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Old 16th August 2014, 05:28 AM   #991
Corpius is offline Corpius  Netherlands
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Default problem solved

I was able to pin down the problem I was having with my DAC. I must have been plagued by ground loops. The problem always occurred when I connected the DAC to a earthed wall socket. There is no problem when the wall socket is not earthed. The computer I use for playing media is also earthed so that's probably how the ground loop could be completed.

I solved the problem by replacing the metal spacers by plastic spacers. That way the circuit ground becomes fully isolated from the chassis and the chassis can stay earthed. For safety reasons obviously.

I still need to take a look at why the ground at the spdif connector connects to the circuit ground... despite of the pulse transformer.
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Old 17th August 2014, 02:01 PM   #992
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I found your problem, and it seems it would be the case for every kit. Good finding by the way! I measured my own DAC and I found the same problem as you, the coax input connector gnd is indeed connected to the Dac ground defeating the purpose of an insulation digital transformer at the input.

The cause is simple and it is an error in the PCB design. Look at the included picture. In this type of RCA connector, the two large connector mounting studs (circled in black on the image) are connected to the rca gnd. The correct PCB would have these two studs insulated from the PCB gnd.

To correct this is a little tricky for the some of you guys. The pcb been to faces, these two pins are grounded on both top and bottm sides, meaning that we cannot just cut the pcb around the stud on the bottom face to cure the problem. First you need to unsoldered the connector (that is the tricky part) without damaging the board, cut the pcb ground around each mounting stud, double check with an ohmeter the insulation, the re-solder the connector. That will correct the problem.

WARNING: If you're not sure of your soldering skills and/or don't have the proper tools, you can damage the PCB...

I'm doing it right now.
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Old 17th August 2014, 02:18 PM   #993
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False alarm, I removed the connector, and the two studs came out of the connector casing. I can see that inside the plastic the studs are not touching, so they are insulated. Looking closely at the under of the PCB, there is the answer. A piece of wire was added between two pads, one being the PCB gnd. This jumper ground the coax ground. We need to remove this jumper. Confirming right now...
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Old 17th August 2014, 02:28 PM   #994
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I can confirm that removing the jumper cure the problem. Simple as that. The input transformer is correctly installed on the PCB, and it doesn't seem to be any error of the pcb. I don't know why this jumper was installed

In some coax input implementation there is a small cap (47pf here) between the primary ground side of the connector, see schematic. This is something we may try as well. Just install the cap in place of the jumper.
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Last edited by Algar_emi; 17th August 2014 at 02:39 PM.
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Old 17th August 2014, 03:14 PM   #995
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I haven't found the time yet to look at why the connector ground connects to the DAC ground, so you beat me to it! Good work!
I also noticed the jumper wire underneath the PCB in this post from quanghao. My first impression was that it was needed to overcome some difficulty in PCB design, but perhaps it was there for testing purposes and quanghao forgot to remove the jumper before shipping the DACs. Who knows?

Can you confirm that the coax input still works with the jumper removed?

Now about the coax input implementation. There already seems to be some circuit in use at the connector ground side that implements a cap and some resistors. I not at home right now, so I can't check the exact circuit.
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Old 17th August 2014, 04:09 PM   #996
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Yes it is working right now. Check the input schematic included, all the circuitry is on the secondary side, not the primary. This Wima red cap is the 10uF input cap on the schematic. I reversed engineer all the dac schematics by the way
Very nice design all around.
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Last edited by Algar_emi; 17th August 2014 at 04:25 PM.
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Old 17th August 2014, 06:35 PM   #997
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Quote:
Originally Posted by Algar_emi View Post
Yes it is working right now. Check the input schematic included, all the circuitry is on the secondary side, not the primary. This Wima red cap is the 10uF input cap on the schematic. I reversed engineer all the dac schematics by the way
Very nice design all around.
Right, so it's not on the primary side.
Wow you've reverse engineered all the schematics! You sure have put a lot of effort into it! Are you willing to share those schematics?
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Old 17th August 2014, 06:49 PM   #998
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Not really, first they are not mine to disclosed, Quanghao has the intellectual rights, second there is no warranty my schematics are without errors, there are grey zones for some smd parts for example, etc. I don't want to hear about what I may I figure out wrong, or argue on my own choice of parts and/or values. You have the PCB to look at it. It is all in plain site. You'll have a chance to appreciate the PCB routing, supplies decoupling and grounding. When you know what to look for you can appreciate the design quality. The next best thing would be to have a 4-6 multilayers PCB, but the cost of it is probably prohibitive for the DIY market. Not mentioning that to work on such PCB is a nightmare if you want to repair, change/upgrade it.

Just to let you know that with the already disclosed information (look at Dac version 1 and the AD1864 DAC threads as well), and Quanghao block diagrams you should have a petty good idea of what is going on. For example the complete digital input section is about the same as the one already shown for version 1. There is just a few better/different LC supplies filtering added, and parts re-numbering, the basic concept is the same. Same thing for the clock buffer and Amanero insulator. Look also at part datasheets, it was very informative.

It took me just a few hours to get the complete DAC circuits, and about one day for the different IV pcb sections. Feel free to do the same. It was fun

Last edited by Algar_emi; 17th August 2014 at 06:58 PM.
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Old 17th August 2014, 08:09 PM   #999
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perhaps I'll do that one day, but I'm too busy with other things at the moment. I guess that it's all a matter of priorities I do fully understand why you are not willing to share. No hard feelings from my side.

the design quality is indeed outstanding. I enjoyed this dac since the moment it arrived at my home. Nevertheless some problems need to be solved to make it even better. That's DIY!
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Old 17th August 2014, 09:37 PM   #1000
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Indeed. Main reason I did it was to be able to support it in the future and how I can adapt some of it in my own projects.
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