Hi Richard,
I suspect the reason floating the ground wire doesn’t work in this case is that the inputs of your amp are transformer coupled, so a ground path is not established between the sub and the amp via the preamp, since the preamp’s ground is isolated from the amp’s ground via those transformers. If the grounds of the amp and preamp were not isolated, as is usually the case with most other designs, a ground path having reasonably low impedance would be established between the sub and the amp via the preamp-to-amp interconnections in combination with the AC safety ground wiring of the preamp and the sub.
For the same reason connecting the black wire to a ground point on the preamp would presumably not work either.
While Lalit has very graciously offered to let you try the speaker-to-line level converters he will be receiving, I don’t think it would be safe to use them with your particular amp. I would envision that most such converters would attenuate the level that is provided to their signal input, but would route the ground that is provided to their input directly to their output. Since your balanced amp has full-amplitude signals on both its + and - output terminals (rather than the amp’s - output terminals being connected to its circuit ground), the result of routing both output channels of your amp through such converters to the left and right channel RCA input connectors of a single sub would almost certainly be that the full amplitude signals on the negative output terminals of the two channels are shorted together by the sub’s internal ground. Definitely a no no.
Given that the amp does not provide RCA input connectors, or any other circuit ground connector, the one **possible** alternative I can envision to attaching the sub’s black wire to a chassis screw, as Jeff suggested, would be to solder the black wire to the ground shell of a 1/8 inch mini-plug, while leaving the signal pin(s) of that plug unconnected, and inserting that plug into the amp’s remote on/off jack.
This assumes three things, though:
1) It assumes that the ground shell of the remote on/off jack is internally connected to the amp’s circuit ground. You would need to confirm that with Jeff.
2) I suspect that it wouldn’t matter whether the mini-plug is a stereo type or a mono type. Again, though, that should be confirmed with Jeff.
3) It assumes that you presently have nothing connected to the remote on/off jack. Although if you are presently using it you could probably solder the sub’s black wire to the ground shell of the plug that is being used, if assumption (1) is confirmed by Jeff.
Good luck. Best regards,
-- Al
