Replacing 8-ohm tweeter with 4-ohm - diyAudio
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Old 1st September 2009, 03:48 PM   #1
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Default Replacing 8-ohm tweeter with 4-ohm

Hello, new to forum. I've got an original set of ProAc Super Tablettes which use a discontinued SEAS 19mm tweeter crossed at 5kHz. One of them fried from an apparent power surge. Having looked around DIY forums, it would seem that replacing both with Vifa XT19's would be a more than appropriate substitution.

However, the XT19's are only available in 4-ohms, while the SEAS 19's are 8-ohms. Obviously a 4-ohm resistor is required to match the impedance for crossover values to remain the same, but the XT19's are only 2dB more efficient than the SEAS 19's, and putting a 4-ohm resistor in series would drop the output by 3dB. I'm concerned about how drastic it would be to fudge the resistor values to match sensitivities better while moving the HPF frequency slightly. I use these as studio monitors, otherwise I wouldn't hesitate to fudge them.

FYI I was really shocked to learn how high the x-over frequency is on these. I'm particularly sensitive to harshness in the 2-4k range, and these speakers are the least fatiguing I've ever heard despite being exceptionally detailed. I always credited the tweeter for that! Skan-Speak makes one hell of a good mid-high cone!

I'd appreciate reading about anyone else's experiences like this. Thanks!
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Old 1st September 2009, 08:10 PM   #2
Stu is offline Stu  Australia
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1 dB would almost be close enough... but a 4 ohm resistor will drop the output by 6 dB. Not a lot you can really do about that... there isn't another tweeter you could try? That high a crossover frequency opens up your options a bit.
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Old 2nd September 2009, 01:06 AM   #3
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Well in my experience (and pro audio convention) -6dB represents a halving of perceived volume, but half power output results in a -3dB change in actual output, i.e. halving perceived output requires reducing by 6dB either power or sensitivity (or a mixture thereof), a reduction of 3/4 of the actual output. Adding a resistor in series equal to the speaker impedance should result in half actual power output, or -3dB.

Is there something specific to hifi speakers I'm missing?
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Old 2nd September 2009, 01:47 AM   #4
Stu is offline Stu  Australia
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Quote:
Originally Posted by Zigakly View Post
Well in my experience (and pro audio convention) -6dB represents a halving of perceived volume, but half power output results in a -3dB change in actual output, i.e. halving perceived output requires reducing by 6dB either power or sensitivity (or a mixture thereof), a reduction of 3/4 of the actual output. Adding a resistor in series equal to the speaker impedance should result in half actual power output, or -3dB.

Is there something specific to hifi speakers I'm missing?
3 dB represents a halving of power, 6 dB represents a halving of voltage.

A resistor in series equal to the driver impedance will halve the voltage, so that's 6 dB and a quarter of the power. The reasoning being that halving the voltage also halves the current and P = IV.
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Old 9th September 2009, 07:38 PM   #5
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Ok, that makes sense. Adding a speaker in parallel to a system results in more current, but same voltage, hence only a 3dB increase.

So why the hell doesn't Vifa offer the XT19 in 8-Ohm? Sure they're being used in cars, but it's a cabinet mount on it, not a car mount...
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Old 9th September 2009, 11:14 PM   #6
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http://www.sengpielaudio.com/calculator-Lpad.htm
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 9th September 2009, 11:41 PM   #7
Jay_WJ is offline Jay_WJ  United States
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Solen sells some Seas 19 mm silk domes, which should work as a drop-in replacement for your Proac:

http://www.solen.ca/pub/cms_nf_catal...che.php?id=500

Or you can simply replace the voice coils:

http://www.madisound.com/catalog/pro...roducts_id=813

-jAy

Last edited by Jay_WJ; 9th September 2009 at 11:51 PM.
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Old 22nd September 2009, 09:02 PM   #8
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Bah, I'm a moron. It's a Scan-Speak D2008. Madi has replacements for $77. No voice coil replacement possible, it's a sealed assembly.
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Old 22nd September 2009, 09:59 PM   #9
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Zigakly View Post
it would seem that replacing both with Vifa XT19's would be a more than appropriate substitution.

However, the XT19's are only available in 4-ohms, while the SEAS 19's are 8-ohms.

Obviously a 4-ohm resistor is required to match the impedance for crossover values to remain the same
XT19 is a nice tweeter

But to just ad a resistor?
It may not be THAT simple

Anyway, doing what you suggest, you may not need as much as 4ohm resistor
8ohm drivers are usually around 5-6ohm
And the XT19 may be very close to 4ohm
Maybe you only need 2ohm res

But as said, it may not be that simple
May work fairly ok though
Or at least better than a total silent tweeter
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