I am trying to record audio output from a Yamaha LS9 board in a Yolo Live stream box with SD card.
The line input is extremely noisy (expected), so I'm running a balanced output XLR cable from the LS9 output bus to an XLR input on a Sony HD cam (don't remember the model, but let's call it A). The HDMI output of the cam is connected to the Yolo. Settings for the audio input on the cam: Line, 0 dB attenuation, manual volume adjustment.
For an A/B comparison, an identical camera (call it B) has a Sennheiser wireless receiver operating on batteries as the audio input.
Power for the cameras and the Yolo comes from a common UPS. Audio was monitored with headphones plugged into the Yolo.
Noise levels for audio both cameras were the same (basic white noise with a extremely faint 60/120(?) Hz) with the Sennheiser on and the XLR cable unplugged. These noise levels do not change with an adjustment of the camera audio input volume dial.
Here's the problem--When I plug the XLR on B into the LS9, there is a small but noticeable increase in noise at a frequency approximately a minor 3rd interval higher (72/144 Hz?). This buzz/noise occurs even if the LS9 is unplugged and all input items are off. (I haven't tried unplugging all inputs and other outputs.) Plugging the LS9 in, then powering up does not change this extra noise.
What could be causing the extra buzz, especially considering it happens whether or not the board is on or off, but doesn't happen until the cable is plugged into an output.