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Old 3rd August 2009, 09:18 PM   #1
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Default Do I need a Zobel ?

I have read and studied that the Zobel will create a flat impedence curve making the XO circuit perform as designed.

How do you decide if it is warranted to have a Zobel on either tweeter or woofer ?

I would assume that if your XO point is suficiently high on the tweeter (above the resonance freq) then you may not need one?

On the woofer, would you say that if you XO point is low enough such that there is little rise in the eimpedence at that point you can get away with out it ?

Thanks in advance.
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Old 3rd August 2009, 09:23 PM   #2
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The most compelling reason to use a zobel on a passive crossover network is to provide a flat impedance as seen by the amplifier. Especially with tube outputs with little to no feedback, performance of the amplifier is optimized with a flat impedance vs frequency.

That's my understanding.
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Old 3rd August 2009, 09:37 PM   #3
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Suppose I have a woofer whos impedence curve rises as the frequency rises (I think this is typical). Suppose that I choose an XO point far to the right within that area.

Also, suppose I have not used a Zobel circuit.

What is the effect? Will volume increase as the frequency increases towards the XO point?
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Old 3rd August 2009, 09:56 PM   #4
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I've never bothered to Zobel a woofer, just choose a woofer that doesn't have a skyrocketing impedance at resonance.

Now midtweeters can be a problem that most people overlook/ignore... you don't want an "near-unloaded" tubeamp at HF

Cheers!
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Old 3rd August 2009, 09:58 PM   #5
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Zobels don't help with impedance rise at resonance, only the rise above resonance which is due to the inductance of the voicecoil.

You "decide" if it's warranted by looking/measuring the impedance curve and acoustic output and playing about with the crossover.
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Old 3rd August 2009, 10:32 PM   #6
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I thought the Zobel was to help keep the amp stable by ensureing a more constant load for the amp. So if you remove the Zobel your amp might start ringing or even go into oscillation and burn up.
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Old 3rd August 2009, 10:47 PM   #7
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Here is an impedance curve I pulled of some website... I drew a red arrow at my proposed XO point. I can see that the impedance is rising here but my XO will also be dropping the output off at 12DB.

Will this counteract any negatives of this rise in this region ?

Click the image to open in full size.
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Old 3rd August 2009, 11:49 PM   #8
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As has been mentioned before, a zobel network is used to counteract the impedance rise created by the inductance of the voice coil.

A zobel network is basically another tool a designer has available to help them to shape the acoustic output of the driver via the crossover.

If you need one or not depends on the driver/s being used.

That being said, textbook crossover calculators assume a perfectly flat impedance and perfectly flat frequency response.

If you were working with a wide bandwidth driver that was reasonably flat..

http://www.tymphany.com/832873

..one of those for example. And you decided you wanted your crossover frequency to be at 2khz. Obviously this drivers frequency response extends well beyond that point. If you were to put a text book 4th order electrical slope into practice, you'd end up reasonably close to your desired 4th order acoustic goal.

But you can go one step further and that's to apply a zobel network across the drivers terminals. The effect will be to flatten the drivers rising impedance thus making the text book crossover see the impedance it was designed to work with as well as a flat frequency response, getting you one step closer to your goal.
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Old 3rd August 2009, 11:56 PM   #9
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Quote:
Originally posted by cbdb
I thought the Zobel was to help keep the amp stable by ensureing a more constant load for the amp. So if you remove the Zobel your amp might start ringing or even go into oscillation and burn up.
Yes, that is another form of zobel. Different from the one in the topic here.
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Old 4th August 2009, 02:08 AM   #10
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What are the negatives of using the zobel ?
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