I think neyman and jazzyvee are actually seeing two different problems here.
Neyman, if this noise problem occurs somewhat repeatedly after the bass has been in use for some time, that might actually be a problem with one of the preamps overheating. They don't draw much power and shouldn't get hot to where it affects the sound, but it's not impossible. The interesting thing for you to check is to look at the quality of output from your DS-5. If you can read DC volts between pins 4 and 5 (V+ and V-) you should see between 30-40 volts. If it's more than that, that may be the problem. I think this is unlikely to be the problem if your equipment has no mods, but with a homemade unit, or a unit operating with a different line voltage than it was made for, this may be a problem. The easiest way to test this is to open the DS-5 and trace back the wires that connect to pins 4 and 5. While you're there, you can also test to see if you have any AC voltage - it should be 0, even on the most sensitive range of your voltmeter.
If the voltage is too high on V+/V-, then you would want to take a look at the transformer to make sure it's the right part.
If the supply voltage to the bass it too high or noisy, then the preamps may overheat or show other problems. Unlike an overheating power amp, they will probably return to normal seconds after you unplug the power supply, which is why a quick change to the 1/4 plug (an 18V power) seems to be an immediate fix.
You might also check to make sure that that 20K resistors that sum the individual outputs to mono are both present. If you connect the outputs directly, this shorts out the internal preamps which would be another condition that could cause high power draw (in this case, it's more likely that you'll get no output at all).
The classic way to test this problem is to wait until the problem is occuring (it may not happen when you're not actually playing the bass, although that's an interesting test too). Do this with the backplate that has the EQ board off the bass. When the humming is happening, you want to shoot the three preamp chips with a can of cold spray or even just one of those canned air bottles. If the hum goes away when you spray the chip (do each one separately), then this is the chip that's causing the hum. But the problem is more likely to be the power supply than the preamps, I think.
Jazzyvee, I think you're more likely to be suffering from a bad cable or bad connector. You've only got noise when the audio is travelling through the 5-pin cable, so I think this is much more suspect. I would also check those 20K summing resistors to make sure they're properly in place. A problem there might explain what you're seeing with the Stereo/Mono switch.
The Alembic stuff is quite different than what most techs would have seen in the wild, and rare enough that they have nothing to compare it to. If you work with active EMGs (which also have individual preamps in the pickups), you don't need the summing resistors, so they might have changed the values or left them out when servicing the box.
Hope this helps,
David Fung
