BA preamp ground hum on DI and other problems

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Hwidow
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Joined: Mon Mar 02, 2015 8:27 am

BA preamp ground hum on DI and other problems

Post by Hwidow »

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
Djani BA 50Hz hum 1 .JPG

(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
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Hwidow
Posts: 10
Joined: Mon Mar 02, 2015 8:27 am

Re: BA preamp ground hum on DI and other problems

Post by Hwidow »

Update: After I reviewed few things with Joe's help both of my mic pre's are working fine. The DI-s on both channels are making horrible noise when I plug the guitar into them, no it's not my playing unfortunately :) Guitar was plugged in but not played. Instrument is passive and I've tested with another guitar, the active one and few different set of cables, always same result.
I've made a few short recordings of that noise, with me turning gain from zero to max, but cannot attach mp3 files here.

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Joe Malone
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Re: BA preamp ground hum on DI and other problems

Post by Joe Malone »

Hwidow wrote:Update: After I reviewed few things with Joe's help both of my mic pre's are working fine. The DI-s on both channels are making horrible noise when I plug the guitar into them, no it's not my playing unfortunately :) Guitar was plugged in but not played. Instrument is passive and I've tested with another guitar, the active one and few different set of cables, always same result.
I've made a few short recordings of that noise, with me turning gain from zero to max, but cannot attach mp3 files here.
HI Dani
The second DI CANNOT be wired directly as you have done. It must switch from mic in to DI in otherwise it will overload and be affected by phantom power.

Fully discount all wires to the the DI without the IDC header and test the DI on chancel one plugged into channel one and then into channel two and see if work ok in both channels without noise.

Fix second DI tracks and refit IDC header and use ribbon cable to connect to channel two and you should be working if no fets on the DI have been damaged.
Joe :-)
JLM Audio
Capturing Audio without Injury

Hwidow
Posts: 10
Joined: Mon Mar 02, 2015 8:27 am

Re: BA preamp ground hum on DI and other problems

Post by Hwidow »

Tested that right now, disconnected second DI and tested only the first channel, the noise, this time less loud is still there.
Happens on both channels when I switch the DI 1 into preamp board 2 connector. Wonder how did it worked ok for so long like that, hooked directly onto XLR connection.

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Joe Malone
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Re: BA preamp ground hum on DI and other problems

Post by Joe Malone »

Hwidow wrote:Tested that right now, disconnected second DI and tested only the first channel, the noise, this time less loud is still there.
Happens on both channels when I switch the DI 1 into preamp board 2 connector. Wonder how did it worked ok for so long like that, hooked directly onto XLR connection.
The only things left is the 48v power supply has either gone very noisy or 0v is not on the case.

Have you tried cleaning up the wiring and putting the lid on an bolting it into a rack to see if it is ok when it is back in one piece?

Do any of the resistors on the DI look burnt slightly?
Joe :-)
JLM Audio
Capturing Audio without Injury

Hwidow
Posts: 10
Joined: Mon Mar 02, 2015 8:27 am

Re: BA preamp ground hum on DI and other problems

Post by Hwidow »

Hi, thanks for the input
All resistors on the DIs look clean and like new.
All is the same when lid is screwed back, (this preamp isn't in a rack, it's usually on the table top, near console, outputs hooked to a fully balanced patch bay).
Just to make it clear, now this roaring noise is present only when I plug the guitar into DI on ch 1, (DI on ch2 is disconnected)
The Mic Pres are pretty clean and silent even when gain is maxed with condenser mic, and very little noise present when gain is maxed with dynamic mic.
So I conclude that mic press are working fine.

And I've noticed that same noise is present on mic pre in first few seconds when gain is boosted from zero up, then it disappears.

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Joe Malone
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Re: BA preamp ground hum on DI and other problems

Post by Joe Malone »

Hwidow wrote:Hi, thanks for the input
All resistors on the DIs look clean and like new.
All is the same when lid is screwed back, (this preamp isn't in a rack, it's usually on the table top, near console, outputs hooked to a fully balanced patch bay).
Just to make it clear, now this roaring noise is present only when I plug the guitar into DI on ch 1, (DI on ch2 is disconnected)
The Mic Pres are pretty clean and silent even when gain is maxed with condenser mic, and very little noise present when gain is maxed with dynamic mic.
So I conclude that mic press are working fine.

And I've noticed that same noise is present on mic pre in first few seconds when gain is boosted from zero up, then it disappears.
HI Dani
I would fix the 2nd fet DI so you can see if it does the same thing as the other DI.

Do you know anyone who would have a spare 48v smps to swap too as I think this will be the problem?

Or remove the link in the BD681 behind the switches and put in a 47 to 100ohm resistor in place of the link and see if the DI is then quiet noise wise?
Joe :-)
JLM Audio
Capturing Audio without Injury

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