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Old 12th May 2004, 06:22 PM   #1
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Question Zobel Network question

I just built a sealed enclosure for two 15" subs and the plotted impedance rises from its nominal 1 ohm to over 6 ohms @ 40hz. The subs are running on a class D amp that will put out next nothing at 6 ohms.

I have plotted the impedance curve in bass box pro using a zobel network of 1.25 ohms and 4000 micro farads and the curve totally flattens out.

I know this subject has been beat to death, and yes I've searched long and hard on the subject................but, is there any good reason why I cant use one?

It seems to me that it would be useful for a sub in this situation because the impedance rise happens at 40hz where there shouldn't be any roll off.

If it is a plausible idea, what kind of power ratings do the resistor and cap need to handle a 1200 watt setup

I appreciate any help or info you guys can give me.


Thanks in advance,

Sean
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Old 12th May 2004, 10:05 PM   #2
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Your amp doesn't care what the impedance of the speaker is. You don't need a zobel. All your speaker cares about is volts, not power....unless it's too much power While your speaker may have a 6 ohm impedance at resonance, it is also 6 times as sensitive at that frequency, so everything evens out.

If you put in the zobel, you will just be dumping all of the "extra" power your amp is putting out through the resistor and Cap - which is dumb. A resistor and Cap are the wrong type of zobel for a resonant peak anyway.... Even the correct type would just act as a radiator for heat, so there is no reason to do it.
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Old 12th May 2004, 10:48 PM   #3
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Thats exactly what I thought. I know when they are used in a crossover its so the crossover point doesnt change, not to "gain" any power to the speaker.

Thanks for the response.
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Old 13th May 2004, 12:16 AM   #4
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Just to keep matters clear, an impedance correction network as talked about here is NOT called a Zobel.
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Old 13th May 2004, 01:08 AM   #5
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Quote:
Originally posted by Bill Fitzpatrick
Just to keep matters clear, an impedance correction network as talked about here is NOT called a Zobel.
So, is it only a Zobel if it's attached to a tweeter? Or is there some other (probably obvious) distinction I'm missing...?
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Old 13th May 2004, 01:34 AM   #6
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As frequency increases the impedance of a speaker increases due to the inductance of the voice coil. A Zobel is an RC network placed across the driver. The impedance of this network decreases as frequency increases, thereby compensating. A Zobel may be placed across any driver; tweet, mid or woofer.

This is not to suggest that the above is the only use for a Zobel.
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Old 13th May 2004, 01:49 AM   #7
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Quote:
Originally posted by Bill Fitzpatrick
As frequency increases the impedance of a speaker increases due to the inductance of the voice coil. A Zobel is an RC network placed across the driver. The impedance of this network decreases as frequency increases, thereby compensating. A Zobel may be placed across any driver; tweet, mid or woofer.

This is not to suggest that the above is the only use for a Zobel.
I assume you're replying to me... yes, I understand the function of a Zobel network; where I'm confused is how what the original poster described differs from what you just described.
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Old 13th May 2004, 03:55 AM   #8
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Quote:
Originally posted by HeatMiser

I assume you're replying to me... yes, I understand the function of a Zobel network; where I'm confused is how what the original poster described differs from what you just described.
I was just babbling incoherently.

Actually a question for Sean is in order. Are you bi-amping? If you are passively crossing over, what is the crossover frequency and what slope are you after? What is the next speaker up in the frequency chain?
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Old 13th May 2004, 05:27 AM   #9
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If one wants to call an RLC shunt filter an "impedance conjugate network" rather than a zobel (which is according to Bill limited to RC impedance conjugates - although it is also used as a term for RL conjugates in amplifiers) I guess one may feel free to be pedantic.

Notice that the OP is describing a rise in impedance at 40Hz. This is well below any inductive rise unless his woofer has a 20+mH inductance....

Since the OP posted: "Thats exactly what I thought. I know when they are used in a crossover its so the crossover point doesnt change, not to "gain" any power to the speaker." I would say he understands somewhat the crossover benefits of having an RC conjugate. Seeing that it is a 1ohm/1200W system, it is most likely a Car system, and those are almost exclusively actively crossed, and rightly so. Imagine the power being sunk into a typical subwoofer -valued passive inductor coil with a DCR of perhaps half an ohm, when using a 1200W amp into a load impedance of 1ohm.....

In my take on the question, it seemed the OP was concerned about a loss of power. I merely pointed out that any extra power delivered would go through the Zobel - (Zoebel?) ;-)
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