Use backward DSL filter as phone jack extender from laptop

I'm using an inline DSL filter as an extender for the phone jack on my laptop. That is, rather than frequently plugging and unplugging the phone line to/from my laptop, I plug and unplug to/from a DSL filter connected to the laptop. I do this because it's much easier to replace the filter/extender rather than fix any problems on the laptop due to wear and tear.

I am not making use of the filter functionality, only it's ability to physically extend the jack by a small distance. I can do the same for about 14 bucks by purchasing a 6-inch length of phone cable (male plugs on both ends) and a female-to-female adapter. The important thing is that the 56Kbps dialup signalling is preserved in both directions, /and/ the laptop's modem is not unduly stressed or damaged. I am assuming that this occupies the normal voice spectrum (upto about 3.5KHz).

In order to use the DSL filter instead, the filter must be used "backward" i.e. with the mail end plugged into the laptop (it should plug into the wall), and the female end connected to a phone cord that plugs into the wall. I'm not sure if that is OK. For one thing, I'm not sure if the filter is made of entirely passive, linear components. A simple concept circuit at

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shows that it is certainly not symmetric; and there is no reason for it to be, considering that the system in which it sits is not symmetric (see circuit at
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its functionality is not symmetric:

What a Microfilter does* ------------------------ A microfilter performs two main tasks. First, it prevents high frequency noise from appearing at the telephone and potentially decreasing voice quality. Second, it prevents the telephone equipment from interfering with the modem by preventing harmonics, overtones and changes in impedance caused by the telephone equipment.

*From
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Be that as it may, we still know in its /normal/ use, bidirectional conveyance of the voice band is preserved. I am hoping that this is still true when it is hooked up in reverse, with supply voltage being sourced from the "home" side (which I plug into the wall, as mentioned above). This also means that any high frequencies coming from the modem on the laptop gets bounced back to the modem, as per the filter's normal function. I do not know if this affects the modem, as the designer might not have anticipated this unusual scenario.

I wonder if those with experience in circuitry for interfacing to telephone lines can comment on the risk of this approach (long term risk to the laptop, that is).

Thanks,

Fred

Reply to
fred ma
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I wouldn't worry about it. It's hard to see how a low-pass filter will do anything more hostile than filtering out high frequences you didn't want anyway.

Reply to
USENET News

Hi Fred,

No, DSL filters are not symmetrical and the schematic you sent does not correctly represent what's inside a DSL filter. I do not have a correct one handy, and it obviously changes from manufacturer to manufacturer. However, those I had a pleasure to dissect all had transistors in them, clearly making them NOT symmetrical.

Although it clearly looks like a classical example of over-engineering to me, I have a suggestion: Just undo the screws that hold the halves of the filter's body together, get inside with a soldering iron and simply shorten the path between the corresponding center pins of the male and the female connectors. BTW, you cost estimate on the off-the-shelf components required to achieve the same effect is way high. Well, maybe with shipping it is correct, but in any case any nothing-over-$1 store would carry all you need to construct such cord

Reply to
Dmitri(Cabling-Design.com

Thanks, Dmitri. I suspected there might be amplifiers in there. The filter I got doesn't have screws, so I should be prepared to bust it, if required, by prying open at the seams. I'd also have to purchase a soldering iron. So the simplest route is to go to the dollar store, as you suggested. The prices I got were from an electronics shop in Canada, with taxes estimated and included. Thanks for pointing out the dollar-store alternative.

Fred

Reply to
fred ma

Some of the early SBC DSL filters had female jack on the 'input' side also and came with a short (3") cable. Very nice since you could use them with a long cord in places where the filter didn't fit in well near the jack. The new ones, as you mentioned, have the cord permanently attached.

If you are this worried about it, why are you fooling around and asking questions, just go make a 3" cord and use it with a female-female coupler. Presto, guaranteed no active components and no violation of the signal.

-- Gary Breuckman

Reply to
Gary Breuckman

Um, that question wasn't posted as a joke, so it isn't "fooling around". It was posted so that I can determine whether the worry was warranted. If not, I'll just use the dongle. It was from posting that I found out that this was not OK. From that, I decided to do as you suggested, which is to make my own extender. The parts really add up at the electronics store (and I have to get a crimper, which I was hoping to avoid since I hardly ever have a need for it). Some of them can be gotten from a dollar store, but not all. If the question had been answered otherwise, going with a free dongle would be far preferable.

Fred

Reply to
fred ma

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