#5 Hum Bug

[David Rowe]

With some telephones 50 Hz mains hum can be heard in the analog phone, especially while dialing or during a call when the line is otherwise quiet. I can reproduce this bug on three MP01s (2 V1.2 and one V1.1) with generic black phones Rael supplied at the 2nd workshop. Two samples of this phone produce the problem. Three other models of analog phone tested have no hum.

I have heard similar hum on early IP04s, which uses the same FXS chip-set. On the IP04 the problem went away by changing phones or using a grounded power supply (like a laptop PS). On the MP01 a laptop PS has no effect on the hum.