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Old 9th August 2007, 08:46 PM   #1
germpod is offline germpod  United States
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Default Two 8ohm speakers make 4, how to fix

I am making a center channel out of two Fostex fe103 full range drivers. When I hook them up it will drop them down from 8ohms to 4ohms or 16ohms which I do not want. How do I fix this problem and would 16ohms be doable for the center channel? Most center channels have two woofers and a tweeter, so how do they make the two woofers not drop the impedence to 4ohms?

Any help is appreciated since I am new to this.

I browsed the forum for a while but did not find an answer to this problem.

Thanks.

Ed Robinson
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Old 9th August 2007, 09:10 PM   #2
cuibono is offline cuibono  United States
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I've seen it done with musicians bass cabinets : using four speakers total, wire two in series, then the other two in series, then parallel both groups. It gives the original impedance. But I doubt this is how you want your center channel to end up...

Maybe someone here has built a center channel speaker you could copy...
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Old 9th August 2007, 09:10 PM   #3
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D00d!

The answer comes down to whatever is driving this. If the power amp in your setup cannot drive 4 ohm speakers, then you need to go to 16 ohms. Don't worry about the volume not matching, even if you have a surround receiver that's all in one box, it has settable levels for each channel.

BTW, why 2 drivers? It's my number one audio pet peeve that people take the function of the centre channel, to provide a point source to anchor voice to the screen, and then just throw that out the window and design one purely based on the cosmetics of a speaker that lies flat. Argh!
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Old 9th August 2007, 09:13 PM   #4
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Hi Ed,

Without using resistors you can't get to where you are going. Your choices are 4 or 16.

There's nothing wrong with 16 ohms but my question really is why do you want two woofers in the centre channel? An ideal set up is to have all speakers identical.

I think the dual woofer came into play when the manufacturers realized that a great many persons were going to be laying the centre unit on it's side and that a woofer either side of the tweeter looked better. It certainly isn't necessary.

EDIT: The others were posting while I was typing.
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Old 9th August 2007, 09:59 PM   #5
chrisb is offline chrisb  Canada
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hmm, given a few more replies posted, one might see a pattern emerging

If you're dedicated to using 2 drivers in your centre channel, and the installation allows for it, you could always try bipole.

The other option that will still utilize 2 drivers would be to build a pair of monopoles, and place them immediately on either side of the screen. The phantom mono image thus created will likely be more accurately anchored to the centre of the video screen.

Of course the matter of impedance remains, so if the motive behind the concern is a Surround receiver uncomfortable with low impedance loads, then I'd opt for the series connection at 16, and forget about it.

all of this sage advice from someone who long ago gave up on the whole multi-channel HT mess

the most satisfying sound I've ever had in my own video system was with a simple 2 channel rig - Fostex/Heil AMT bipole speakers, and EL84 P/P amp - musical and natural enough sounding to forget it's there, and just enjoy.
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Old 9th August 2007, 10:28 PM   #6
germpod is offline germpod  United States
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The center channel is for a friend, I only need my two stereo spekers myself.

I will try both the sixteen ohm load and only using one speaker since I made the enclosures seperate so we can fiddle with them better. I am glad to hear the 16ohm load should not hurt anything.

Thank you all for the information.

Ed Robinson
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Old 11th August 2007, 12:49 AM   #7
owdi is offline owdi  United States
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You may be able to get away with wiring them in parallel. According to the specs listed at madisound, the fostex 103e has an Re of 7.45 ohms. That is pretty high, and would make this more of a 10 ohm nominal driver, rather than an 8 ohm.

Two in parallel drop you down to 3.725 ohms. If you are using a baffle step compensation circuit, add the RE of the inductor to 3.725. This could easily get you above 4 ohms total (6 ohm nominal), which many receivers can handle.

Dan
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Old 11th August 2007, 03:05 AM   #8
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my question is are these the Shielded FE-103s? If a CRT type TV is being used, you'd want a shielded driver to prevent picture distortion.
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Old 11th August 2007, 10:59 PM   #9
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If the amp is 4ohm stable, good chances are parallel could work well - if, like in most cases, the tweeter has a much higher sensitivity than that of the woofers, then that could work in your favour.

Certainly worth a try IMO
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Old 11th August 2007, 11:35 PM   #10
Svante is offline Svante  Sweden
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16 ohms is not as bad as it may sound, remember that two series connected drivers have the same sensitivity as one driver (ie dB@2.83V straight in front of the drivers).

If the amplifier can handle a 4 ohm load, you will gain 6 dB sensitivity by connecting them in parallel, though.

PS The efficiency (the ratio between acoustic output power and electrical input power) is the same in both cases, and twice as high (+ 3 dB) compared to the single driver and at low frequencies.
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