Active Balanced Line Splitter

[chilidawg]

So awhile ago I built one for a friend.
It distributes 1 balanced line input signal to 6 balanced line outputs.

Here's the schematic.

It works great, and then last week he told me that:

1. When nothing is connected to the input, there is a loud constant buzz at the outputs.

2. When the XLR cable plugged into the input but the other end of the cable is not connected to anything, there is a very loud constant buzz at the outputs, touching the cable or the metal body of the XLR connector can also cause more buzzing sound at the outputs.

I put everything inside an ABS enclosure, and the metal body of the I/O connectors have no physical connection to each other, nor to the signal common ground.

PCB is a dual layer design with a bottom layer ground pour. I kept the balanced signal routing symmetrical, equal length of copper traces.

Anyway, I spent 2 days investigating the "problem", but I can't find what is wrong, especially because the "problem" is not there when input is connected to a proper source, such as the balanced output of an audio mixer/audio interface, or just simply the stereo headphone output of a cellphone, and the splitter is putting out 6 perfect copies of the audio signal.

Perhaps someone can explain it to me why, and if there is anything can be added to the circuit to sorta auto mute the outputs when nothing is connected to the input.

[x-pro]

100k input is a lot, unless the input connector is a “combo” form factor, where there is also an XLR + jack, with connected input grounding contacts.
Try lowering the resistor to 10-22k.

[chilidawg]

EDIT: Lowering the 100K input resistor as suggested by x-pro, to 10K did the job! No more buzzing at the outputs when input is not connected to anything. I can't believe it, lol

100k input is a lot, unless the input connector is a “combo” form factor, where there is also an XLR + jack, with connected input grounding contacts.
Try lowering the resistor to 10-22k.

спасибо x-pro!

[loudthud]

What are you using for a power supply ? Most power supplies will have a very large common mode signal at Mains Frequency if there is no connection from the power supply to the Safety or Earth ground. You can measure this, simply connect the Black probe from a DVM to the Earth or Safety ground from your Mains outlet and the Red probe to the Common between +15V and -15V. Set the DVM to AC Volts. You should get Zero Volts if there is some ground reference connected to the output of the power supply.

Edit: I didn't see chilidawg's reply before I posted.