|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
87.115.179.111
In Reply to: RE: Comments on your post (sorry - a bit long) posted by Ugly on January 27, 2017 at 21:52:30
Many times the most effective solution is a bit of both differential and common mode filtering
Thanks for an interesting post and links. I admit that I don't get how the Type 1 / Type 2 distinction or the notion of "back-door noise" adds to what is already well known. It's been several years since I looked at Henry Ott's paper. Who knows? I might get more from it this time . . .
Whatever, JR used isolating transformers because they allow high capacitance across live and neutral without excessive leakage currents (see link). He specified split-bobbin transformers because they were easy to get for a "cheap and cheerful" project and for their low inter-winding capacitance. In practice, they get pretty hot and tend to buzz. Because I had suitable toroids to hand, I used them instead, fitting those with inter-winding screens on the input side. (Don't know why but grounding the screens on both transformers regularly trips the ELCB.)
As you suggest, designing and measuring decent filters seems beyond many audio designers, let alone us hapless DIYers. Once I grasped the importance of clean power for audio, I did much as you said and "tried stuff". After lots of "stuff", much of it discarded, my rules of thumb became:
1. A dedicated power line for digital kit and another for analogue. With care and a bit of luck, both will have lower impedances than the typical home supply (esp with UK-style "ring" circuits) and be relatively free of crud.
2. No cheapo SMPS PSUs on the above. If you need one for e.g. a data server, run it from a separate line and, if possible, connect via the latest fad, optic fibre. (It is a fad but a very effective one if done right.)
3. Instead of JR's high values of C across L&N, fit C-L-C filters between the transformers. I copied a commercial mains-rated design several cuts above those IEC socket things. By using one isolator per line-level device (~15 watts each), no real need for large coils.
Hopefully, unlike JR's circuit, I'm addressing both noise modes and, by using separate isolators & filters for each device, helping to prevent noise from one device affecting another. I don't see how a simple connecting strip or system-wide filter can do that. At least in my system, the difference that several discrete isolators makes is very apparent even though I'm using decent PSUs.
Feel free to criticise.
D
Follow Ups:
It sounds like you are already using a tuned 2 stage line filter with common mode filter first stage and differential mode filter second stage. That sounds like a really good starting place for trying out in various locations.Not that you didn't already think of it or maybe already even doing this but since you didn't mention it a couple potential tweaks for tuning the common mode performance of the first stage come to mind.
Maybe running the CLC in a balanced differential CLC configuration with respect to live and neutral will help enhance common mode rejection performance of the first stage without sacrificing the second stages differential mode performance. Ie you'd still have the same X caps spanning line and neutral but the balanced configuration would have an a separate L (but half the L as compared to an equivalent single ended version of the filter) between the caps on both the line and neutral.
You could also try adding, in a balanced manner, 2 identical and appropriately rated Y caps. One from from live to earth and one from neutral to earth. I'd try variations on where they belong, ie on the transformer primaries, transformer secondaries, after the CLC??? Some where around around 1nF ought not put you up against code violation most locales and is probably a fair place to start experimenting. Adding the Y's like this is to try and enhance common mode noise rejection of the first stage.
The only other thing I might add is that remembering to swap the order of the stages with respect to source and load to see which way works better during experimentation is another thing that may very well be worthwhile.
Edit: maybe there is a use for a damping resistor from line to neutral after the CLC to enhance it's performance.
Edits: 01/28/17 01/28/17
You could also try adding, in a balanced manner, 2 identical and appropriately rated Y caps
Thanks again for your reply. I did ponder doing as you suggest but decided I was perhaps making things a bit over-complex what with five (hopefully) separate devices. I've also got the units buried out of sight and difficult to get to. The temptation to fiddle is reduced but I might just experiment with a spare.
maybe running the CLC in a balanced differential CLC configuration with respect to live and neutral will help enhance common mode rejection performance
H/W the circuit though I didn't fit the 0v link or the 1M resistors.
Because there's a 240V > 24V transformer on the input, the filters are working at low voltage. I didn't see rating as a worry and used a pack of cheap Y-rated 220nF caps. (I'm aware that some places would need X-rated caps were they at mains potential.)
OTOH, currents are higher at the lower voltages even if no isolator is in practice passing more than about 3/4 of an amp. I guessed that 3-amp-rated Murata coils with DC resistance 0.03R would be fine. Well, they're not getting hot . . .
maybe there is a use for a damping resistor from line to neutral after the CLC to enhance it's performance
Agreed but I didn't feel confident in calculating a value so I left them out. Again, I could experiment.
D
I enjoy chatting about this stuff. No need for thanks here. I appreciate your sharing your secrets so I can see how much I'd like to try myself.
Imagine your transformer in place of the common mode choke...This is what I was envisioning Note the Y caps orientation with respect to earth. It might be worth playing with to try and increase impedance to common mode noise.
Though, regarding your filter schematic, I haven't seen anybody using an inductor on earth that way leading me to wonder what it's function in the circuit is. Presumably it can't do much until there is appreciable earth current which in my locale is strictly forbidden above a tiny level.
With double insulated, tinyish transformers, even the authorities presumably wouldn't have any problem with you not using special safety rated parts.
...including the two Henry Ott books in my collection. "Noise Reduction Techniques in Electronic Systems" has been a go-to reference since the 1980's.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: