Ground Loop & how to connect shields...

[jilski]

Hey people,

I've read that great article about connecting grounds and star ground systems on the Rane website and built a little project, a parallel high-shelf filter similar in concept to the kush audio clariphonic.

Well, I've had this thing in use for a while and everything seems nice, except for one weird thing: When I screw the metal case together and screw it to the rack rails, I get that loud 60hz hum. As soon as the unit is on a table or if I don't really fasten all the screws in the enclosure, it works flawlessly.

Here's a picture of the unit open:

The white daisy chain at the pins 1 of the xlr connectors leads to  the bottom left corner of the circuit board, which is the star ground center. Both the power supply ground and the audio ground connect there (and only there). However, on the picture, I did not connect that to the chassis. Theres that extra pin on the xlr sockets that Im using and the Rane paper suggested not to connect that to anything, so I didnt.

Since I thought that might be the problem, I tried connecting the same ground to chassis, but depending on where I connected it, to one of the screws of the xlr connector, or to a switch that has contact with the chassis, the hum gets a bit better, or worse, but it comes back as soon as I screw everything together so the different panels connect to each other.

I know this might be hard to debug, but maybe I'm missing some essential info here or something I'd like this unit to be working flawless, even if I screw it together proper and tight... right now half of the screws are loose and it won't hum.

 

[JohnRoberts]

It sounds like your chassis ground is corrupting your audio signal.

I can't tell much from your picture but it doesn't look like enough wires coming from the XLRs. There should be 2 audio wires each; audio + (pin 2) and audio - (pin 3), with pin 1 tied to the chassis at each jack.

If you just tie all the chassis grounds and audio low leads together, that is like mixing the drinking water with the sewage. The sewage doesn't care but the drinking water is compromised. 

JR

 

[Audio1Man]

Looking @ the photo and your notes I see some things that it may be.  John has touched on some of my comments.

1. A full schematic would help to resolve any issues.
2. What is the GAIN of the circuit?
3. Is the chassis painted? It may not be fully common ground and the cover makes a ground to some parts of the circuit.

4. XLR'S Pins #1 & #4 strap and chassis ground with star lug (Short wire lead)
5. XLR pin #2 & #3 twisted leads to inputs. #2 Hot and #3 Low common ground.
6. XLR pin #2 & #3 twisted leads to outputs. #2 Hot and #3 Low common ground.
7. The AC or DC power input must be floating. If it is connected to the chassis change the connector type.
8. The wiring of the PS can be a problem.
9. Show the single ground spot to the chassis.
Duke

 

[ruffrecords]

Check this out:

https://drive.google.com/open?id=0B_n67A1hN3qtSzNkQUhhdHR1TkE

Cheers

Ian

 

[lassoharp]

but maybe I'm missing some essential info here or something

Just in case, since it wasn't stated - when the unit is in the rack, is there adjacent gear that's turned on while it hums?  I know I can't put my DIY Pultecs within 3 RU of my converters or they pick up crazy hum.

 

[gswan]

You may need to hard wire the frame ground (from the front panel) directly to the earthing star point. This is often done using the metal parts of pots which make contact with the panel.