I have a NAD C372 with a Paradigm powered sub. The NAD does not have a sub out. Per NAD, the best way is to connect it from the right pre out to the input on the sub. It's been working great for about 6 months now.
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If the LFD has pre-out jumpers you can insert the sub there, using a Y-cable (aka splitter, aka one in, two out) there, instead of the jumpers. Run one of the splitter outputs back to the main in on the amp, and one to the sub, from each channel. If the sub only has one input, you will need a Y cable at the other end as well, to sum the L & R channels. Or if your sub has high (speaker) level inputs, you can run the speaker output to the sub and then to the main speakers.
If the sub only has one input, you will need a Y cable at the other end as well, to sum the L & R channels.No, in this case Michael's almost invariably sage and astute advice is incorrect. Doing that would result in the L & R signals to the main speakers also being summed.
If you are just using one sub, and it does not provide separate inputs for L & R, you'll have to connect to just one channel, which may represent a significant sonic compromise. A way around that would be to connect some sort of buffer stage between the pre-outs and the sub, and sum its outputs together.
If I connect it to the left or right channel, won't it create a different load across l and r channels and thus affect the sound?Not if the input impedance of the sub is far higher than the output impedance of the outputs of the integrated amp that are being used to drive it. That will be the case if the powered sub provides speaker-level inputs and is driven from the speaker-level outputs of the amp. It will USUALLY be the case, also, if the powered sub provides line-level inputs and is driven from line-level outputs of a solid state integrated amp (or preamp).
The best way to connect a sub to an integrated depends on the available I/Os on both units. The OP specified that his LFD integrated has no pre-outs or sub-out. However, he doesn't mention which sub he's using or what I/Os the sub has.
Some subs have a single mono LFE RCA input. Some have left and right RCA inputs. Some have speaker-level inputs, some have speaker-level inputs and outputs. Some have stereo pair RCA inputs and outputs, which are useful with a tape or effects loop.
If you have a stereo pre-out pair and the sub has a stereo RCA input, use that. If you have a stereo RCA output but the sub only has a single mono RCA input, you can't fix that with a Y-adapter. You'd need a stereo-to-mono mixer of some sort.
If your sub has speaker-level inputs, use those. IME they sound surprisingly good and you don't have to turn the sub up as much, which should keep noise level down as well. I have found that--like anything else--the sound quality improves with better speaker cables. In other words, use something better than lamp cord.