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Old 21st March 2006, 09:26 PM   #1
Klimon is offline Klimon  Belgium
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Default 2.5-way impedance riddle

Hi,

I'm planning on building a 2.5-way speaker = simple two-way + additional woofer running alongside the main woofer below 500hz to fill in the baffle step loss. As both woofers have 8ohm impedance and will work together (in parallel) below 500hz, the impedance there equals 4ohm. Is this a problem? If so, what should I do to fix it?

Thanks
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Old 21st March 2006, 10:04 PM   #2
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Impedance isn't a one-number property, the "nominal impedance" is. Impedance is that chart that came with your driver that shows a big spike at the lower frequencies. You are worried about your impedance dipping in the region where it is actually spiking up.
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Old 21st March 2006, 10:25 PM   #3
lndm is offline lndm  Australia
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Does your amp have a low output impedance (ie does it have feedback, is it solid state)? If so, it should have little problem dealting with this generally speaking. Impedance variations can make a speaker a difficult load for any amp, especially those with a high output impedance.

Even still, build first, fix this after (just make sure you don't overload you amp in the meantime).
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Old 21st March 2006, 11:18 PM   #4
Klimon is offline Klimon  Belgium
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@Leadbelly: You've got a point there ; I rest assured

@Indm: Depending on how efficient the speakers will turn out (using late sixties Isophon alnico loudspeakers in d'appolito +0.5 woofer mounted on the backside as bipole; most probably between 92 and 96db/w) either fleapowered sets or mini-pp's without NFB ---- I've read before about this or that speaker being 'an easy load' which seems especially important for small NFB tube-amps. Could you draw my a picture why speakers with a relatively flat impedance are prefered with these amps? No nfb tube-amps have high output impedance and low damping factor, but how can I visualise what happens when this tube-amp is mated with an easy or a difficult impedance-load?

Thanks for you replies

Simon
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Old 21st March 2006, 11:33 PM   #5
lndm is offline lndm  Australia
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Two things.

1. Your frequency response will resemble your impedance plot. ie, an impedance bump will bring about a f/resp bump.

2. Consider the effect of the elliptical load line that a complex load gives, (triodes are noted for handling this better than tetrodes).

I once stared in amazement at a real time/pink noise frequency response plot I was taking. I thought I had left the rig configured for my las test (impedance magnitude). But no, I had a reasonably flat f/resp combined with the effect of low Zo
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