Passive preamp grounding questions

I'm planning to build a passive preamp-- simple thing with just an input selector and stepped attenuator. It will have no active components and no gain. Though I think I will try adding Tangent's Linkwitz crossfeed (also passive) with an additional DPDT switch to optionally  bypass it completely.

My question is, where should I ground the enclosure to? On my amp, the case is connected to the common L+R input ground. But on the planned preamp, there will be multiple inputs. Should I connect all of the input ground wires together and tie the case to that?

Thanks.

Is there any advantage to going ungrounded rather than tying the case to the output ground?

What if I tied the preamp case to my PSU's case (i.e. AC ground)?

edit for clarity: The PSU is a σ11 I'm using with my headphone amp, it won't be supplying anything to the preamp.

Thanks again for your help.

Hello again, fellow DIYers.

So I finally finished the casework on my preamp and will be wiring it up next. But there might be a problem. My selector switch is a rotary 2P3T type. (And not 3P3T.) Meaning I can switch between the three pairs of L + R input channels but not the input grounds.

3 pairs of RCA jacks --> 2P3T selector --> (optional crossfeed circuit) --> stepped attenuator --> RCA output jacks

Now I think I could just tie all of the input RCA ground tabs together, and still have a functional preamp. But what are the potential problems of combining the input grounds?

I've done some googling and these are the relevant pages I found:

http://audiokarma.org/forums/showthread.php?t=70124

http://www.diyaudio.com/forums/parts/72652-selector-switch.html

http://gilmore2.chem.northwestern.edu/projects/showfile.php?file=cmoy7_prj.htm

Seems to suggest that combining the ground among the various inputs is okay, or even preferable sometimes. Just want to make sure there isn't something in my planned configuration that makes it an exception.

Thanks.

I've got it mostly all wired up now. And it works! Yay. My first DIY build that didn't come from a ready made design.

Some progress pics:

UPDATE:

This is mainly for the benefit of others who might search for this in the future. But of course comments are welcome.

I now see a disadvantage with combining the ground of all the inputs. If any one of my sources has a noise problem, then I will hear the noise no matter which source is currently selected with the rotary switch. For example, I tried connecting my Yulong D100, AMB γ2, and Creative Xmod to the preamp. Nothing really noticeable during normal listening, but at high volume levels with no music playing, I can hear noise. It was the loudest when I switched to the Xmod, and still audible with the other two. But after I unplugged the Xmod from the preamp, the noise was gone.

Anyway, this is not a serious problem. The noise is not audible at normal levels. And it's really the fault of the Creative Xmod. But maybe I will get a 3P3T switch so I can try toggling the input ground too.

Not related to the above, but I also grounded the case to the input/output ground channel.

Thanks. Okay, I managed to borrow a decent camera, so here are some more pics.

Here it is in between my M³ and σ11. The amp is topless because I'm currently working on adding better ventilation to the lid.

Lots of DIY interconnects at the rear.

In order to isolate all the RCA jacks from the chasis, I needed fairly large panel holes so that the shoulder washers could fit. These were larger than my hand drill could make, so I used a file. As you can see, my filing precision leaves much to be desired. :P

Interior of the preamp. Steel plate mounted with blu tack. Directly below in the σ11 PSU is the transformer. I didn't have any noise or hum, but the steel is there to put my magnetic interference paranoia to rest.

The RCA jack grounds are tied together with stripped hookup wire. I used Navships wire throughout.

The crossfeed is mounted onto the base of the chassis with a single standoff. The circuit's output ground is wired to the ground of the RCA jacks. The input ground is wired to the chassis. Both are directly connected on the circuit. I used two types of DPDT switches. The blue one (Mouser: 642-5646A) costs more and is larger. I used it for the master bypass switch. In hindsight, I should have used that for all three positions. The smaller red ones are harder to flip.

I used a 100K log stepped attenuator from Vintage Audio Labs (eBay) for volume control. The volume knob is temporary and was pulled out from my Creative Xmod. Still waiting for my eBay knob to arrive.

The selector switch is actually 2P5T, though only three positions are actually used. I got it from Mouser (part number: MB00L1NZQD). It's got a narrow 3.17mm shaft, so your standard volume knobs won't fit. I'm using a Kilo knob from Digikey.

Hope that helps.