inverting summing amp grounding/wiring interconnect?

[rhythminmind]

I'm working on a summing 990 summing amplifier design, while testing i've become confounded by a grounding behavior. I'm hoping the GroupDIY masterminds might know why if I disconnect the input shields to my summing network all goes to hell. The network ground bus & chassis is grounded via the rack (tested) & is connected via output pin 1 to the summing amp grounds. All other hardware has audio & or chassis ground.
What I really don't understand is why the shield needs connection from the patch-bay to the summing inputs for the amp to be happy. Why isn't the network bus/chassis ground serving the same utility?

 

[rhythminmind]

But works wonderfully with this. I just want to fully understand before moving on.

 

[CJ]

grounding issues can be mystical and defy normal intuition,

so you experiment and find the best way,

sounds like you have it down with the shields connected, so run with that,

 

[bruce0]

I don't know about the "all goes to hell" part,

I am not sure about this...  but I see one potential problem.  I don't know if it is because you have not shown all of the schematic, or I don't understand the circuit.  The center of my question is, what keeps the 2 op amps putting out a signal that is anywhere near ground?  Slight input offset from any mixed component, and combination of various factors ... (waveforms/gain differences/slew rate) could easily get an out of control DC bias on the balanced pair.

Ungrounded and isolated the differential amps might handle it... but if you ground them, you are sending potentially large DC offsets into the next piece in the chain.

Is it a virtual earth circuit?

There are various approaches, is there a full schematic of the mixer.

Does the problem occur if you ground the equipment to the wall circuit?  Is all of the equipment cases grounded?

Can you put a voltmeter (not your fingers) between the equipment cases?

 

[nielsk]

In addition, your drawing shows the shield audio "buss" tied to chassis AND to the signal ground, which is also tied to chassis, making a ground loop