Accidental ringer

[JPKNHTP]

-JPKNHTP

[dyarker]

Either the wall wart made a connection between circuit common and earth ground, the wall wart is faulty and feed 60Hz AC to the phone line through your circuit, or half wave rectified power wasn't filtered by the battery and got onto the red line.

That wall wart needs to be hi-pot tested. Any leakage at all from 120VAC side to low voltage side is EXTREMELY DANGEROUS to you, the guys that work on the telco equipment and the equipment!

The user end of a phone must always be floating, completely isolated from any grounds or ground referenced voltages.

You are lucky you didn't blow your circuit or trigger over-voltage protection at the telco office.

Cheers but be careful,

[JPKNHTP]

-JPKNHTP

[Chris Smith]

That or an optical isolator.

[richfloe]

Not only do you not want to make either side of the line common to ground (or anything else), you dont want to place anything across the line that will show DC continuity as this will cause the line to be tied up. Any large capacitance across the line is also a problem.

The isolation transformer is a great idea, just make sure you put a capacitor in one or both of the lines, something small, 0.1uF or smaller with a voltage rating greater than 100v (the telco sends 90vac at 20hz to ring the phones). A higher impedance primary would be good as long as you get enough drive out of the secondary to feed your amplifier.

Do note that when the phone rings, you are going to hear a very loud 20hz coming out the speaker. Perhaps a stage or two of high-pass filtering will help. The capacitor feeding the primary of the transformer will help some by itself.

Rich