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A 16ohm driver with a DCR of 7.8 ohms? JBL 2441


JL Sargent

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Is it normal for a 16ohm driver (JBL 2441) to have a DCR of 7.8 ohms?

What would the normal DCR be for the same driver with an 8 ohm diaphram?

Lastly, can it really be a 16ohm driver with a DCR of 7.8 ohms? Seems fishy to me.

Driver impedance not very straightforward to me I guess.

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The d.c. voice coil resistance you state seems about correct. That is the impedance at zero Hertz. And at that freq, resistance and impedance are the same.

The difficult thing is that manufacturers publish a "nominal" impedance, as one number. Sometimes that is all that is published. Sometimes the d.c. voice coil resistance is given. And in rare cases, the manufacturer will show a graph of impedances.

But impedance changes quite a bit with frequency. That is why the graph is needed. There is, as I recall, a standard which essentially says what single number is okay to publish as a nominal. Roughly the mean or median in the freq range where the driver is intended to operate. We obviously do not use them at zero Hertz. Therefore d.c. voice coil resistance is always different and lower than the nominal.

Let me suggest that you go to the JBL website, and "Pro" section. There is a sheet for that driver mounted on horn but it doesn't show impedance. But then look at the family of horns used and you'll see the electrical impedance graph on the same one as the acoustic output.

Generally speaking, putting a driver on a horn will increase its electrical impedance in the region where the horn is working. This is an indication that electrical power is going toward making sound.

We can model with with two resistors. A 10 ohm (voice coil) in series with a 0.1 ohm (air load). Only 1/100th (1%) of the voltage is experience by the 0.1 ohm.

But let us replace the 0.1 by another 10 ohm. Now the voltage is equally divided between the two resistors. Our load resistor gets half of the applied voltage. (50%) We have a 50 times increase in voltage being delivered to the air load.

You can point out that the current in the second example is about half the first one (10.1 ohms versus 20 ohms) . Fine. But if we look at voltage times current, as power, we see that the power in the air load resistance is still 25 times higher, in the second case. .

Isn't that WEIRD? Yes, but true. By making the overall resisitance higher, we've allowed more power into the air load resistor. That air load resistor is the power being sent into the horn and out into the room.

Wm McD

Wm McD

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That really helps me alot. So the driver coupled to the horn making sounds creates impedances that represent "real" impedence values for that driver across its usable frequency range? The DCR is really not that useful in determining what the crossover "sees" in the driver I guess? Thank you.

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John is, of course, correct. Every piece of the speaker system, electrical, mechanical, and acoustic, contributes to what we call that electrical impedance "seen": by the amp.

If we have a good understanding of how those all combine we can make some some good estimates of what is going on just based on very careful analysis of the electrical impedance at the input to the driver. This is how so many of the characterists of a woofer can be infered by the electrical tests leading to the T-S parameters.

My point was that the acoustic part tends to increase electical impedance. Which is a good thing. This is not part of the T-S tests, though. They go to measurement in free air.

But this might not be answering your basic questions.

I've taken a snap shot of the JBL data on the driver mounted on a horn. Again, see the JBL webite.

Reading the graph is like taking the SAT's perhaps. What does this graph show? Here JBL is showing two things.

One is acoustic output of the horn in dB. This is the top curve with measurement units on the left Y axis. Boy this combination of horn and driver is very good.

They are also showing electrical impedance. This is the bottom curve with measurments on the right Y axis. This is tough to read and we have to mentally squit a bit and fill in the blanks.

1) Does the electrical impedance at 0 Hertz show on this graph? Nope. We only get down to 20 Hz. But we see a trend where it is fairly flat and we can extapolate that it is not going much lower. So there we see that 7 ohms is a pretty good estimation of where it is going at zero.

2) Even though the right axis does not have all the divisions we'd like, we can see that the electrical impedance is varying widely. And we see that it is someplace around 16 ohms, on the average.

3) But, maybe you say: I know that a passive crossover circuit has to be made in accordance to the electrical load at that cross over freq and probably for a freq range of 2:1 on either side of the freq.. What do I use? Here the graph says . . . you are in deep trouble.

4) The thing in 3) is why designers use Zobels to even out the impedance load. But then we need to know how to make up those Zobels.

Wm McD

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post-2552-13819454853118_thumb.jpg

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Looking at that graph seems you could cross that driver/horn at 500hz with a Khorn or other suitable bass bin going two way and be in pretty good shape without a tweeter.

Other than the two camels backs the impedance seems pretty steady to my untrained eye. I would say the average impedance on that combo closer to 8 than 16 ohms though. It spends alot of time below 10 ohms. Maybe Im seeing it wrong. It looks like it should be treated as a 8 ohm speaker on the crossover circuit. What am I missing?

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"in actual application you're going to use it above 400 hz, so block that portion of the imp curve including the first huge peak. "

Really?

Add a swamping resistor or a notch filter. Newer JBL speakers all use notch filters, despite their complexity and expense.

Now why would they do that?

The impedance peak below the crossover point interacts with the crossover, causes distortion, and excursion problems that can result in damage.

The swamping resistor is the no-brainer, no-cost way to improve this.

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The traditional way to specify a compression driver impedance is to use twice its DC resistance.

Newer JBL 8R compression drivers should measure 3R2 ±8%

http://www.jblproservice.com/pdf/Studio%20Monitor%20Series/4430LR.pdf

Newer JBL 16R compression drivers should measure 8R ±8%

http://www.jblproservice.com/pdf/SR-Series/SR4732.pdf

In practice, the 8R JBL 2425/26H has an impedance minimum on the JBL2370A horn of about 5 ohms at 5Khz, and a DCR of 3R2.

http://www.xlrtechs.com/dbkeele.com/PDF/Keele%20(2004-10%20AES%20Preprint)%20-%20Maximum%20Efficiency%20of%20Compression%20Drivers.pdf


"the maximum nominal efficiency occurs when the reflected acoustic load resistance equals the driver 's voice-coil resistance and the maximum true efficiency occurs when the reflected acoustic load resistance is much higher that the driver’s voice-coil resistance. "


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