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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment

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Old 24th Jan 2021, 6:35 pm   #1
rambo1152's Avatar
Join Date: May 2010
Location: Manchester, UK.
Posts: 2,539
Default An active xDSL micro-filter

I have posted about this in "Modern Technology", but I want to explore the implications for our vintage telephony.

I've had this filter for ages and I've been using it on my VDSL setup on the basis that it's the best one I have, after all, it has a good few semiconductor devices in it, and not merely passive, so it must be good, right?

After examining it closely, I'm not at all sure of that.

Hear is the item, it's quite a big beast.

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As far as I can ascertain it was made by LEA (

The four devices that I thought were transistors turned out to be BT134-600 triacs.

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I reversed engineered, it and produced this schematic.

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So, as I see it, as a filter, it's very basic . Just two small inductors one in each leg and a common-mode choke. I have better filters with more ferrite, if weight is any indication.

So what of the rest of it?

I don't know why they needed this symmetrical arrangement of four triacs to do the job, but what seems to be happening is if the phone or other POTS equipment is on hook, then the triacs switch off, and isolate the phone port from the incoming line. But why? What part of "on-hook" don't I understand?

Well, you might say, some phones are designed to abstract a small current from the line when on-hook to retain number memory or even recharge a small battery. But If this filter prevented that, it would hardly be a Good Thing.

So all in all, I'm a bit perplexed. Do you agree with my interpretation of what's happening?
Can you enlarge on it?
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Old 25th Jan 2021, 9:47 am   #2
dagskarlsen's Avatar
Join Date: Jan 2008
Location: Hakadal, Norway
Posts: 534
Default Re: An active xDSL micro-filter

I doe not understand the need o 4 triacs, for the most 1 would do the job, 2 will make the switching symmetric and that may reduce noise from things along the wire after the filter, so OK.
Experience from the really old days with modems.. are that even the on hook phone with a ringer reduced the speed of the data.

I do not now, but I guess that you are right about a better filter would do that, but that ma again reduce the ring.current, and the voice level. Charging batteries in the phone is not that common, and with those triacs... forget it.

Still this is the most advanced filter I have seen so far, and it looks like DSL is not the the "solution of tomorrow".
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