[R-390] ground loop solution

[Drew P.]

We-i Li wrote:

"Firstly, thanks to all who offered opinions and advice re my problem. To reiterate, the hot and neutral AC supply lines were not crossed. The 3 to 2 AC adapter was put away in my toolbox.

What I did do was get a small audio 1:1 isolation transformer advertised as a *ground loop isolator*, and said to have wide frequency response. It is  sold in car stereo shops for around $20.

It worked eliminating 95% of the annoying noise."

Good suggestion. The $20 is not a lot to spend for a solution complete with connectors.  But if you are a cheapskate like me, there is a good solution available from common salvage.  This would be the telephone line isolation transformer taken from a scrapped modem.  I have used these to good effect for audio isolation.  These are 1:1 with a rated impedance of around 600 ohms.  I tested one at a level of a few dBm and it was flat within about 1 dB from 20 Hz out to 60 KHz.

Drew