Two chassis preamp grounding

[Jaytor]

I am building a preamp with a separate power supply chassis. I searched for threads discussing this topic, but didn't find one that was a good match to what I am planning.

My power supply chassis will have a custom transformer to drive four dual-rail supplies (filtered, but not regulated) for analog circuits and one single-rail regulated supply for digital circuits. The IEC ground will be connected to the chassis ground. I am currently planning a 14 conductor umbilical so all supplies will have their own ground connection, so all these supplies will be floating with respect to chassis ground and to each other.

The preamp chassis will use four dual-channel UltraBib regulators - two for each channel (one supplying +/- 17v for Salas DCG3 amps, and one supplying +/- 10v for op-amp circuits).

I'm assuming I want to have a single star grounding point in the preamp chassis to provide a common grounding point for each of these separate circuits. Do I need to somehow connect the preamp chassis ground to the PS chassis ground? If so, I assume I will need a 15th conductor in my umbilical and connect through a ground-break circuit, but I'd rather avoid this unless it's really necessary since I'd have to use a bigger umbilical connector.

Thanks.

[mhenschel]

I'm grappling with a similar question only I figured on creating two (stereo) +/- feeds from four secondaries. To connect signal ground back to the PSU safety ground I came to the same conclusion you did: I'd need extra ground conductors.

Perhaps this will help?

Building a Pearl 2

[Jaytor]

Thanks for the reply and link. I want to keep the grounds for the separate supplies all isolated until they are connected to a common ground point in the main chassis to avoid a ground loop, so it seems like the safest approach is to add an additional connection in my umbilical that takes the star ground point back to the power supply chassis and connects to the PS chassis through a ground break.

[mhenschel]

In his Pearl 1 project Wayne chose to connect the signal and power ground planes together on the board. Just sayin'

[Phloodpants]

Earth ground the chassis of your power supply and preamp, but leave the transformer's outputs floating until they get to your preamp's power supply. The transformer secondary coils have no need to connect to anything except the inputs of your bridge rectifiers.

Since you have five separate secondary coils, you don't need to do a center-tapped bridge configuration, and there is no need to connect the coils to star-ground at all. Just use 5 bridge rectifiers, and connect their outputs straight to the filter caps. Then go to star-ground.

[Phloodpants]

Originally Posted by mhenschel

Perhaps this will help?

Building a Pearl 2

This is spot-on.

**EDIT Spot-on only in respect to how the bridges and caps are configured. DO NOT float your earth ground with a ground loop breaker as shown in this thread. Always connect earth ground DIRECTLY to the chassis, on it's own bolt with a lock-washer.

[Jaytor]

Yeah, that's what I'm thinking, with the star ground in the preamp chassis. The question I wrestling with is whether this star ground needs to be connected back to the power supply chassis through a ground break.

The preamp chassis will have a ground reference from connections to input devices and amps, so perhaps this is all that is needed.

[Phloodpants]

Nope, you'll just form a loop that way, from the preamp chassis, through this wire, out the chassis ground and back to the preamp through it's chassis ground.

[Jaytor]

I'm not sure if I'm missing something with the Pearl 2 reference. It looks like he is connecting the power supply ground to the chassis through a ground break, but otherwise passing it to the second chassis where it provides a reference.

In my case, should I connect all the power supply grounds together in the power supply chassis and just run a single ground connection to the preamp chassis? I was thinking it would make sense to keep them separate and only connect them at a common point in the preamp chassis.

[Phloodpants]

It might not actually show up as noise in the system, but it's currents that don't need to be flowing at all.