External enclosure power transformer safety ground connection

[fripholm]

Hi all

I've just finished my version of a discrete FET based mic preamp. It's running from a bipolar +/- 17V power supply which is inside the unit. For the lack of space in the main enclosure I decided to put the 2x18V power transformer in a separate metal box. The AC output of the power transformer including its center tap goes to the preamp using 3-wire cable. At the moment safety ground is tied directly to the metal box but there is no direct connection to the center tap which eventually becomes GND in the preamp. See the diagram for clarification.

Using this setup the preamp has extremely low noise and there is no hum at all but I'm wondering if it's a good idea safety-wise to NOT connect safety ground to the center tap and with this to the main chassis. Most of the grounding scenarios I've seen connect the safety ground directly to chassis when using just one enclosure but what happens if there are two?

 

[fripholm]

Here's a shot of the finished pre 

 

[PRR]

In general, all touchable metal should be bonded back to the wall-outlet 3rd pin for safety.

Strictly you should run a 4th conductor from lump to preamp.

I'd think it would be OK to bond the PT CT to PT chassis and let the CT be the ground.

 

[Andy Peters]

In the box with the transformer, you need to tie the secondary center tap to the chassis, which is connected to the mains safety ground. This establishes that point where it all comes together. Otherwise, the audio common floats with respect to the chassis, and that's bad.

And, as PRR suggests, running a fourth wire, call it "chassis," from the transformer to the preamp enclosure ensuresthat the preamp chassis is grounded in a shield sense. You could probably make do with a three-conductor-plus shield cable between the transformer box and the preamp box. Preamp XLR pins 1 all go to the chassis, nowhere else.

-a

 

[fripholm]

Thanks guys. Will try a 4th wire.