View Full Version : What's impedance matching?

06-01-2005, 04:41 AM
In a passive crossover for a 2-way component set, is impedance matching ever used? I bought new 6.5" components and this is what my dealer told me about the included passive crossovers:

- For the tweeters, the capacitors, inductors and resistors in the XO not only form a 2nd order filter, but also provide impedance matching to help smooth out the natural impedance curve of the tweeter.

- If I were to go active w/my system, I should leave the tweeter's caps, coils and resistors inline with it... impedance matching = better sound.

- But, if I went active and the tweets were hooked directly up to an amp, the amp would see a more erratic impedance and more resonances would result without the passive XO... worse sound.

- Passive components would also provide a fail-safe for the tweeters in case full-range ever hits them. My deck's crossover will still control high-pass frequency.

So, how much of this true? My dealer is an electronics grad, but is also a persuasive salesman. Will my tri-amped system sound better if I kept the passive XO for the tweeter? Any input would be appreciated. thanks in advance.


06-01-2005, 10:46 AM
Well, he isn't lying persay......what he is talking about is called a zobel network. It flattens out the impedence curve to present a consistent impedence to the crossover, and to control natural resonances.

Now, when running active, impedence has no affect on the crossover point (since the crossover is in the preamp stage). So, obviously you won't have to worry about it affecting that. And as he said.....the impedence curve would no longer be flat. But, that isn't necessarily a bad thing, in and of itself. Most speakers are more sensitive as they increase in frequency; and the increasing impedence can help counteract this to give a flatter frequency response. As for the "full range" problem....it shouldn't be a problem. So I wouldn't worry about that.

Like Jmac said; I'd try it both ways and see what you personally think.

06-01-2005, 12:45 PM
I agree with squeak, also theres one other thing to keep in mind...

At least in a homemade passive crossover of 2nd order or higher (12db/octave +) if you unplug one of the speakers, at some frequencies the amp will see a short circuit of close to zero ohms that could cook the amp.

Premade passive crossovers may have something to prevent this, but it may be worth checking in to.

06-01-2005, 03:14 PM
I should have mentioned that my tweeter crossovers boil down to essentially a 12uF cap and 2.7 ohm resistor inline, and lastly an inductor across the tweeter output terminals (crossover freq approx 2500Hz). Because its a very simple 2nd order circuit, I tend to not believe there is any impedance matching involved - wouldn't there be more components involved if there was?

I don't really want to try with/without crossovers because I would have to open one up and disconnect all of the woofers components first. If there is no impedance matching, I will simply forget about the passives and hook the tweeters stright to the amp. Please comment!

06-01-2005, 04:51 PM
I will simply forget about the passives and hook the tweeters stright to the amp.

I'd say go ahead and try it and see what you think ;)

Anyways...like I said in the PM, if there is a resistor, then there is in all likelyhood a zobel network because a simple 2nd order HPF only consists of a cap and inductor, no resistor needed. However, a resistor is used in a zobel network :)

06-01-2005, 06:16 PM

Here is the tweeter section. Is there such thing as a series Zobel network? Most Zobels I came across are a capacitor and resistor in parallel with the driver.

06-01-2005, 06:54 PM
You make that drawing yourself, or did you get it somewhere?