Why are they doing this?

Just to let you know about REL subs as I have a pair of Stentors III connected to monoblocks. I use the red and yellow together for each sub on the + terminal of both right and left monoblocks so each Stentor is connected to one monoblock each. I do not connect the black wires. You only need to connect the black to ground if you have a hum.

01-31-14: Coxhaus
... You only need to connect the black to ground if you have a hum.

While with some amplifiers and subs not connecting the black wire will work, in general I would strongly recommend against it. As I'm sure everyone here realizes, for current to flow a complete circuit is required, in this case from the amplifier to the sub and back to the amplifier. Without the black wire being connected, instead of being through that wire the return path would be from the sub's circuit ground via some design-dependent impedance to its AC safety ground, then through the AC power wiring to the amplifier's AC safety ground, then through some design-dependent impedance to its circuit ground. In addition to creating a susceptibility to pickup of low level hum and noise, I would expect that to result in at least a few cases (depending on the internal grounding configuration of the sub and the amplifier) in hum that is loud enough to be destructive.

And particularly so in the situation where there is initially a low level hum, which the user then sees fit to try to eliminate by putting a 3-prong to 2-prong cheater plug on the power cord of one of the components. Thereby leaving the grounds of the sub and the amp "floating" relative to one another, with AC "leakage" paths (e.g. in power transformers) being the only means of signal return. I would not want to be anywhere in the vicinity in that situation.

Regards,
-- Al

Al, you responded to Albert's point about connecting a self powered sub to the linestage/pre as follows:

"[C]onnecting at line-level, which would require purchasing additional interconnect cables, possibly purchasing splitters and introducing them into the signal path, and possibly spending several hundred dollars or more on a buffer, while losing the possible sonic benefit of providing the same signal to both the subs and the main speakers, and introducing the possible sonic effects of the splitters or buffer, if needed, and of the additional interconnect cables ...."

Of course I get your points, which are fair. Just a few clarifications please. First, in my case, I suspect the sonic downsides may be acceptable because my linestage has two (2) XLR Main Outputs -- one for the power amp and the other for whatever turns you on. In my case, it's the self-powered sub. So the linestage's output signal did not have to be split. Instead, as mentioned below, it needed to be summed.

Second, while there may be some qualitative sonic costs to the hook-up just described, I suspect that it's minor because my sub is configured to blend into the fronts at about 40 Hz and below, which is the roll off point of the fronts. So .... the main signal from the linestage is direct, i.e., no artifacts, to the power amp and then of course to the speakers.

Third and last, I had no choice. My sub is NOT designed to accept an input signal from the power amp. Hence, a hook-up of some sort had to be made from the linestage/pre.

As you may recall from my posts about this issue some years ago, my impedance buffer had to solve three (3) problems at one time: (a) asymetrical loading of the linestage (i.e., XLR -- linestage to amp; RCA/SE linestage to sub); (b) summing the L and R channels without shorting the linestage's Main Outputs because I am running a single sub; and (c) increase the impedance presented to the linestage or else chance overloading the linestage's outputs, thereby causing possible sonic degradation.

Ergo, a custom impedance buffer designed and made by Tom Tutay (located in Ft. Walton Beach, FL). Considering Tom's fee, which was extraordinarily reasonable, he solved a lot of problems for a paltry sum indeed.

Cheers,

Hi Bruce,

Yes, of course I well recall the solution you arrived at, revolving around Tom Tutay's buffer. And it is certainly an excellent solution in your case.

What precipitated my comments that you've quoted is that the wording of your initial post above, the perhaps also Albert's initial post following my response to it, might have been construed by some to be implying that Mrschret should abandon the speaker-level approach he was trying to implement, and go for a line-level approach instead. You perhaps meant no such implication, and Albert subsequently made clear that he didn't, but your and his initial wording, it seemed to me, could easily have been interpreted that way.

Given particularly that your sub is pretty much just filling in the bottom octave, and assuming that your interconnect cables at the preamp outputs are not especially long and do not have particularly high capacitance per unit length, I see nothing less than ideal about your setup. (The reason I mention the last point is that if the two XLR outputs are both used, but are not individually buffered within the preamp, the interaction of preamp output impedance with the TOTAL of the capacitances of the cables attached to BOTH sets of outputs may affect the high frequency components of the signals sent from one of those output pairs to the main power amp).

Best,
-- Al

Thanks Al. I suppose my first post had more to do with my surprise that the OP's sub could be hooked up to the power amp outputs. Admittedly I am not familiar with his sub. I thought most subs are self powered like mine. But even if not, if this were my rig, as a threshold matter, given the complexity of optimizing amp and speaker compatibility, I would have been somewhat circumspect about throwing another load into the mix - namely a sub.

You raise a couple of interesting points about my current interconnect hookup. In fact, the Ref 5's Main Outputs are not buffered. That's why using a "Y" connector to sum the L and R channels was not an option, ergo one of the little problems that Tom's buffer addressed.

The other point deals with combined output capacitance. Certainly, the combined resistive impedance side is covered. Just checked the ARC web site about the capacitance side of the house. ARC advises that maximum capacitance should not exceed 2000 pF.

In my case, the Ref 5 Main One I/C run to the amp is about 10 feet. The other I/C run to the buffer is about 5 feet. Not sure how combined capacitance is calc'd in this context, but even if its summed, to exceed 2000 pF would mean that capacitance per foot would be roughly 135 to 140 pF per foot. That impresses me as quite a bit. I'll double check the specs on my Kimber Heros and come back if an issue. But I suspect not.

Thanks Al.

Cheers,