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now with added coils . . .)

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



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