Sub woofer cable help please...

Sub cable

I recently purchased a single sub woofer cable, but I have two outputs out of my preamplifier. I am ok going into my subs single input but coming out of the preamp is an issue. I have two female RCA outs of my pre amp and I need a y adapter with two male and one female. Do you know where I might find one or any suggestions? As it stands now I can plug it in but only to the left or right output from preamplifier. And then into the one input on the sub. Thank you...
You can get either this one:

or this one:
You might check with your preamp manufacterer first. I did the same thing and once I plugged in the "Y"-adapter to my extra set of outputs, it bridged both channels, (and both sets of outputs) to mono.

If the extra set of outputs are just paralled off the first set, this will happen.

I had to go with an outboard subwoofer crossover that took the stereo signal from the preamp, then converted it mono out to the sub.
Thank you Mofi and Delux...
I appreciate the comments.
I agree with Mofimadness. In fact, I am doing the exact same thing with my rig. If youy check some of my threads, you can see some of the technical issues that I had to overcome, but some of these issues could be unique to my set up.

By way of quick summary, I have an ARC Ref 3 pre which has two main outputs. Main 1 connects to my power amp. Main 2 connects to my self powered sub. It appears that your situation may be a little simplier than mine because it looks like you're working entirely with SE/RCA hook ups. In my case, one hook up was SE and the other balanced.

Now . . . as Mofi suggested, check with your preamp manufacturer to see if you can use a Y connector to sum the left and right channels for the preamp output feeding the sub. If not, you will need a crossover to do that for you. One problem I had, which again may be unique to my set up, was that I had to show my Ref 3 a combined load that was not less than 20K Ohms. The problem was the two loads (i.e., Main 1 - amp and Main 2 - sub) that I presented to my Ref 3 summed to be less than 20K Ohms. So I had to purchase a custom made impedance buffer that summed the Main 2 channels and raised the load presented to the Ref 3 to make everything work properly.

So . . . the take-away from this post is: (1) see if you can sum the channels into mono for the sub with a Y adapter. If not, look into using a cross-over that will sum the channels into mono for the sub, but leave the other output in stereo for your fronts. (2) make sure you don't overload your preamp by presenting too low a combined load. In both cases, discuss these issues with your preamp manufacturer.

BTW: just a suggestion, before I switched over to the custom impedance buffer, I used to sum my Main 2 outputs with a Paradigm X-30 controller. Check the Paradigm listing here on A'gon. The X-30s come up for sale from time to time and sell for roughly $150, plus or minus. The X-30 will sum your channels without "shorting" your preamp, and also help fine tune blending your sub with your fronts by giving you the flexibility to adjust LF rolloff, loudness and phase. FWIW

Good luck and let us know how you nake out.
The Paradigm X-30 is exactly what I am using. It works great and has controls on the front of the unit for easy setup. This unit solved my hook-up problems.
Thanks Bif... I feel like you wrote a term paper :)
I have sent an email to Steve McCormack and Kris at smc audio and will see what they say, better safe than sorry. At first I thought it was a stupid question, I am glad I asked now!
thanks again! I really appreciate the help.
Glad to help. Although I doubt you will have to deal with the preamp loading issue that I had, just in case, the formula is:

L1 x L2/L1 + L2 + CI (Combined Impedance)

L1 = Main 1 load impedance
L2 = Main 2 Load impedance

So, for example, if L1 is 300K Ohms and L2 is 20K Ohms, CI = 300K x 20K/320K, or 18.75K Ohms. In my post above, I mentioned that the Ref 3 pre should NOT be presented with a load that presents LESS than 20K Ohms. Therefore, in my case, because the recommended load should not be less than 20K Ohms, the Reg 3 would be overloaded because the CI is only 18.75K Ohms. Overloading my Ref 3 could result in denigrated sonics and could prematurely age the tubes. FWIW
Thank you again I really appreciate it!
Good comments above.

I would add, though, that IMO even if the two sets of preamp outputs are separately buffered it would still not be good practice to sum the two channels together for the sub with a y-adapter. In saying so, I recognize that many people do exactly that, with reasonable results.

To the extent that the signals on the two channels differ, the output stages that drive the two channels will fight each other, conceivably with adverse sonic effects. That would be particularly likely to occur if an instrument producing deep bass tones were off to one side (i.e., essentially in just one channel).

A more technical way of putting it is that the output stage of each channel will have to drive the stereo component of the signal into a load impedance equal to (actually, slightly less than) the output impedance of the stage driving the other channel, which will be very low compared to the impedances it is designed to drive.

-- Al
Thank you Al and all the rest who have posted. I appreciate the help as it turns out there was more to this than I anticipated.
Correction: In my post above, I wrote the formula to compute Combined Impedance is:

L1 x L2/L1 + L2 + CI (Combined Impedance). I should have used an equal sign in the formula, sorry. As corrected:

(L1 x L2)/(L1 + L2) = CI (Combined Impedance)
Thanks Bifwynne for the update!
I just received this advice from Kris T SMC Audio.

"What you need is a summing cable that takes the two outputs and combines them into a single cable. (2 x RCA male to 1 x RCA female.  A summing cable uses resistors to isolate the two outputs.  A regular y connector should not be used."
Does anyone know where I can buy one?
I am not aware of any off-the-shelf resistor y-adapters. Creating one yourself probably wouldn't be too difficult.

However, a lot more information, and some analysis, would be necessary to choose a suitable resistor value (even if off-the-shelf resistor y-adapters were available). The value would be dependent on whether or not the two sets of preamp outputs are separately buffered, what their nominal output impedance is, how that output impedance varies with frequency, the input impedance of the sub, and (if the preamp outputs are not separately buffered) the input impedance of the main power amp.

The X-30 that was suggested, or something comparable, sounds to me like the way to go.

-- Al
If the powered subwoofer has high level inputs (speaker level) try connecting it this way. I run my pair of subs in this manner ( off of main amps speaker terminals ) and it works great.