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Old 30th July 2012, 05:20 PM   #11
wicked1 is offline wicked1  United States
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Ok, thanks.
I do have it built on breadboard for now, all the components stuck in w/ untrimmed legs, and while I called my grounding scheme "star", it was messy. I was assuming the unbalanced load was the issue, but it was just a guess. Since everyone is thinking it's ground related I'll rebuild it neatly.
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Old 30th July 2012, 05:32 PM   #12
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by DF96 View Post
The second image (fig 3.2-4) shows you how not to do it. It is not a clean power supply - it uses the classic dirty CT noise injection which I have been telling the OP to avoid.

The first image shows why it is wrong. You ground a PSU at the clean/quiet end, not the dirty/noisy end or somewhere between.
This is the author's explanation why 3.2-4 is the right choice: "In figure 3.2-4, the internal ground buss is collapsed into a point, forming a local star ground. Bringing everything to a point forces us to make a connection to that point – no more multiple-point connection over which a noise voltage could form. Before, we had two connections: a high noise one connected to safety ground and a low noise one connected to signal reference. Note that now the high noise point is directly connected to the power common where it can be connected to safety ground to drain the AC leakage current, and the low noise point is directly connected to the power common where it can be connected to signal reference. "

Edit: I do agree with the dual rectifier approach: this forces you to connect the low noise side of the PSU to the star ground.

Last edited by jitter; 30th July 2012 at 05:36 PM.
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Old 30th July 2012, 05:44 PM   #13
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by wicked1 View Post
Ok, thanks.
I do have it built on breadboard for now, all the components stuck in w/ untrimmed legs, and while I called my grounding scheme "star", it was messy. I was assuming the unbalanced load was the issue, but it was just a guess. Since everyone is thinking it's ground related I'll rebuild it neatly.
Well, with a breadboard you almost automatically end up with a bussed ground.
Would you be willing to try to build the PSU as in fig. 3.2-4 from post #9 and see if that does the trick?
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Old 30th July 2012, 05:46 PM   #14
DF96 is offline DF96  England
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The author's explanation is wrong. He has directly injected charging pulses into his ground. If everyone reads and follows this misinformation it might explain why so many people get their CT grounding wrong. He seems to be more worried about a small amount of noise/interference from the mains supply than the much larger problem of charging pulses.
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Old 30th July 2012, 07:43 PM   #15
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by DF96 View Post
The author's explanation is wrong. He has directly injected charging pulses into his ground. If everyone reads and follows this misinformation it might explain why so many people get their CT grounding wrong. He seems to be more worried about a small amount of noise/interference from the mains supply than the much larger problem of charging pulses.
The charging pulses are the source of the noise/interference.

I don't agree with you and that became even more apparent to me when I noticed the images attached to post #9 were of the PSU alone, not of the whole device. The power common is not yet connected to anything in that image and the "star" is local, as the text I quoted says. I'll explain below why I disagree.

Quote:
Always ground a PSU from the clean end, not the dirty end, and don't mix up the ends into one ground. A PSU should really use a ground bus, even if one end (the correct end!) then goes to a star.
It is the bussed ground that is the cause of noise, not the cure for it. See fig. 3.2-1, the noise from the bussed PSU affects the signal reference.

A current through a resistance creates a voltage, Ohm's law. The lower the resistance of the path the current pulses must travel through, the better.
Fig. 3.1-1 shows the "bussed" ground that creates a higher voltage noise (created by the current pulses) than necessary. Fig. 3.2-4 minimizes this and avoids the connection across a noise voltage (noisy side to safety earth and quiet side to signal reference).
With earthed appliances (as we DIYers are forced to make because we cannot guarantee our creations are compliant to double insulation standards ) this is even more important because this noise voltage can drive a groundloop. From the same article: "Loops aren't bad, it depends on what's in the loop. Unless there is a voltage generator to drive a current around the loop, or radiated current into the loop, it is merely a parallel path. Consider the parallel shields of a left and right channel stereo cable."

The more I think about it, the more fig. 3.2-4 seems right. Esp. if you realize that the star drawn there is not the main starground. What seems contradictory is that the PSU star is both the low and high noise point. Left of the star is the high noise point (here flow the current pulses), right of the star is the low noise point (the current pulses don't flow here). In the star itself the currents in the loops left and right of it obviously flow together, but since this local star is just one very small very low resistance point, the current pulses can't create appreciable noise that would affect power common and signal reference.
Attached Images
File Type: gif Image4.gif (10.7 KB, 108 views)
File Type: gif Image3.gif (11.5 KB, 107 views)

Last edited by jitter; 30th July 2012 at 07:57 PM.
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Old 30th July 2012, 08:47 PM   #16
jcx is offline jcx  United States
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I simmed a sitation where even balanced Class A load gives CT current pulses - you have to locally "star" the CT and resevior Cap's "dirty gnd" leads - and use a single branch from that junction for the "clean gnd"

Marsh headphone amp from Linear Audio
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Old 30th July 2012, 09:21 PM   #17
DF96 is offline DF96  England
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Quote:
Originally Posted by jitter
It is the bussed ground that is the cause of noise, not the cure for it.
No, it is the connection between dirty ground and safety ground which causes the problem. Safety ground should be connected to the clean ground. The solution is very simple, and to be honest I am getting tired of how many times I and others have to keep saying it: keep charging pulses in tight loops away from grounds, and connect the ground to the clean end of a PSU.

Connecting the transformer CT and all the caps to a star, whether it is a PSU star, safety star or (even worse) signal star just creates problems. You can minimise the problems by getting the connections in the right order at the star, but much better to avoid the problems altogether by doing it right: CT-caps ground should not go to the star.
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Old 30th July 2012, 09:26 PM   #18
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Quote:
Originally Posted by DF96 View Post
No, it is the connection between dirty ground and safety ground which causes the problem. Safety ground should be connected to the clean ground. The solution is very simple, and to be honest I am getting tired of how many times I and others have to keep saying it: keep charging pulses in tight loops away from grounds, and connect the ground to the clean end of a PSU.

Connecting the transformer CT and all the caps to a star, whether it is a PSU star, safety star or (even worse) signal star just creates problems. You can minimise the problems by getting the connections in the right order at the star, but much better to avoid the problems altogether by doing it right: CT-caps ground should not go to the star.
an simple image will help us
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Old 30th July 2012, 09:30 PM   #19
jcx is offline jcx  United States
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my sims often include the picture...
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Old 30th July 2012, 09:30 PM   #20
DF96 is offline DF96  England
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OK, I will try to find one.
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