HELP! Blown ground wire in two pre-amps

OK I'm not sure where to ask this so I'll start here. Background: I'm running an Oppo into Conrad-Johnson pre-amp, out to Mac 601's, then to two Rel Subs and lastly into Revel Ultima Salon 2's. Apparently something in the system blew out the ground in the pre-amp (actually 2 different CJ's: one a solid state the other a tube). Both pre-amps had been working fine on different occasions (yeah I "tinker" with components). I'm using a different pre-amp now and don't currently have a problem (pun intended). Any ideas on what might do something like that, or what I should check in the system? The room is in an older part of the house so the plugs may not be grounded, but I don't know if that would be the source of the problem.  I think I'm going to have an electrician out here to check the plugs and ground them if they are not grounded (they have the prong input - I don't think a ground was connected), but I'd like to be sure it's not something else. I'm not all that technical so any of you electrically educated folks sound off and help me here. I appreciate any advice you guys/gals may have.
Ag insider logo xs@2xnab2

First, some further information would be helpful:

1) Please clarify exactly what you mean by "blew out the ground."

2) It appears that you are using the speaker-level inputs of the subs, and connecting them to the outputs of the amps. Please indicate exactly which terminals on the MC601 the red, yellow, and black wires from the corresponding sub are connected to. Properly connecting a powered sub to the outputs of an amp having autoformers, like the MC601, can be tricky.

But pending that information it seems to me that the problem may be caused by a combination of two factors, namely the possible lack of a ground on the outlets that you mentioned, and the manner in which the subs have been connected.

I’m guessing that you are connecting the red and yellow wires from each sub to one of the + output terminals of the corresponding amp (which is fine), and the black wire from each sub to the corresponding – output terminal of the amp (which together with the possible lack of a ground on the outlets may be causing the problem, as I explain below). For example, you may be connecting the red and yellow wires to the 4 ohm + terminal and the black wire to the 4 ohm – terminal.

If the AC outlets do not have their ground pins grounded then what may be going on is as follows:

Given that the amp uses autoformers at its outputs, and provides separate + and - terminals for each of its three taps (8, 4, and 2 ohms), the – output terminal of the amp that is being used is probably not connected to the amp’s circuit ground, and when music is playing it probably has a large signal voltage on it relative to the amp’s internal circuit ground (which in turn is connected to the preamp’s internal circuit ground via the interconnect cables). The sub’s black wire that is connected to that terminal is almost certainly connected to the sub’s internal circuit ground, which in turn is most likely connected within the sub to the safety ground wire of the sub’s power cord through some low resistance. The sub’s AC safety ground is connected to the preamp’s AC safety ground via the outlets (or via a power conditioner or power strip if you are using one). If the AC safety grounds of the outlets are in fact not being grounded the large signal voltage that is being applied to the black wire of the sub’s speaker-level input when music is playing would then cause a large voltage to develop between the preamp’s safety ground and its internal circuit ground, causing the damage.

So in addition to having the electrician check the wiring of the outlets, it may be a good idea to contact McIntosh and ask for their recommendation as to where on the amp the sub’s black (ground) wire should be connected. The answer may be that it should be connected to a chassis screw on the amp, not to a – output terminal.

-- Al

Quick response: First: Thanks! Second: I like your reasoning. The subs are hooked up per Rels instructions which are as you suggest. As for the action of the sub back into the preamps: that really sounds right. The preamp's ground acted as a fuse protecting the rest of the circuitry of the preamp (apparently). I will also check with McIntosh.

Thanks again Al!

Any other thoughts are welcome!