do it the way you suggest--since you don't have speaker level ouputs on the preamps, you have to use the preamp rca outs to the low level input on your rel.
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You definitely do not want to use a Y-adapter to attempt to form a summed mono signal from the Left & Right unbalanced outputs of your preamp.
I assume your REL sub does not provide Left & Right line level inputs. Given your active speakers, I would try to find an NHT X2 filter (ListenUp.com may still have some) and high pass your main speakers. The X2 will provide a proper summed mono output for the sub.
If you can't find an X2, Hsu has a subwoofer crossover. I have 2 M&K bass management controllers (one unbalanced, one balanced) that I'm no longer using which would work.
Addendum to previous post... There are subs that have 2-channel bass management built into them. One good example is the SVS SP-12 Plus. I use one in my office system with active speakers. I use the balanced connections.
Why do I not want to sum the left and right? Isn't that effectively what the REL does when connected through its high-level input wired to the left and right speaker terminals?
DRubin -- The high-level input does sum to mono, but it does so via active circuitry which presents a very high impedance (i.e., a negligible load) to the left and right channel inputs. But summing to mono by putting a y-adapter on low-level signals, which simply shorts them together, is generally not good practice, and may or may not work depending on the circumstances.
First, it will not work if those same inputs are fed to the main active speakers, or to a power amp driving the main left and right speakers, because it will cause a mono signal to be fed to those speakers. Also, the same thing will happen if the preamp provides two sets of outputs which are simply jumpered together internally (i.e., not separately buffered), and you feed one set of output jacks into the y-adapter and the other set of output jacks to the power amp driving the main speakers.
Those situations are not applicable in this case, because you are proposing to use the balanced outputs to the active speakers, and the presumably separately buffered unbalanced outputs for the sub. However, it is still not good practice, and may not work with some preamps, because since you would be shorting together the left and right unbalanced outputs, you will have their driver circuits in the preamp fighting each other. Depending on the preamp output impedance and drive capability, that may or may not work, and conceivably could even cause damage.
Is your REL one of the models that has two line-level inputs, one with 12 db gain and one with 0 db gain? If so, the best approach would be to use a 12 db attenuator in series with one channel, into the input that has 12 db gain, and the other channel directly into the input that has 0 db gain.
If your REL has only a single low-level input, then Bob's suggestions sound well worth looking into. Or else let us know what preamp you are using, and perhaps one of us or someone else can determine whether shorting its two unbalanced output channels seems advisable.
Let me answer some of the questions raised:
The pre is a GamuT D3
The sub is a REL R-205, which has only one low-level input
Samujohn: I'm not following the relationship between the balanced speaker connection (which fixes a hum problem I had running unbalanced) and the sub puzzler. Will the sub problem be resolved somehow if I connect the Quads unbalanced?
What I am doing currently is running the sub off one channel only. That's obviously not a long-term solution, but I'm amazed at how well it works, and the REL and Quads go together very well. I'm reluctant to throw more money at this with something like the Velodyne box unless I decide to stick with the Quads. I was really just hoping to find a workable solution to get up and running. If I were to spend some more, another approach I would consider is getting a second R-205 to run from the other channel. Stereo subs are supposed to be great anyway.
I appreciate everyone's comments, thanks.
DRubin -- I found the manual for the D3i on the Gamutaudio site, and I assume its specs are similar to those of the D3. It has an output impedance of only 75 ohms. Given that, and in the absence of specific design information on the driver circuitry, or credible assurance from the manufacturer, I would not want to short the two output channels together. You might want to contact Gamut and see what they have to say.
I notice that the sub has both a normal low-level input and an LFE low-level input. I have little familiarity with multi-channel stuff, but perhaps someone more knowledgeable about it can comment on whether it might make sense to send one channel into the LFE input, and the other channel into the normal low-level input. It looks like there are independent volume controls for each on the sub.