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Old 29th October 2010, 11:48 PM   #1
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Default 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
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Old 30th October 2010, 04:02 AM   #2
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Yeah, I made many dozens of great bang-buck party/rock speakers with RS (Tandy) drivers way back when, but the closest I've seen to them recently are the GRS series in the latest PE Sale catalog.

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Old 30th October 2010, 07:05 AM   #3
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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.
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Old 30th October 2010, 09:42 AM   #4
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Would you set the XO in a similar region??
Diagram for clarification please, I'm not to hot with series XOs
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Old 30th October 2010, 10:35 AM   #5
sreten is online now sreten  United Kingdom
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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.
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Old 30th October 2010, 10:55 PM   #6
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Well if I was able to repair my cheap active sub that's what I would be using; just that we have other financial constraints ATM, renovations always take priority.
When we start on the lounge room extension maybe I'll get room for some IB subs.
SWMB Obeyed wont allow yet another amplifier until we get the work finished
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Old 30th October 2010, 11:52 PM   #7
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"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:

Click the image to open in full size.

Series version:

Click the image to open in full size.
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Old 22nd November 2010, 05:19 AM   #8
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Thanx for the replies and help.
I found a pair of these on ebay USA
postage would be much more tha woofers would be worth but these are very good drivers for my desired application, I used 3 of the 8 inch version in parallel for my sons R&R box

Radio Shack 40-1349 Subwoofer 10" Dual Voice-Coil 120-W - eBay (item 160509388638 end time Nov-28-10 07:53:39 PST)

Now if only I could find some in OZ
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Old 24th November 2010, 10:03 AM   #9
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I have one of those Radio Shack 12in still in it's box
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Old 24th November 2010, 11:07 AM   #10
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Quote:
Originally Posted by sreten View Post
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?
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