I have just registered to this forum after Google brought me here. I have read a few posts that seemed related, but they did not offer useful info. (I am sorry if I posted this in the wrong section though.)
I have a DAP Audio IMIX 7.1 in my set-up and something appears to be not quite right with it.
This mixer is powered by a transformer (which delivers 2 x 18V AC), the transformer only having a two prong power plug. The IMIX is in a rack with other units (Roland U220 that is powered by a two-prong plug, and an E-MU Vintage Keys and E-MU Proteus2 powered both by three-wire plugs to properly grounded socket). Due to the conducting nature of the metal chassis and the metal rack all these devices grounds are all interconnected.
The problem started by noticing that there is a ground-loop-hum in the output and the LED VU meter of the mixer was not working. I discovered that when I remove the IMIX from the rack, i.e. its chassis not connected to the common ground, everything works fine.
Before I removed it, I first wanted to see what happens if I unscrew it, breaking the ground connection to the chassis, and as the unit was still turned on, I noticed sparks! I have dabbled enough with electronics to know that is not a good thing. If not connected to the chassis, the audio output is without any noise and the LED VU meters are also functional.
So the next thing I did to diagnose this problem, I connected a voltmeter between the chassis of the IMIX and the rack, and I am measuring a 32V AC voltage potential. That seem, literally, quite shocking.
To eliminate the idea that the problem is caused by another unit in the rack, I turned them off one by one to see if the problem disappears. It didn't. Interestingly, the 32V AC can also be measured when connecting any of the audio signals from *any* units in the rack -- even with all the other units turned off and even their plugs removed from the socket. What the heck?
I am surely missing something as this doesn't seem to make sense to me. I would really appreciate a bit of insight as to what is exactly going on or why this could happen, and what a next step would be to try to figure this out.
(Obviously, a work-around to get a properly functioning mixer with a clear signal and LED VU meters, would be to isolate the ground of the device from the rack -- but then there would still be a 32V AC potential to ground from its chassis which doesn't feel right to me at all.
)
Thanks in advance for any responses.
Best regards,
Hugo
(Edited to add the transformer voltage of 2x18V AC -- which seems suspiciously close to the 32V AC that I'm measuring.)
(Edited again to add, that after some more reading and searching, I happened to come across this remark note in totally different DAP Audio mixer manual:
"Equipment which has unbalanced inputs and outputs may need to be isolated from the rack to prevent earth loops.".
Does this imply that I should just simply indeed isolate it from the rack and be done with it and don't care about the 32V AC?)
I have a DAP Audio IMIX 7.1 in my set-up and something appears to be not quite right with it.
This mixer is powered by a transformer (which delivers 2 x 18V AC), the transformer only having a two prong power plug. The IMIX is in a rack with other units (Roland U220 that is powered by a two-prong plug, and an E-MU Vintage Keys and E-MU Proteus2 powered both by three-wire plugs to properly grounded socket). Due to the conducting nature of the metal chassis and the metal rack all these devices grounds are all interconnected.
The problem started by noticing that there is a ground-loop-hum in the output and the LED VU meter of the mixer was not working. I discovered that when I remove the IMIX from the rack, i.e. its chassis not connected to the common ground, everything works fine.
Before I removed it, I first wanted to see what happens if I unscrew it, breaking the ground connection to the chassis, and as the unit was still turned on, I noticed sparks! I have dabbled enough with electronics to know that is not a good thing. If not connected to the chassis, the audio output is without any noise and the LED VU meters are also functional.
So the next thing I did to diagnose this problem, I connected a voltmeter between the chassis of the IMIX and the rack, and I am measuring a 32V AC voltage potential. That seem, literally, quite shocking.
To eliminate the idea that the problem is caused by another unit in the rack, I turned them off one by one to see if the problem disappears. It didn't. Interestingly, the 32V AC can also be measured when connecting any of the audio signals from *any* units in the rack -- even with all the other units turned off and even their plugs removed from the socket. What the heck?
I am surely missing something as this doesn't seem to make sense to me. I would really appreciate a bit of insight as to what is exactly going on or why this could happen, and what a next step would be to try to figure this out.
(Obviously, a work-around to get a properly functioning mixer with a clear signal and LED VU meters, would be to isolate the ground of the device from the rack -- but then there would still be a 32V AC potential to ground from its chassis which doesn't feel right to me at all.
)Thanks in advance for any responses.
Best regards,
Hugo
(Edited to add the transformer voltage of 2x18V AC -- which seems suspiciously close to the 32V AC that I'm measuring.)
(Edited again to add, that after some more reading and searching, I happened to come across this remark note in totally different DAP Audio mixer manual:
"Equipment which has unbalanced inputs and outputs may need to be isolated from the rack to prevent earth loops.".
Does this imply that I should just simply indeed isolate it from the rack and be done with it and don't care about the 32V AC?)
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as there is hardly a resistive current path to gnd a small current creates a large voltage.
so instead of measuring the voltage, you can measure the current by setting to meter to AC mA/A range. it will be quite small. this current however flowing through the ground summing rail of you mixer can still create a hum.
most of the ground current is created by switching power supplies that have relatively large filtering caps to ground to pass the EMI standards. that current has a noise switching naure, easily picked up by the mixer's summing rail.
so instead of measuring the voltage, you can measure the current by setting to meter to AC mA/A range. it will be quite small. this current however flowing through the ground summing rail of you mixer can still create a hum.
most of the ground current is created by switching power supplies that have relatively large filtering caps to ground to pass the EMI standards. that current has a noise switching naure, easily picked up by the mixer's summing rail.
No, that´s irrelevant, just chance.
OK
ground loop? Ok, a common problem.
VU Meter not working?
Weird. Completely unrelated circuits. Are you feeding signal into mixer? From which source? Try a "floating" one such as a Smartphone.
Certainly but not so tiny current through mains filtering can cause small sparks.
What do you call chassis? Rack rails?
What else is mounted on that rack?
Are they connected to mains?
Turning them OFF does NOT disconnect them from Mains, at all.
Not too clear.
I would dismount all units from rack, put them side by side but not touching each other on , say, a wooden table, not interconnected at signal level so no possibility of joining Grounds that way, plug all into Mains, all OFF, and measure voltage between Mixer and each unit and also between them.
A picture will make layout clear.
Then turn all ON and repeat test.
Please post results.
Please don´t.
PS: as MAAC suggested, verify continuity between 3 pin power supply ground (or center tap) connector on Mixer back panel and its chassis.
I expect continuity (as ion near zero ohm) but, who knows?
I would also measure voltage present, if any, between that power supply connector (any pin) and a real confirmed ground.
Be careful, a multimeter probe may slip and short it.
Also measure voltage (if any) between every chassis and a confirmed Ground.
Asd you see, we are testing all possible combinations
This really sounds like the power supply may have slightly excessive capacitive coupling across the mains transformer. It's only 25 VA, but who knows what the thing is built like. I'd try turning the mains plug around and seeing whether that makes any difference.
Given all the unbalanced ins and outs on the mixer, I'm not surprised it would be kept floating by default. Not sure what the point of a rackmount mixer with loads of unbalanced I/O would be - once the case gets grounded by the installation you'll be inviting lots of ground loops from devices that aren't floating by themselves. I presume that's where the OP's hum issues were coming from.
I think this mixer is only useful if you have lots of CD players, keyboards and other such floating, standalone devices. In a rack I'd expect lots of ground loop issues.
Given all the unbalanced ins and outs on the mixer, I'm not surprised it would be kept floating by default. Not sure what the point of a rackmount mixer with loads of unbalanced I/O would be - once the case gets grounded by the installation you'll be inviting lots of ground loops from devices that aren't floating by themselves. I presume that's where the OP's hum issues were coming from.
I think this mixer is only useful if you have lots of CD players, keyboards and other such floating, standalone devices. In a rack I'd expect lots of ground loop issues.
Do you connect all the following stages via balanced XLR cables?
Only strict balanced operation will not hum. Once you go unbalanced into an amp or xover it cancels the common mode noise suppression and hum will arise.
The unit is meant to be grounded via rack rails, when you isolate it you will get some tickling sensation when touching. You might also damage any input devices if they come in contact with ground.
Only strict balanced operation will not hum. Once you go unbalanced into an amp or xover it cancels the common mode noise suppression and hum will arise.
The unit is meant to be grounded via rack rails, when you isolate it you will get some tickling sensation when touching. You might also damage any input devices if they come in contact with ground.
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