Ground loop problem with an L-pad?
Not sure if this thread should be in this forum or another forum like extremely low cost theory, but just a continuation on L-Pads:
I just built an L-Pad as discussed above and put it inline on a line level output cable which is then split to 2 mic level stereo inputs (L+R joined together), one going to a JVC HDD camcorder mic input, and the other to a Sony mini-disk recorder mic input. The Sony minidisk has both line level and mic level inputs available, the camcorder only has mic level. Hence, I decided to take the single line and split between two devices at mic level. (Apparently that's OK to do.)
The problem is that now the mic in recording on the camcorder is doing really good with the L-pad attenuated voltage, but the minidisk recorder is now recording a low hum all of the time on its mic input -- not sure if this is a ground loop effect caused by the L-Pad -- but doesn't seem to be a problem on the camcorder recorded signal. (Why?) The minidisk did fine before with its line level input and no L-Pad, but to save hassle with cables I'm now trying to use mic level on both. The PA is some 12m away on a different power point from the 2 recording devices, in case that is the likely cause of ground loop effects.
Some designs call for a capacitor to be placed in the line also to 'block DC' -- I'm not sure if that's the problem here, or whether it's a ground loop effect where the ground level on the PA is different from the other devices etc.
A commonly repeated design on the web appears at, for instance, Line level signal to microphone input adapter
which is what I've done -- -40dB atten circuit. The author points to possible hum problems and suggests a further ground isolating circuit as a solution.
Fixes that occur to me are:
1) just insert the L-pad on 1 line only out of 2 after the split -- send a mic level signal to the camcorder, and a line level to the minidisk, as these both seem to work without problems -- will this work or will the 2 devices somehow interact? I can easily cut my existing dual cable on one plug and insert my L-Pad on it inline, which then lets the cable be used for line/line or mic/line as options by just plugging in the L-Pad. This would be my preferred option for elegance at present.
2) use this capacitor thingummy as a DC uncoupler on top of the basic voltage divider, but it may not resolve the problem. thoughts?
3) use a ground isolating circuit which is an isolating 1:1 transformer or something by the looks, only $20 or so, but yet another device on the line -- is this really necessary or the best option?
The line so far looks like circuit 1 on the web link above, where the mono mic level output then gets split to 2 mono lines each terminating in a 3.5mm stereo plug with L+R joined together on each.
I'm doing this on the cheap for a community organisation without much money, so far I've built these custom cables for under $20 total. Each week there's a new wrinkle on this problem which I have to try to solve for the next week...
thanks in advance for any help