Why are they doing this?

Al and Mrschret ... I also use a self-powered subwoofer which is connected to my linestage via a 330K ohm impedance buffer. As just stated, the sub is connected to my linestage ... not the amp.

Why can't Mreschret connect her REL sub to his/her pre/linestage?? Why mess with the amps.

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,

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,