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Old 4th July 2003, 04:41 AM   #1
JeremyD is offline JeremyD  United States
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Default Reducing impedance rise?

Is there any real way to reduce in-box impedence rise? Through some testing, I found out that I am not delivering half the power I though I was due to impedence rise. I'm wondering if there are any things I can do to help minimize this.
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Old 4th July 2003, 05:09 AM   #2
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in what context? did you want to correct impedance rise, or have speakers without it?

Le will play the biggest role. the impedance of a speaker is a combo of it's resistance, "acoustic impedance", and inductance.

inductance will rise with frequency becasue that's what it loves to do.

so you could buy loudspeakers with low Le ratings. some loudspeakers use metal rings to reduce Le. there are ribbon tweeters that are effecitively resistive completly.

if all you need is correction, a "zobel network" can work. note that this filter is used to correct this rising impedance so the riseing impedance doesn't affect crossover preformance.

which speakers are you worried about?
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Old 4th July 2003, 07:47 AM   #3
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I think large value (> 1.5mH) inductors also cause impedance rise problems? - also effectively "preventing" full power delivery from an amplifier?

eg. most people think "add an inductance compensation (RC) network to fix impedance rise". Sure - this is the case for the xover (so that it sees a reasonably constant resistive load from the driver), but if you are using large value inductors in-line with the woofer - you will still cause impedance increases, effectively reducing possible power an amp can provide.

I'm using Speakerworkshop (a very good program IHMO) to "play" with crossovers, BDS, inductance compensation, resonance, contour / notch, attenuation etc... to see the effect on inductance, phase and frequency response.

Dave.
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Old 4th July 2003, 08:28 AM   #4
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Quote:
in-box impedence rise
What exactly are you talking of ? Do you mean the impedance rise at the upper end (that's what the former two posts deal with)?
Or are you probably talking about the impedance increase at the diver's resonance ?

Regards

Charles
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Old 4th July 2003, 08:47 AM   #5
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Or perhaps he is referring to driver impedance increasing due to heating in the voice coil....
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Old 4th July 2003, 10:02 AM   #6
Ron E is offline Ron E  United States
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If you mean the rise in impedance at resonance, you can minimize it by stretching a layer of slightly porous cloth or fiberglass over the back of the driver basket. This causes mechanical damping to increase (decreases Qms - and Qts)and causes the impedance peak to diminish.

Since it sounds like you have an SPL contest-type situation, this might not be such a good idea, because of heat transfer concerns, etc

Generally, speakers are not "power sensitive", they are "voltage sensitive". The decrease in power delivered is compensated by the fact that the woofer is more efficient at and near resonance. This means that even if you could deliver more "power" by reducing impedance, you would not gain any SPL, if that is what you are after....

You really need to explain your problem/needs better. Are you just upset that your 1200 watt amp is only delivering 200 at resonance, or ?
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Old 4th July 2003, 10:58 PM   #7
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for the inductor part, well, for a woofer this is intentional as a crossover. if you don't wanna reduce power at high freqeucies you should run the woofer full range! i'm what kinda crossover design your reffering to. impedance rise as i understand the term is the inductive reactance of a woofer becoming a large portion of impedance, cuaseing impedance to rise as frequency rises.
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