Sorry to say but you have your understanding back to front. An ADSL filter is an accurate name for an ADSL filter because its job is to filter out the ADSL signal. An ADSL filter is essentially a low pass filter (although designs are often more complex than that) that lets the low frequency voice signal through and blocks the ADSL.

I know your post was well intentioned to help people better understand how ADSL works. My correction is not personal I'm just trying to avoid further confusion caused by incorrect information.

My original post was prompted by problems with my adsl connection - BT standard faceplate, Zen 8000 Active - Speedtouch 580, - which has been fluctuating quite a lot since teams of BT engineers worked on overhead lines in the area last December. Severals teams were in the immediate area for six weeks or more.
Now waiting for BT to investigate - initial report within six working hours promised - must say I am more than a little apprehensive about outcome.
Trying to eliminate any possible problems on my side of connection. Any advice would be appreciated. Could any one tell me whether Micro Filter problems are a very likely cause of my massive drop in download speeds? Have filters on four extension sockets, one DECT and three analogue 'phones. All filters are Speedtouch ADSL filters and were supplied by them when I bought the router.
Thanks for any advice offered.
Best regards
Ben

So, as you sell both Pressac and XF-1e based filters, which design is better? My line isn't marginal, but did recently have a (now repaired) line fault which prompted me to look into my modem and filter.

I always thought the BT one was the best, but is the XF-1e really better? Would I see any change like a tiny increase in SNR levels or anything, and most importantly, will I still be hearing a small hissing sound on the Voice side from my ADSL?

Filters do fail, you need to eliminate whether one of them has gone faulty.

I've got both types at my new property (the XTE was out of stock when I got my ADSL connected). The line at this property is running a solid 8 meg connection (Zen 8000 Active) and TBH I didn't notice much difference between the two filters though the SNR is marginally better with the XTE, but that could be attributed to better contact in the socket and a few other things.

Unfortunately all the data I had to compare them has been lost due to HDD failure in my Linux server.

I'll grab one of them then! I don't hear any hiss when using my Speedtouch dangly filters, so a replacement Faceplate filter should do the job nicely!

You shouldn't hear any noise on the line, if you do it is probably because the ADSL signal is leaking through the filter. The XTE-2005 is a better design than the Pressac version. We only sell the Pressac one because some people ask for it and some places sell them at rediculous prices, so we may as well supply them at a reasonable price.

Yes I was aware that was the reason I was hearing hiss from the DSL signal. Will be ordering an XTE-2005 later on this evening. Still pondering whether to replace my ADSL2MUE with a ST585v6 though...

Quite often the cause of the hiss is the phone. Many phones have electronic amplifiers in them (especially DECT) which have automatic gain controls in them to prevent feedback. Some filters do allow a small amount of the HF ADSL signal to come through, and this is heard as a hiss in these phones. If you still get hiss with the XF-1e (which I believe is the best filter out there), then you may need to consider new phones.

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Be aware that some telephone equipment, DECT handsets and FAX machines for example, can interfere with the ADSL signal.

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I think you'll find that it's usually the other way around, especially when "cheap and cheerful" ADSL filters are in use.