The Sennhiser EW300 manual specifically says to only use one output at a time. I guess they are not isolated or buffered from each other.
I was told more than once by Sennheiser support on the phone that you could use both outputs at the same time. When I first was wondering if it could be used that way and when I encounters this noise problem. I couldn’t find in the EW300 manual where is says to only use one output at a time. But I didn’t notice before that it says that they are in parallel or I interpreted that to mean that they both had the same signal but I guess I assumed that they were buffered from each other in some way. I can’t find a schematic online for this to see what it indicates. I think there is a spare receiver (but it might be a EW100) and I could probably open that up and see what it looks like.
Below is a copy and paste from the EW365 G3 instructions for use PDF. That seems to imply that you can use both outputs at the same time when they say “and/or”. But if it is just a hard wired connection between the 2 connectors I could see how that might be a problem. I would assume that this would mean that pin2 of the XLR is connected to the tip of the ¼” and pin 1 of the XLR is wired to the sleeve of the ¼”. But what makes me think that this isn’t true is when I have a ¼” TS (unbalanced) cable plugged into the jack I get the noise on the ¼” but not on the XLR. It is a puzzle to me.
Keep in mind that I said this happens when I am just connected the 1/4” output to a battery powered headphone amp and the XLR to the house system.
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Connecting an amplifier/mixing console
The receiver’s ¼” (6.3 mm) jack socket and the XLR-3M socket are connected in parallel.
Use a suitable cable to connect the amplifier and/or the mixing console to the ¼”(6.3 mm) jack socket and/or the XLR-3M socket .
Via the operating menu, adjust the audio output level (“AF Out”) of the receiver to the input of the amplifier or mixing console (see page 26). The audio output level is adjusted via the operating menu and is common for both sockets.
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But with all of this said. I have it working with the Y cables (hardwired split) out of the XLR connectors on the receivers so this is now a low priority for me.
Thank you for all of your suggestions and I at some time will try to get to the bottom of this.