... maybe the manufacturer is refering to hooking up a device that grounds its negative wires of the left and right channels to the chassis, would be a problem for a balanced amplifier. I’m just guessing.Yes, one of the two major potential issues is along those lines. I’ll discuss that issue first, but be sure to see the paragraph near the end of this post that begins with "HOWEVER" for a very important point regarding the other one.
In good designs circuit ground (also referred to as signal ground) will often be connected to chassis through a low value resistor, commonly somewhere between 10 and 100 ohms. In some cases circuit ground and chassis may even be connected directly together (i.e., through zero ohms), although that is poor practice because it results in susceptibility to ground loop issues.
If the component has a three-prong power plug chassis will usually be connected to AC safety ground.
And as you realize a fully balanced amp has a full amplitude signal on its negative output terminal as well as on its positive output terminal. (And in all fully balanced designs I am familiar with that provide RCA as well as XLR inputs, that is true even if the RCA input is used).
So in cases where BOTH the sub and the amp have three-prong power plugs, connecting the negative output terminal of a fully balanced amp to the negative speaker-level input of a sub (which is presumably connected to the sub’s circuit ground in most cases) would result in the signal that is present on the amp’s negative output terminal being sent through some unknown but probably low impedance in the sub to the sub’s chassis, then to its AC safety ground connection, then through the power wiring to the amp’s AC safety ground connection, then to the amp’s chassis, then through some unknown but probably low impedance in the amp to the amp’s circuit ground. Resulting in the circuit in the amp which drives its negative output terminal probably being loaded by somewhere between 0 ohms (a direct short) and 200 ohms. If that value is low enough, obviously the amp may either be damaged or forced into a protective shutdown mode. Also, one or both of the resistors might blow as a result of having to handle more power than they are rated to handle.
I’ve seen a number of cases here where people using REL subs have reported doing that, and getting away with it, presumably because the resistors I referred to were high enough in value to prevent a problem. (I seem to recall that one person measured the resistance between his REL sub’s negative speaker-level input terminal and its AC safety ground connection as being 100 ohms). But it is poor practice at best.
In your case, though, since the sub has a two-prong power plug you’ll most likely be ok ***in that respect,*** as I and Sfall have said.
***HOWEVER,*** it occurs to me that if you are connecting BOTH channels to a SINGLE sub a different issue is likely to arise. Namely that the negative signal of each channel would be connected to the circuit ground of the sub, and the two signals would thereby be connected to each other. A definite no-no!! (I hadn’t thought of that possibility when I wrote my previous post).
I don’t know if the amp manufacturer is making a warning to the connection of high level (speaker wires) or low level (rca) input connections to a sub, maybe either can pose a problem if the sub has a common ground.The caution in such cases just pertains to a speaker-level connection, not to a line-level connection.
Good luck. Regards,
-- Al