BA preamp ground hum on DI and other problems
Posted: Sun Mar 29, 2015 11:24 am
Hello all,
recently my dual mic preamp/DI BA with 1:4 JLM input transformers and 1:1:1 output transformers and JLM 99V opamp started to output some hum which sounded like a ground noise at around 50Hz. (for couple of years was working flawlessly)
I could hear it on headphones and when gain is boosted a little higher, but with condenser mic it was fine as it was too low comparing to signal.
Joe kindly advised me to check for 0 Ohms between Internal 0 and metal case with power supply unplugged, which I did check and it was fine.
But when I plugged a cable into DI input then the hum got lot worse. So I decided to open it up and disconnect both DI-s.
The hum was then gone.
But I obviously made a mistake while disconnecting DI boards and now my first channel doesn't work at all. I can hear it crackles a little when I turn the volume knob and I can hear some noise when gain is at the max but no mic signal neither from condenser neither from dynamic mic.
So if anyone can help, my questions are why the DIs are causing the hum, and what could have I do wrong at the input to disable signal coming in. (I've checked the cables and connectors of course, they look fine) I'm writing right after a long recording session sorry for eventual mistakes.
Here is the picture before disconnecting the DIs
(you'll noticed that second channel DI is (was) connected via its own side connectors on the board for power and signal, because back then, I soldered its ribbon connector wrongly and then damaged the pcb too much to solder it back so I connected it to be active permanently.)
Thanks in advance, cheers
recently my dual mic preamp/DI BA with 1:4 JLM input transformers and 1:1:1 output transformers and JLM 99V opamp started to output some hum which sounded like a ground noise at around 50Hz. (for couple of years was working flawlessly)
I could hear it on headphones and when gain is boosted a little higher, but with condenser mic it was fine as it was too low comparing to signal.
Joe kindly advised me to check for 0 Ohms between Internal 0 and metal case with power supply unplugged, which I did check and it was fine.
But when I plugged a cable into DI input then the hum got lot worse. So I decided to open it up and disconnect both DI-s.
The hum was then gone.
But I obviously made a mistake while disconnecting DI boards and now my first channel doesn't work at all. I can hear it crackles a little when I turn the volume knob and I can hear some noise when gain is at the max but no mic signal neither from condenser neither from dynamic mic.
So if anyone can help, my questions are why the DIs are causing the hum, and what could have I do wrong at the input to disable signal coming in. (I've checked the cables and connectors of course, they look fine) I'm writing right after a long recording session sorry for eventual mistakes.
Here is the picture before disconnecting the DIs
(you'll noticed that second channel DI is (was) connected via its own side connectors on the board for power and signal, because back then, I soldered its ribbon connector wrongly and then damaged the pcb too much to solder it back so I connected it to be active permanently.)
Thanks in advance, cheers