Component grounding

@atmasphere  -- No no, I meant the preamp section of the integrated. It has a feature that let's you bypass the preamp section and use the output control on the DAC instead. When I did this, the hum came back. I don't know what that means, but you're right, the issue must reside with this particular amp because I've never had a hum otherwise. 

@williewonka  -- Exactly. When I unplugged the amp from the Shunyata (using the non-modified power cord) and plugged it into the free receptacle on the wall duplex (the Shunyata occupying the other receptacle) the hum disappeared. So that would suggest . . . what? That the isolated zones on the Shunyata are not so isolated after all? 

The neutral side of any circuit should be at zero volts, but very often the neutral side may have some residual voltage due to the design of the circuit. If this voltage is not "wicked away" to a proper ground then connecting that component to another component may introduce hum.

In your case it would appear that the neutral of one of the components plugged into the shunyata is not at zero volts and the "isolated zones" on the shunyata are doing a great job at isolation (even the ground), but this allows the component with the non zero neutral to propogate the hum via the interconencts.

Can you plug everything into a single Zone?

Or perhaps experiment with different groupings of components in each zone e.g.

- pre-amp and source in one zone power amp in another

Try different permutations to see what works

Hope that helps - Steve

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This got me thinking. Several months ago I opened the DAC to try a specialty fuse. To free the cover, you have to disconnect the chassis ground.

--As I wrote earlier, when I disengaged the preamp section on the integrated and engaged the output control on the DAC, the hum came back. Also, over the last few weeks as I've been trying different combinations in order to try to identify the culprit, I noticed that the hum kicked in when I connected the interconnects from the DAC to the integrated. It didn't happen when the components were merely plugged in and powered on. (Although this still doesn't explain why the hum didn't happen with either of the other two integrateds that I tried.) --

Anyway, this morning I decided to open the DAC again to check whether I reattached the ground chassis correctly. I discovered that there was no shoulder washer. Whether I lost it or it was never there, who knows. So I installed a nylon washer and powered up again. 

No difference. Still a hum. 

I also tried connecting the integrated and DAC to a single zone. No difference. 

So perhaps the integrated has a bad chassis ground?