A technical question for this community

Whether or not a problem will occur depends on the internal grounding configuration of both the sub and the amp, particularly in terms of the impedance through which their internal circuit grounds and AC safety ground are connected.

By any chance does the M&K sub have a two-prong power plug? If so, you’ll most likely be ok since its circuit ground wouldn’t be connected to AC safety ground.

When a sub and a balanced amp both have three-prong power plugs, as in the case of most modern components, the usual recommendation for a speaker-level connection is to connect the sub’s negative input terminal to a circuit ground point on the amp, if possible, and if not to connect it to a chassis screw on the amp. Connecting to a chassis screw might result in hum, again depending on the internal grounding configurations of the components, but will usually work ok. A circuit ground terminal would be preferable, though, if one is provided on the amp. Or, alternatively, if the amp has an unused RCA input connector a circuit ground connection can be implemented by soldering the negative wire from the sub to the ground sleeve of an RCA plug, leaving the center pin unconnected, and inserting the plug into the RCA connector.

***However,*** your statement about connecting the sub "in and out" leaves me in doubt as to whether any of this would be applicable to what you are trying to do. I assume you mean that the amp would be connected to the sub, and outputs of the sub would be connected to the main speakers. If so, are those sub outputs high-pass filtered, or are they just a directly connected pass-through? Also, I’m not sure what you mean by "this setup currently is not being used in balanced mode." Can you clarify these points further?

Regards,
-- Al

Hi Almarg, the sub has a two prong power plug however the amp uses a three prong plug, maybe I'm ok there as you mentioned, thank you for that.

Yes I would be connecting  the speaker wires coming from the amp to the high level inputs of the sub and then the speaker wires would be connected to the high level ouputs of the sub.

On the other question, let me clarify, although the amp and preamp are a  balanced design, it is not being used in  balanced mode but just using the rca outputs of preamp to the rca inputs of the amp. I am not using the balanced feature of either the preamp or power amp. If this makes a difference.

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.

Almarg, 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.

" On the other question, let me clarify, although the amp and preamp are a balanced design, it is not being used in balanced mode but just using the rca outputs of preamp to the rca inputs of the amp. I am not using the balanced feature of either the preamp or power amp. If this makes a difference. "

If everything is running in SE mode, you're OK. Use the RCA or speaker cable connections. As for the 2 prong PC, you just have to try it. If you get a hum, then deal with it. Most likely, you won't have any issues.

... 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