Grounding for summing/mic pre/compressor unit

[Robotski]

Hi!
I'm trying to get a grip on the subject of grounding for my project, but there's too many contradicting advice around and I can't figure out what to do.
"Absolutely avoid having any ground conductor handling both signal and power" I've read many times, yet I've also read "pin 1 on input and output XLR's shall be connected to the chassi".
At the chassi I've connected my mains power (230v) GND, so what do I do with this information?
Also it's recommended to keep the L and R channels' grounds separate, so this is also contradicting...

My build project is a 3U rack case including:
- Input for 16 channels on two DB25, going to individual mute switches for 5
stereo channels and 3 mono channels, to
- 16 channel passive summing to stereo, to
- Dual input transformers, to
- Dual mic pre's, to
- Dual output transformers, to
- Gyraf GSSL bus compressor, to
- Balanced L/R outputs on both XLR's and TRS (I have just split the signal for
XLR/TRS with nothing in between, I think this is ok...?)

I'm too stupid for this, how the hell do I do proper grounding for everything?
I should keep audio gnd, power supply gnd, and mains gnd separate I've read, but at the same time I should connect XLR pin 1 to the chassi...? That doesn't make sense.

Thanks for reading, hope someone can help me with this!

Cheers,

/T.

 

[MarcelvdG]

You passively mix signals, then put them in a transformer and then in a microphone preamplifier? Sounds quite suboptimal with respect to noise.

As long as everything is balanced, just connect it according to the AES48 standard:

-Pin 1 (or whatever the ground pins may be called on the non-XLR connectors) grounded straight to the chassis
-Safety ground also connected straight to the chassis
-Chassis connected to the PCB ground at one and only one place

This doesn't prevent ground loops, but it prevents ground loop currents from flowing through the internal circuitry. All external interfaces are (hopefully) unaffected by ground loop currents because everything is well-balanced.

AES48 is a bit of a compromise between hum suppression and keeping the shielding intact to protect your circuitry from RF interference - but I assume you want both.

 

[Robotski]

Yes it's the input transformers for the preamps (JLM Neve clone), with VERY short wiring after the summing circuit. After passive summing there must be a preamp for gain make-up since the level is very low. Would you say it's better to omit this transformer because of noise levels?

But by connecting the audio inputs to chassis I'm connecting the audio gnd straight to the mains gnd... which is said to be bad since the mains gnd is not "clean" and therefor will cause noise.
What about doing this connection but inserting a resistor between audio gnd and chassi? I've heard about this practice somewhere.
Is this for the inputs only or is it the same for the outputs?

 

[MarcelvdG]

Regarding noise, I'm worried about the passive mixing, not about the transformer. It would be better noise-wise to give each microphone its own transformer and preamplifier and do the mixing afterwards. Then again, that would also be far more expensive and maybe the present set-up is good enough for you.

If you could make the whole thing double insulated, you wouldn't need mains earth at all.

 

[Robotski]

It's not microphones that will be mixed, it's balanced line level signals from studio outboard units like compressors, eq's and also subgroup bus outputs from my DAW.