Is this the reason mains filtering falls short of the performance of mains regenerati

[Neil McCauley]

I received this email yesterday and not being technically competent enough in this specific field to be able to know one way or the other, I've placed it here to invite comments. Thank you. HP

"I haven't got a problem with mains filters done properly but to do it properly you need to do calculations so you know what result you will get beforehand. As all mains filters are passive you have to optimise them to the load. Load them too much you will loose voltage and increase output impedance. Under-load them and you can actually get a gain in radio frequencies rather than an attenuation.All this assumes purely resistive loads which no hifi gear is, they all have reactive loads which then just makes the matching of filter to equipment even more complicated."

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[StanleyB]

Before I started my own business I used to be head of the R&D department of a big company that once controlled 20% or more of the electrical accessories market. One of the things we also designed were mains filters and our only competitors in that field (mains filters) at that time were Belkin. So I would hazard a guess that I am reasonably qualified to comment on this subject.

The first thing to note is that there are two types of filtering: surge & spike. The other thing to note is that for the European market these things have to conform to a number of approvals, including CE, EMC, etc. One of the tests they have to pass is that they work as described on the can. The other one that they have to conform to is suppression of RF frequencies.

The article emailed to you is, I assume, aimed at the DIY guy who wants to build his own filter. Or it is from a company who wants to sell you a mains filter. My advise: ask them to see a copy of the CE and EMC tests carried out on their product. Also ask to see a copy of their insurance cover in the event the product fails. No test results and insurance? Then stay clear. Customers and Trading Standards could take you to the cleaners.

I know of one case where a house burnt down, the carpet in a church caught fire, and another one where a customer received an electric shock from the earthing tag on a mains filter. The pay outs were massive in each case, but luckily the items were approved and there was insurance cover.

By the way, just about every mains operated electronic product now has a filter built in as part of the EMC regulation. If you ever open up an item, just look at the area close to the mains transformer. There are either two round and blue caps or two rectangular yellowish caps soldered there.

[Primalsea]

Hi, Actually it was me who sent the email. There was no hidden motive just a bit of a rant. I do feel better for it now but probably not over it just yet

What you say about surge and spike filters is bang on but I was referring to Mains RFI Filters that use series inductors and parallel capacitors to make a Low Pass Filter. Some of these do also have surge and spike protection as well in addition.

My gripe was that passive filters such as these really have to be optimised for the source output impedance and the load input impedance which is a figure that will change with frequency. Not only that nearly everything hifi is a reactive load, some kinder than other though I must admit.

This means that unless the calculations are done or tests have been run previously you don't really have any idea whats going to happen. This means any company selling mains filters doesn't have any idea of the real world performance their customer is going to get (could even make things worse) unless the filter has some complicated load sensing that controls some mechanism for adapting the filter for the load.

Well, I'm obviously still not over it!

[Cotlake]

Hummm, well several of us from the World Designs forum have built what was popularly known at the time (2003) as the 'Swampy Filter', designed by Dave Marsh of Triode Systems. The web address/company doesn't appear to exist now and web address has now been occupied by DIY Hi-Fi Supply. However, several of us found this filter useful particularly on digital components. We stopped using it on our valve amps because it flattened the sound. It filters spikes, surge and RFI. You could build either a simple single stage or more complex dual stage filter for an exceptionally nice price. In my case, I have some extremely noisy appliances on my mains and overall things are sonically not good. The Swampy filter does an excellent job at dealing with all this. I'm still in contact with Swampy and could facilitate build information and circuit on these filters if there is any interest. On a basic filter application, something on the back of an amps IEC socket can be very effective and the below link will give you a cheap yet quality option.

http://wduk.worldomain.net/forum/showthread.php?t=149

Best wishes,

Greg

[Mike]

On a basic filter application, something on the back of an amps IEC socket can be very effective and the below link will give you a cheap yet quality option.

http://wduk.worldomain.net/forum/showthread.php?t=149

Best wishes,

Greg

Hi Greg,

Do you not find that the 'basic filter' in the FAQ section also has a bit of a flattening effect on valve amps?... I tried it on my '88' and soon removed it.

Cheers...

[Ali Tait]

Will second Greg's comments on the "Swampyfilter" It's an excellent performer for I think about 35-40 quid for the bits.Doubt there's anything commercial at anywhere near that price.I seem to recall someone testing it against some quite pricey commercial stuff,and it easily outperformed it.Can bring mine to Chester if anyone wants to hear it.

[Cotlake]

Hi Mike,

Well actually no. I've used it on the Pre II units, KaT88, 2A3PSE and currently my 300B PP clone. As you know my kit has been played publically away from home. I've never found any flattening effect myself and I've never had any comment from others to suggest that. You might be right if applied to a basic construction. I don't know. As I'm sure you're aware, all my kit is well tweaked and this might make the difference. Having said that....I still don't know, but certainly I'm very happy with my sound

As an afterthought, might your experience be down to which components you chose for the tweak? Not intending to be rude but I just wonder if that might be a reason why you didn't get on with it.

Further thoughts. Max Nightingale (Max N on WD) rates this tweak as the single most cost effective tweak he has ever done. He applies it to all his builds and believe me, he is no skimper. His DIY builds a seriously expensive and quality products.

Best wishes,

Greg

[Mike]

Hmmm... Fair point.

But!

It was pretty much almost a standard WD88XL (I say 'almost' because I've not yet fitted the large RIFA caps in the HT supply) apart form a bit of tube rolling.

HOWEVER!.. I don't seem to have any problems with mains noise these days. I used to suffer the odd click and pop here and there, but found fitting ferrite's to all the stuff around the house (fridge-freezer, heating pump, computer crap etc..) had a bigger effect than mucking about with that sort of thing on my HiFi eqmt. I'm certain much of it depends on whether (note correct spelling this time! ) or not there is actually a problem that needs addressing in the first place.


Cheers...

[Primalsea]

Hi Greg,

I haven't seen a Swampy filter so can't really comment on it other than a few rough guesses. Did it have any inductors??

Mains filters come in various flavours the one on the WD website is very tolerant of load as they just use a VDR to clamp voltage spikes and capacitors in an effort to give a low impedance path to ground for high frequencies. These are useful but at the bottom of the filter ladder in terms of performance. The next step is to use series inductors as well as the caps, this is where the problems can occur. Its likely the swampy was well designed and worked well with relatively low power consumption items that had a relatively constant current draw when being operated (CDP's). However as you said it didn't work too well with amps possibly because of the reasons I ranted about above.

Filters can work very well when done properly but theres no such thing as a one size fits all.

[Cotlake]

Hi Paul,

Yes agree. The WD option is certainly useful but very basic in function. It does however seem to do alot of good for several people.

I've forwarded this thread to Swampy. He may join in himself or just release stuff to me to post accordingly. I only have hard copy print that I thankfully saved 5 years ago. I could scan and post I suppose but as Swampy is talking to me on this at the moment I'll leave a while and see what he comes up with. Watch this space.

Best wishes,

Greg