, I did connect the grounding pins on the DAC and the XMOS. I think I've tried every possible combination by now, the only thing I can think of now is a faulty DAC board. Maybe I'll try to resolder all the pins...

In addition, I tried a number of opamps also - LM4562, NE5532, OPA2134, etc. No effect.
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XMOS->PCM1794->LME49720->BUF634(x2)->KRK KNS08400
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