Can I connect a 100W subwoofer to the 40W+40W rear speaker outputs of a stereo?

[epigramx]

It's for low quality requirements in an old car so high audio quality is unimportant, just to not suck totally. It has a new stereo that outputs 4 X 40W to speakers but only the front ones are connected (or exist).

Can one just stick the outs of the rear speakers to a 100W subwoofer and get power to it that way? To be honest I do not even know if it's "allowed" to connect the positives and neutrals of speakers like that.

[Byonnem]

Why not power it with 1x 40W? Underpowering a speaker does not hurt it. However it could possibly damage the amp. (but if that is the case then 2x 40 is still underpowering!)

I do not see how connecting multiple outputs to one speaker could be a better idea.

[themadhippy]

Dont connect the outputs together unless you want to buy a new amp, wire the sub to 1 channel only .if you want to experiment, and do it at your own risk,  try the speaker wired across both the + outputs,dont connect anything or link the - outputs.

[james_s]

Most head units use an internal amp that is already using a bridged configuration so you cannot bridge multiple outputs. you need to filter the higher frequencies out before they get to the sub anyway though. Amplifiers are dirt cheap these days, just use a sub amp, most head units already have a line level sub output.

[Audioguru again]

You cannot connect audio amplifier outputs in parallel, even if they have the same signals (stereo often has completely different signals). If one amplifier plays an output but the other amplifier plays it with opposite phase or plays nothing then both amplifiers drive a direct short circuit and might be destroyed if they do not have a protection circuit.

If you connect a speaker to only the + wire of both amplifiers then identical signals from both amplifiers (usually bass sounds) will be cancelled and the amplifiers will be overloaded when signals have opposite phase.

A simple solution to your requirement is to use two subwoofer speakers, one for each rear amplifier channel. 

[Zero999]

Yes, you need two subwoofers, if you want to use both outputs. Another possibility is a dual coil subwoofer, which can be connected to both outputs at once.
https://www.google.com/search?q=dual+coil+subwoofer&ie=utf-8&oe=utf-8&client=firefox-b

[james_s]

That still doesn't solve the problem of needing to filter out the higher frequencies before they get to the subwoofer.

[Zero999]

That still doesn't solve the problem of needing to filter out the higher frequencies before they get to the subwoofer.

An inductor will do that.

[themadhippy]

An inductor will do that.

And waste  a fair bit of the already limit power  for  only  a  6db slope

[Yansi]

Then add a capacitor, to get 12dB/oct.

[james_s]

Why not just spend $20 or so on a small sub amp and use the actively filtered sub line out that almost every head unit has?

[Zero999]

It might not even be necessary to cut the high frequencies to the woofer, which could have a poor treble response anyway.

[AVGresponding]

It will have a poor treble response.

Generally crossovers are only needed where you need to power share between different frequency range speakers on the same output. Using them with a single speaker just wastes power.

As has been already mentioned, don't connect outputs together, they are almost certainly already bridged. Though some can be connected together depending on the amp configuration, this almost always involves bridging front and rear on each side.

Dual voice coil subs are easy enough to find, this is probably the simplest solution.

[Audioguru again]

Almost every speaker "shrieks" at its cone-breakup frequencies. You do not want to hear the shriek frequencies of a sub-woofer.
Here is a typical small 4 ohms 100W sub-woofer:

[AVGresponding]

Given the OP's intention to use it in lieu of rear speakers, I'm not sure it would be hugely noticeable, especially if it's going to be in the load area, covered over with a parcel shelf etc.

One could always stick some old chewing gum on the dome/cone joint to interfere with this resonance   

[Bassman59]

Why not power it with 1x 40W? Underpowering a speaker does not hurt it. However it could possibly damage the amp. (but if that is the case then 2x 40 is still underpowering!)

You've got it exactly backwards.

Underpowering a speaker could indeed hurt it -- IF you drive the amp into clip, so the speaker sees a lot more power than if the amp output was clean. And this overheats the voice coil, which causes failures. As for whether this condition can damage the amplifier, that depends on the amplifier design. Even inexpensive amplifiers have some kind of thermal overload protection.

If you have a 40 W amplifier and a speaker capable of handling 100 W and you only demand 20 W from the amplifier, nothing is going to break.

[Jwillis]

How much do you want to spend ?

2 ways you can add your subwoofer to your stereo .
You can "sum" the left and right together using the below diagram.This is a passive circuit and won't require additional power but its not perfect. It may reduce the volume and theirs a possibility of cross talk. Easy but won't give the best results.

The other way is to get an inexpensive subwoofer power amp. Theirs quite a few available on Ebay and come in various power ratings.
Heres a few examples https://www.ebay.ca/sch/i.html?_from=R40&_sacat=0&_nkw=subwoofer+amplifier&_pgn=2&_skc=50&rt=nc
Hope this helps
 

[Zero999]

Why not power it with 1x 40W? Underpowering a speaker does not hurt it. However it could possibly damage the amp. (but if that is the case then 2x 40 is still underpowering!)

You've got it exactly backwards.

Underpowering a speaker could indeed hurt it -- IF you drive the amp into clip, so the speaker sees a lot more power than if the amp output was clean. And this overheats the voice coil, which causes failures. As for whether this condition can damage the amplifier, that depends on the amplifier design. Even inexpensive amplifiers have some kind of thermal overload protection.

If you have a 40 W amplifier and a speaker capable of handling 100 W and you only demand 20 W from the amplifier, nothing is going to break.

Sorry, I don't know what you're talking about. The person you've just responded to is talking about driving a 100W woofer with 40W, which will not cause any problems.

Almost every speaker "shrieks" at its cone-breakup frequencies. You do not want to hear the shriek frequencies of a sub-woofer.
Here is a typical small 4 ohms 100W sub-woofer:

That shriek is only 15dB above the output at 100Hz and is probably not that noticeable, especially if the acoustics of the enclosure, i.e. a car boot, attenuate high frequencies. If it's an issue, a simple inductor will easily get rid of that and won't lose much power if it has a low enough ESR.

[Bassman59]

Why not power it with 1x 40W? Underpowering a speaker does not hurt it. However it could possibly damage the amp. (but if that is the case then 2x 40 is still underpowering!)

You've got it exactly backwards.

Underpowering a speaker could indeed hurt it -- IF you drive the amp into clip, so the speaker sees a lot more power than if the amp output was clean. And this overheats the voice coil, which causes failures. As for whether this condition can damage the amplifier, that depends on the amplifier design. Even inexpensive amplifiers have some kind of thermal overload protection.

If you have a 40 W amplifier and a speaker capable of handling 100 W and you only demand 20 W from the amplifier, nothing is going to break.

Sorry, I don't know what you're talking about. The person you've just responded to is talking about driving a 100W woofer with 40W, which will not cause any problems.

Let's see how much power that amp puts out when driven into clip, which is (unfortunately) very common for subwoofer amps.

[AVGresponding]

If you drive any amp into any speaker hard enough it'll start to clip 

The solution is pretty simple: turn it down a wee bit.

You have to work with what you have, always. The OP has the head unit already and seems only to be considering the implications of using a sub/s on the rear outputs. If they turn it up too high, it'll distort, then clip. That's just part of learning the limits of your system 

[Zero999]

Let's see how much power that amp puts out when driven into clip, which is (unfortunately) very common for subwoofer amps.

I see what you meant now. To answer your question, an amplifier will output a maximum of double the output power when driven into clipping, which will be 80W per channel in this case. This is because the RMS voltage of a squarewave is equal to its peak value and the RMS voltage of a sine wave is equal to V/root(2). R = V₂/R and V_RMS = V_P/root(2).

For example:

Suppose the maximum output voltage of the amplifier is 8V and the speaker impedance is 4Ohms

If the output is a sinewave, without any clipping, the peak voltage of the sinewave will be 8V. Calculate the RMS voltage:
V = 8/(root(2)) = 8/1.414 = 5.657V
R = 4

Calculate the power
P = V²/R
P = 5.657²/4 = 32/4 = 8W

Suppose it's driven into clipping, so the output is a squarewave. The RMS voltage of the squarewave is simply 8V.

Calculate the power
V = 8
P = V²/R = 8²/4 = 64/4 = 16W

See it's double!

In reality the user will not drive their amplifier so far into clipping to the point of it being so distorted, it's a squarewave, giving 80W per channel, a total of 160W. They most likely reduce the volume long before that point and the music won't be playing at a full duty cycle, so it's a non-issue