I really need some help on this one. I was recently asked by someone to take a look at their home theater setup and see what I'd suggest they'd do to spruce it up. Given that their receiver was pretty old (only had Pro Logic), I suggested they get a new one and a new HDTV.

Well, I thought I'd done enough work figuring out their current setup, but I clearly did not. Otherwise I might have asked "where the hell is the signal from the subwoofer coming from?

Here's the setup: they have the usual 5.1 arrangement, with the sub being an in-wall model. I've identified every single wire coming back to the built-in cabinet the equipment is kept in, and there isn't a single cable for the sub. The old receiver just had a pre-out anyway, and there certainly wasn't a spare RCA cable in there.

So my next step was to take the speaker out of the wall and see what was what. I did so, and near as I can figure, the sub is wired in-line with the rear speakers. Is this common? If so, how on earth do you set up the receiver to account for that? I can't even tell how they did that on the old receiver.

I really need some help on this one. Thanks...

Sounds like a kludge. Many subs have a pass-through, with or without filtering the audio. This sort of feature almost makes sense in an old-school setup (before the advent of the modern digital receiver), where you'd cobble together your own satellite/sub system.

When you're moving to a modern 5.1 world, you want to run a separate line from the receiver to the sub. If for some reason that's infeasible, then pretty much the only other option is to tell the receiver that (a) you don't have a sub and (b) your front speakers are "small" while your rear speakers are "large". This will cause the receiver to direct all the low-frequency information to the only place that can handle it: the "large" speakers. (Or, it wouldn't surprise me if some subset of the receivers get deeply confused if you set them up this way.)

What size is the subwoofer?

In my experience, a 10" or smaller subwoofer running in a free-air environment (i.e., not in a sealed box) is pretty happy running full range frequencies. In that case, you could just run it off the rear speaker outputs, assuming that they are not crossed over with a low-end cutoff.

caveat: My experience is entirely with automobile stereo systems, maybe home audio is different.

tanstaafl.

Generally, it is different. In home audio, the subwoofer is a discrete channel, usually with its own separate speaker wire. That's what "5.1" means. 5 discrete audio channels going to 5 full-range speakers, plus a discrete subwoofer channel.

Dignan,

Any system where the subwoofer appears to be daisy-chained with a pass through to the other speakers means that either it wasn't wired for true multi-channel operation in the first place, or that the subwoofer has some kind of internal smarts that is doing the multi channel decoding itself.

This latter case, where the subwoofer has its own internal smarts and does the channel separating itself, is fairly common, so check to see if that's the case here.

Could you not modulate the signals for the sub using a centre tapped transformer balanced transmission line powering both the rear channels?

That's one possible interpretation of "smarts". Although I've seen systems more detailed than that.

I have an old passive sub made by Mission, that takes the left and right channels filters out the low notes and has a left and right out put for the normal speakers. It think this was quite common before active subs came along. This sub has no mains power and is just a speaker in a box.

I have to say the results are not very good, and it needs lots of power from my amp to work.

The thing that strikes me as strange about the setup you have there is that I didn't think the rears were full range, so I wonder if there is some sort of none standard arrangement going on there ?

Cheers

Cris.

Thanks guys. I think the idea to tell the receiver I have no sub, that the front speakers are "small," and the rears are "large" is probably my best bet. Thanks, Dan. Clearly this is far from ideal, but I can't be blamed for how their installer did this initially.

I can't imagine why they did, though. Why on earth would you do that? As long as I've been familiar with home theater setups, the sub has always been discrete. It couldn't have been to save money either, as these people had a pretty nice Denon receiver, a whole-house speaker system, and amps to drive it all. Seems crazy to me that if you're going to run all those wires anyway, you'd run something to the sub separately.

Could it be because they were using an in-wall model that wouldn't be able to get power to, like Cris said? I agree that the sub will probably be less than satisfactory.

Showing your youth again..

Yeah. The obvious question is "did the Denon receiver that they had have a separate subwoofer output?"

Right, I know that. I was talking about the subwoofer itself -- a 10" free-air sub (again, in my car experience, perhaps not applicable to home environment) can sound pretty good playing a full-range (i.e., no crossover filtering) signal, and could thus be connected and run on the same signal path with smaller speakers if necessary, which in this case seems to be the case.

tanstaafl.

Perhaps I am, but it seems to me that you don't have to be very old to remember when surround sound systems became popular in the consumer space. My dad had one installed in 1992, and that most certainly had a separate channel for the sub.

I'm not sure the age of the technology (or myself) has everything to do with this setup. It seems to me that this system was either not set up ideally, or perhaps it's a limitation of using an in-wall sub.

My point exactly! (But, yes, you really meant "young" rather than "old" there..).

Our sound systems here all pre-date 1992, and none of them have sub-woofer outputs. Except for the new one with the PVR, from 2006.

Cheers

In the late 80's, my parents got a hi-fi setup at home. For aesthetic reasons, they got smallish bookshelf speakers mounted near the ceiling (next to air ducts -- not the best sounding idea ever) and went with a massive powered sub (on which they've turned down the volume to the point that it might as well not be on at all). Back then, there were no DVDs, no Dolby Pro-Logic, and so forth. Instead, we had a standard two-channel stereo receiver that had pre-amp outputs. The "professional" installer used a y-cable to merge left and right and feed that output to the sub.

So far as I can tell, Dignan's setup in question follows the same "hey, whatever, seems to work" ethos that reigned prior to Dolby Digital and the modern era of surround sound.

Actually, I did mean "old." I also specifically said "when surround sound systems became popular in the consumer space." I have no doubt that you guys have older "surround" systems, but I highly doubt they were considered "popular" at that time. The sound system I'm working with at the moment couldn't be more than 10 years old, and by then they should have known better, particularly if they knew enough to wire the entire first floor of the home for sound.

There's nothing particularly "bad" about subwoofer systems that predate Dolby 5.1 surround setups, when properly done with low-pass filters (as in Dignan's case).

Even 5.1 isn't magic -- sure, it works incredibly well when coupled with an input source that was created with 5.1 encoding. But for anything else, it has to do the same tricks as the "old stuff" did when creating a feed for the sub.

Cheers