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RE: Comments on your post (sorry - a bit long)

Posted by Ugly (A) on January 27, 2017 at 21:52:30

In Reply to: Comments on your post (sorry - a bit long) posted by Ryelands on January 27, 2017 at 09:37:32:

"I feel pretty sure it's not ground loops I've cured as discussed in the Jim Brown or Bill Whitlock papers as I have no connections between system ground and safety ground."

If I remember correctly the Risch "isolators" are RC filters, right? RC's are for "type 1", aka differential mode, problems.

"Type 2" noise is a subset of the more broad set of problems known as common mode noise.

It's pretty safe to say that in general solutions for differential problems are almost completely useless for common mode issues and vise versa. All systems will have a bit of both differential and common mode noise present pretty much throughout the system. It's only a matter of how much and is it a problem? Many times the most effective solution is a bit of both differential and common mode filtering.

There is only so much that even the best power supply designer can do in design and layout to prevent emi. Mother nature disallows perfection from an emi standpoint. Unfortunately, in some cases, filters are the best option. When all that can be done in these areas has been, you are left with filters to get you the rest of the way. Filters are a necessary evil.

Henry Ott is considered one of the old masters when it comes to understanding EMC issues. Slide 16, in the link about common mode problems from power supplies, gives a visual for the power supply leakage current "ground loop" path from energy transferred via transformer inter winding capacitance and via parasitic capacitance to nearby conductive objects.

As Ott points out, in the slides but also in his several excellent books on the subject, leakage current generally responsible for causing conducted problems. Conducted problems can be handled with filters.

Though an important point to note, by my estimation, is that conducted can easily convert to radiated and vise versa under the proper conditions so getting a filter in just the right location is critical to prevent high frequency conducted problems from converting to radiated energy.

Diff mode problems are easy, you can easily see them with any oscilloscope. With common mode, not so much.

The hard part is figuring out if you have common mode problems big enough to worry about and all it's mechanisms and nature of being. With that info dealing with it is a snap. However, to set yourself up with the necessary gear and knowledge to do well at it is beyond practical for most.

Avoiding problem children at the time of new power supply purchase seems logical to me. But it's not as if the power supply companies I'm aware of are making it easy by supplying emi scan captures or anything useful like that. It's annoying.

After that, most of us are pretty much left trying stuff.

Getting into designing and having success with getting the audio system to sound better via common mode filter application is gonna be tricky for a noob. Getting the current handling capacity, as in power amp filter for example, is easy as getting enough wire gauge for the current. It's getting the filter tuned to and getting enough attenuation at the frequency of noise problem you have, putting the filters in the right spots in the proper orientation is where all the action is.

Since it's tough for us mortals to easily take a look at the problem it's about like shooting in the dark when trying to get the right filter put into the right place.

Some educated guesses are no brainers though in my opinion. For example it's almost guaranteed a PC power supply, even if covered in emi certs, will be injecting some amount of stuff back into the power line as well as onto its other cables. Playing with blocking noise from conducting out of the various cables emanating froma typical PC isn't too risky in my opinion. In fact it seems very popular on this forum now days. Your mileage may vary.

wow guess mine went a bit long too.

• http://www.hottconsultants.com/pdf_files/APEC-2002.pdf   (Open in New Window)

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