circuit needed

Wouldn't this potentially expose the sound card, and therefore the computer, to dangerous Voltages under some circumstances? Lightning strikes and what have you.

--Mac

Eh, computers are cheap.

could you post a schematic? I am not clear how to hook it up

the 600 ohm transformer has 4 wires correct?

two wires on one side of transformer go to line out & line out shield?

two wires on other side of transformer go inserted into red phone wire or else green phone wire? or is it across red & green? (one wire to each)

On Fri, 1 Jul 2005 20:44:07 -0400, "cowboy" wroth:

A 600 Ohm 1:1 transformer is all you need. You could buy one or tear apart an old defunct modem to get one.

One winding connects to the soundcard "line out" and the other winding goes in series with the phone line between the wall plug and the phone you're going to be using.

The transformer is needed to prevent lots of hum on the line and/or interfering with the normal operation of the phone.

It shouldn't make any difference whether the phone is cordless or not.

Jim

James P. Meyer

If you have DSL, make sure it's fitted on the telephone circuit side of the 'splitter'.

British Telecom would go nuts over ppl doing stuff like this though.

Graham

It will inded have 4 terminals or wires.

On the sound card. Sure

The colours in use will depend on your country normally. And possibly also where exactly you're 'tapping in' to the circuit.

I strongly doubt it will work acceptably with a paralleled connection either. The '600ohm' transformer will actually present a low impedance to the line and effectively shunt it making volume levels lower. It'll almost certainly kill any DSL usage.

Graham

far

Right, but your modem is designed to accommodate conditions that might reasonably be encountered on the line. There are applicable specifications and regulations for this sort of thing.

Just hooking up a transformer without any protection circuitry sounds dangerous to me. I'm sure the telco wouldn't like it. ;-)

There is also the question of what happens when you put audio on a line which is not off the hook.

--Mac

Jim, could you look at the schematic at ABSE, that I just posted, and tell me what you think?

On Sat, 02 Jul 2005 03:53:20 GMT, Mac wroth:

Sure. I use a dial-up connection for my computer. You can't get any more connected to the phone line than that.

I use an AC surge protected multiple outlet adapter for the power to all my computer connected equipment. The one I have also has a place to plug the phone cord from the wall in and an outlet to plug a phone cord that goes to my modem. It's a "star ground" for my whole system that makes sure (as far as is reasonably possible) that I don't have any ground loops and everybody is (as far as is reasonably possible) protected from lightning.

Jim

James P. Meyer

On Sat, 2 Jul 2005 00:57:16 -0400, "cowboy" wroth:

Correct.

Get yourself a multimeter to make sure that you have identified the connections on the transformer properly. The "ohms" reading across the terminals will show which ones are part of each winding. Even a cheap, >$10.00 one, will do the job. Line out and shield to one winding.

Cut either the red or green wire on the phone cord and connect the cut ends to the other winding.

Nope. That would sort of work, but wired that way the phone line would would be connected all the time that the phone cord was plugged into the wall and none of your other phones could dial out.

Jim

James P. Meyer

On Sat, 02 Jul 2005 18:44:59 GMT, Mac wroth:

A 600 ohm 1:1 transformer is what most dial-up modems use for coupling to the line simply because it's the safest device available. I've dissected a couple of lightning damaged modems and in every case it's a failure of the protective devices that prevent the modem from working. The transformers and everything downstream of them were completely functional.

An on-hook phone presents a very high impedance to the line. When you put an audio voltage source in series with a very high impedance, the current is limited to a very low value. You simply can't put audio on the line when the phone is on-hook using my suggested circuit.

Jim

On Sat, 2 Jul 2005 12:54:28 -0400, "cowboy" wroth:

I've posted a reply on ABSE that shows what I tried to say in words earlier.

Jim