Effects of using y-cable from pre outs

I have built in single channel sub cable. Older receiver only had 1 single sub output. I'm not about to tear into walls or add more cables. If I upgrade to an integrated amp with stereo pre outs...what effect will using a y cable to connect stereo outs have? Phasing effects? High noise?
The y cable will connect both channels together so that the signal going to the sub, left and right channels will be the same, effectively mono.

Either get an amp with dedicated sub outputs, which should be electrically isolated from the main pre outs (if the receiver has them) and use the y cable, else you will need a mixer to blend the left and right signals to give you a mono feed to the sub.
I don't know about LFE channel in theater applications, but for 2ch, everything below 80hz is mono, anyway.
As Ghostrider indicates, if the pre outs are part of the main signal path (i.e., if they are jumpered to the inputs of the power amp section of the integrated amp), the result would be a mono signal going to BOTH the main speakers and the sub.

If the pre-outs are separately buffered and are not in the main signal path through the integrated amp, using a y-cable will PROBABLY work ok. But I would first check with the manufacturer of the amp, to make sure that connecting the two outputs together will not cause damage or compromise long-term reliability. It's usually ok to do so, but conceivably might not be depending on the design of the buffer stage.

A mixer, with high input impedance and low output impedance, would be the most ideal solution.

-- Al
Investigate an Outlaw ICBM - could be your answer.
Won't a Y-cable used in this manner effectively short the preamp outputs together? Looks like a recipe for disaster to me. As indicated by others an active mixer would work.

Depending on your main speakers, an active bass management controller would be your best solution; like the NHT X2 filter.

I'll also comment that "upgrading" to an integrated amp, in may actually be a step backwards -- it depends on your room. I'd look at buying an AV pre/pro or AV receiver to be used as a pre/pro that has bass management and room mode correction from Audyssey.
Won't a Y-cable used in this manner effectively short the preamp outputs together?

Yes it will, Bob, but as I indicated that will USUALLY be ok (assuming the signals being shorted are separately buffered from the signal path to the main speakers). In fact, the literature on REL subwoofers specifically suggests doing that in cases where a speaker-level connection is not possible.

The reason it usually is ok is that the output impedance of the two outputs is typically high enough to limit the current flow to an amount that the output transistors or integrated circuits can handle safely. As contrasted to the situation, say, where the outputs of two solid state power amps (with their extremely low output impedances) are shorted together, in which case pyrotechnics would be the likely result!

-- Al
The amps I've seen have a "pre-out" in case you wanted to add another external amp. I'm not sure about "jumping". I believe it's a regular "out" that you can control volume with remote and use internal amp at same time.
Al, I understand there may not be enough current drawn to immediately destroy the output buffers. But won't each output see the parallel combination of the other output and the sub's input, which will probably be close to the impedance of the output, so each output will be driving a much more difficult load than it was probably designed? Does this shorten the life of the preamp?

How does it affect the length of the cable to the sub?
Hi Bob,

Yes, it's conceivable that with some output stages that are designed inexpensively and not very conservatively, long term reliability could be compromised. Which is why I suggested that the op "first check with the manufacturer of the amp, to make sure that connecting the two outputs together will not cause damage or compromise long-term reliability."

But with typical quality designs (and many cheap designs as well), I would expect that there would be no problem. First, consider that the majority of the energy is typically in the mono component of the signal, and in driving that component the two output stages would not be fighting each other, so to speak, they would be helping each other. In other words, they would both be trying to put out the same voltage, so they would not be loading each other or drawing current from each other.

But let's take a worst case, and say that each channel is trying to put out a large 2-volt signal that is 180 degrees out of phase with the signal from the other channel. And let's say that the output impedance of each channel is 100 ohms. That would result in each output sourcing or sinking a current of 4/200 = 20ma (20 milliamperes). I would expect that to be well within the limits of what a typical quality output driver can handle on a continuous, long-term basis.

With respect to the more subtle sonic effects that may occur due to the load being more difficult, they shouldn't particularly matter. First, because it is only the bass frequencies that we are concerned with, and just the mono component of the bass. Second, if the output impedance of the preamp tends to vary significantly within the bass region (due, perhaps, to a coupling capacitor at the output), that wouldn't matter either because the variation would presumably be the same in the other channel, so the voltage divider ratio between each driver device and the summed output would always be 50-50.

As far as driving the cable is concerned, that would actually be improved. The source impedance driving the cable would be one-half of the output impedance of each channel, per Thevenin's Theorem.

-- Al

I am using a Pass Labs Preamp which has dual XLR and RCA's out that are active. The XLR output goes to my Pass Labs Amp.

The RCA's are connected to a JL Sub with stereo interconnects from the Pass Preamp into the JL Sub.

Based on some of the replies in this posting would'nt this be a problem ?
Ozzy -- I took a look at the manuals for your Pass Labs XP-10 and your JL Audio F113. As might be expected with equipment of this caliber, all four outputs of the XP-10 (left & right xlr & rca), and both of the rca inputs which you are using on the F113 (left & right), are individually buffered. So none of the concerns which have been discussed here are applicable to your system.

-- Al
Almarg, Wow thanks !