How can I power a passive subwoofer?

Question

I have a perfectly good passive subwoofer in my current living room surround system. I got a new receiver for Xmas and I now see that the output does not provide power to the sub... so, its not working. Is there any way I can connect this sub to another powered output on the receiver? Can I get an external power source to power it? Or, should I just buy a new subwoofer?

Answer 1

Aside from getting an amplifier to power the subwoofer. You could buy a plate amplifier made for specifically for subwoofers, used by hobbyists in the DIY market. The benefit of the plate amps is that they are mono, have all the line level (rca) and speaker inputs, an electronic crossover (most with variable settings) and are not to expensive.

It would require some cabinet mods or just make a case for the plate amp. I would suggest leaving the original cabinet alone and building a "box" to mount the plate amp into, even if it's ugly you can hide the amp/box out of site and still enjoy your sub.

You can find cheaper powered subs than the plate amps, but I don't know what kind of sub you currently have.

Few links below:

Edit #3 (sorry):

As far as hooking it up to the new receiver you could wire the sub in with the center channel speaker. I say center instead of the front mains because with modern surround sound so much of the signal goes to the center channel, while the mains merely get "surround" signals.

One problem with what I'm about to describe is that if you listen to music in stereo (without the surround channels active) the sub won't receive a signal. Also the sub wired with the center channel may be too taxing for the receiver to handle, possibly over heating the amp and causing it to shut down (or even damage permanently).

There are two ways to wire multiple speakers together:

Series &

Parallel

Wiring in series will half the impedance the amplifier "sees" where as wiring in parallel does the opposite, it doubles the impedance. For example two 8 Ohm speakers wired in series will become "one" 4 Ohm speaker as far as the amp is concerned. Or 16 Ohms in parallel. What this does to the amp (most of the time) is increase the output power for series and half the power for parallel (a generalization - but close enough to get my point across).

Most receivers are designed to handle 8 ohm speakers. In fact the RMS ratings provided by the manufactures is for an 8 Ohm load. Many can handle 4 Ohms as well, check your manuals specifications section (you may see power output listings for both impedance loads).

I'm going to focus on wiring in series.

All it is, really, is "daisy chaining" You would take the positive (+) lead from receivers center channel and run it to the (+) input on the center channel speaker. Now using another speaker cable, connect the (+) lead to the (+) input of the center channel and run it to the (+) input on your passive sub.

Do the same with the negative (-) side of things. Receiver - center channel - subwoofer.

You could also run TWO speaker cables from the same output on your receiver, one the the center channel and one to the subwoofer.

Again there will be no signal to the sub unless the center channel is being used in the aforementioned set up.

All said, I would invest in a plate amp or new powered sub.

Hope that helps and good luck!

Answer 2

Eric has the best answer here. However, short of this...or better stated....a temporary solution would be to get a separate amplifier (if you have one on hand) and run a connection from the sub out on the new receiver to any input on the "temporary amplifier". CAUTION! If there is a phono input on the temporary amp, DO NOT use that. There's a phono specific circuit in there that must receive NOTHING ELSE but a turntable cartridge signal.

  Another thing that must be done. Make sure that the subwoofer out is switched on at the new receiver. I also forgot to mention. If this temporary amplifier is an old stereo amp, make sure that you hook up your passive subwoofer to whatever channel that you ran the sub out from the new receiver to. For example, if you ran the connection to one the left inputs, make very certain that you hook the passive sub to the left speaker connection on the temporary amp.  Some years ago, this is the very thing that I did. It wasn't the best solution, but it worked fine until I got an actual subwoofer amp. Hopefully, this was helpful.

Answer 3

Unfortunately, I am sorry to say the whole series parallel thing has been explained wrong. Parallel will cut your impedance in half, series will double it. And parallel will only cut it in half IF the speakers have the same impedance value. Otherwise if the values are different then it becomes an average. Example: (2) 8 ohm speakers will give 4ohm total impedance in parallel and 16ohms in series. If you have an 8 ohm and a 6 ohm speaker you will have 7 ohms total impedance in parallel and 14ohms in series.

I do not recommend these types of connections. You are not going to get optimum fidelity from these kinds of hook ups. As Eric did state you could purchase a DIYers sub woofer plate amp. This would require you to modify the subs cabinet though, as stated.

Assuming you don't have a "B" speaker selection

I recommend you get an impedance matching speaker switch with volume control. (partsexpress.com)

This will insure you connect everything properly. It will also allow you to fine tune your subs volume and turn it off if you don't want to use it. Unfortunately you are also sending a full range signal to your sub as opposed to a low frequency bass signal. This is ok if your sub has a built in crossover to filter out the bass frequencies for the sub.

If you have a line level (rca jack) LFE ( low frequency effect) output or sub woofer output. i recommend one of the following

1) Buy a new powered sub.

2) Pick up a single channel mono block amp. Audio Source makes some very low cost single channel amps which would work well in this situation. Use this amp to drive your passive sub.You can get them at Parts express also.

If you have an old receiver laying around you could use it also.