Kingston
Well-known member
Thank you both of you. That helps me immensely.
OK, I've analysed the unit further.
The unit was being tested using an RME sound card balanced IO. The whole system is connected to the same mains output, so I'm guessing the ground loops aren't coming from there. If I measure the RME IO self noise, there's no hum, just flat low noise floor, about 10-20dB below the preamp.
I also tested whether the noise was coming from the input side, using a resistor configuration suggested by NYD earlier (I think it was using 200ohm resistor between pin 2 and 3). Hum still there. Both channels have exactly an equal amount of hum by the way, ever since moving the PSU out of the rack.
The funny thing I noticed is when I screw the lid on the rack, the hum actually increases about 5-10dB! Almost as if some field was being reflected inside the unit.
All XLR Pin1's are connected to chassis as close to the XLR connectors as I could manage. This is also the only chassis point of the whole unit.
The layout is unorthodox due to the fact I wanted to test if moving the capacitors around would have any effect on the noise, same with the current connection of all audio transformers. Maybe I should post some pictures, although right now it looks a bit of an abomination due to the troubleshoot configuration changes.
OK, I've analysed the unit further.
The unit was being tested using an RME sound card balanced IO. The whole system is connected to the same mains output, so I'm guessing the ground loops aren't coming from there. If I measure the RME IO self noise, there's no hum, just flat low noise floor, about 10-20dB below the preamp.
I also tested whether the noise was coming from the input side, using a resistor configuration suggested by NYD earlier (I think it was using 200ohm resistor between pin 2 and 3). Hum still there. Both channels have exactly an equal amount of hum by the way, ever since moving the PSU out of the rack.
The funny thing I noticed is when I screw the lid on the rack, the hum actually increases about 5-10dB! Almost as if some field was being reflected inside the unit.
All XLR Pin1's are connected to chassis as close to the XLR connectors as I could manage. This is also the only chassis point of the whole unit.
The layout is unorthodox due to the fact I wanted to test if moving the capacitors around would have any effect on the noise, same with the current connection of all audio transformers. Maybe I should post some pictures, although right now it looks a bit of an abomination due to the troubleshoot configuration changes.