Crazy Impedance Curve

[abpea]

I'm modeling an Alpair 6.2P driver in a closed box. When I pull up the impedance curve I get the graph below. Does it cause a problem when the impedance goes straight through the roof??

[Davey]

Most likely you have the wrong information for Le in your driver parameters.

I was looking at the data sheet for that driver and it shows Le of 6.94 microhenries. If I plot an impedance file of a driver with Le of 6.94 millihenries it shows me something nearly identical to yours.

Hope that helps.

Dave.

[abpea]

Davey,

Good catch, thank you sir! You are correct, the difference between micro and milli is significant. See corrected graph below (looks much better now!).

Next time I'll do a better job of keeping my units straight!!

Thank you again for pointing my error out to me, much appreciated my friend!

bp

[Bob Brines]

OK, but that 6.94μH is at DC, correct? Anydriver will have a rising Le with rising frequency. A plot that levels out a the DC Le at high frequency is also very wrong.

Bob

[DBMandrake]

Originally Posted by Bob Brines

OK, but that 6.94μH is at DC, correct? Anydriver will have a rising Le with rising frequency. A plot that levels out a the DC Le at high frequency is also very wrong.

Quite true. Modelling a drivers impedance curve based on Re and Le is an exercise in futility, as the Le is not constant with frequency.

The only way to do it accurately is actually measure the impedance curve of the driver, which is not difficult if you have one already.

[Davey]

Yeah, I have my doubts about the listed value of 6.94uH on the driver data sheet. In fact, if you look at the impedance graph on the data sheet it does show the curve rising above 500Hz. You are correct that a proper measurement of an actual driver should be made to confirm.

Anyways, the gross error at higher frequencies in his simulation that bp was seeing was certainly just a pilot error on data input.
Which, I think, is all he was really asking about.

Cheers,

Dave.

[badman]

Originally Posted by DBMandrake

Quite true. Modelling a drivers impedance curve based on Re and Le is an exercise in futility, as the Le is not constant with frequency.

The only way to do it accurately is actually measure the impedance curve of the driver, which is not difficult if you have one already.

Indeedy- I tend to look at the Le at a couple points on the rising portion of the curve, and select a value based upon the averaging of Re + Le that most closely matches the curve. Given that most of the increasing inductance in all but fullrange drivers (which is of course what we're discussing here) is above the passband anyway, this tends to make a good approximation for lightweight modeling purposes.

[Bob Brines]

Originally Posted by Bob Brines

OK, but that 6.94μH is at DC, correct? Anydriver will have a rising Le with rising frequency. A plot that levels out a the DC Le at high frequency is also very wrong.

Bob

First a minor correction to what I said: I meant Re not DC Le.

Second, and not particularly relevant here, you need to use the Le at the cross-over frequency if you want to try textbook XO's.

Bob

[ODougbo]

Is there any disadvantage using a generator - load resistor - volt meter vs. "software" to measure impedance curve(s)?

[DBMandrake]

Yep, it's extremely time consuming to do accurately

Also most multimeters other than high end ones have a limited frequency response and will be inaccurate above a few hundred Hz.

If you have a PC with a sound card and a couple of resistors that's all you need along with some free software to do it the easy way...