The above circuit is what I am using to convert a stereo signal into mono before going to XLR (the only really viable option just at the moment - not using two D.I. boxes etc). The 3.5mm stereo DIN minijack whatever-you-call-it (somebody tell me ) is connected to a laptop output, and the XLR goes up to our booth via multicore. My question is this: how can I eliminate the nasty hum I am getting? What would be causing it? The "converter box" with the resistors/XLR jack etc in it unfortunately happens to be close to our dimmers, if that is a factor. This cannot be changed - if I fade lights up, there is a fluctuation in the hum. Help please!!! Thankyou all in advance.