Cheap dual 8R woofer for sealed box

SWMBO needs a small passive sub-woofer for the computer
( I have a convert )
I am looking for something affordable ( We are renovating ) with dual 8 Ohm voice coils, in Melbourne if possible, specs are not important and we would be happy with anything around 250 - 350mm.
This will be for a first order low pass so smooth on the top end would be better and early roll-off an advantage.
I have had success with cheap Tandy woofers for this application but they are long gone
 

djk

R.I.P
2001-02-04 4:23 am
USA
As long as you're not driving with a bridge-style amplifier, a single voice-coil driver may be used as well.

The grounds for the main left and right speakers are tied together and run to the hot of the subwoofer. The ground of the subwoofer is returned to amplifier ground.

A series crossover is used, a single cap is in parallel with the woofer, and inductor is in parallel with each main speaker.

The woofer should be selected to be about 3dB more efficient than the main speakers, ie: an 87dB woofer with 84dB mains.
 
Hi,

amp L+ goes to L+ of L speaker, amp R+ goes to R+ of R speaker.
speaker L- and R- are commoned. Each speaker has an inductor
in parallel. speaker L-/R- goes to S+ of the subwoofer. S- of
the subwoofer goes to amp L-/R- whioch should be common.
The subwoofer has a capacitor in parallel.

Its fairly safe as long as low bass is effectively mono.

Another option is a version of bridged mode, one channel is inverted
and one speaker connected in reverse phase, both connected with
series capacitors. The sub with a series inductor is connected
across L+ and R+, this is more bombproof.

Though TBH using a 2.1 plate amplifier is by far the easiest, e.g. :
Dayton MCA2550D 2.1 Channel Class D Amplifier | Parts-Express.com

rgds, sreten.
 

djk

R.I.P
2001-02-04 4:23 am
USA
"Another option is a version of bridged mode, one channel is inverted
and one speaker connected in reverse phase, both connected with
series capacitors. The sub with a series inductor is connected
across L+ and R+, this is more bombproof."

Yes, this is also common practice. In this case the sub should be 3dB less sensitive than the mains (because it receives a +6dB signal due to bridging).

For PA with 99dB mains:

MonoSub1.gif


Series version:

passivesurroundandcenter.gif
 
Another option is a version of bridged mode, one channel is inverted
and one speaker connected in reverse phase, both connected with
series capacitors. The sub with a series inductor is connected
across L+ and R+, this is more bombproof.

Usable only with balanced inputs though.

Passive subs suck, surely you can find a used active sub for the same or less than what it costs to build a passive one?
 
Passive subs suck, surely you can find a used active sub for the same or less than what it costs to build a passive one?

THORNSPAWN I have one of those Radio Shack 12in still in it's box

I would say that depends on the cost of the driver, it Thawnspawn sells it to me for $25- then 2*10mh coils at $17- each and I can be done for $65- all up
Would you accept $25- mate??

Think of it more as a triple box 2.5 speaker set