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Uncle Paul
02-20-2005, 03:54 PM
I'm working on a speaker project that is a mixture of old and new JBL components I've gathered along the way. The goal is an L300ish tower that will be biamplified.

I'm using the following parts:

2235H
LE85 w/ 2370A horn
2405 (Alnico)

I'd like to get some feedback about using the 16 ohm 2405 with the 8 ohm LE85. I'm planning on building a 3rd order Butterworth at 10 kHz using high quality components. No problem taking the impedance difference into account when designing the crossover, however, I am wondering if I would be better off using an 077 with the LE85, or using a 242x with the 2405. Any thoughts on this will be appreciated. Also, is it possible to convert the 2405 to an 077 with a diaphram change?

Thanks all,
Paul

4313B
02-20-2005, 04:12 PM
You can't design a proper crossover network without impedance curves. It is irrelevant what is stamped on the drivers. The 077 and 2405 are "10 ohm" devices and the LE85 is a "12 ohm" device but you will need to run impedance curves on both. The LE85 will have to be bolted to the horn you are going to use as the horn loads the LE85 and changes the impedance curve.


Also, is it possible to convert the 2405 to an 077 with a diaphram change?Same driver, they both use the D16R2405 diaphragm. The LE85 used the D16R2420 diaphragm.

Uncle Paul
02-20-2005, 06:26 PM
Thanks, Giskard!

Now that you say it, having the impedance curve is obvious - if you don't know the impedance at the crossover frequency then it's just a guess. Thanks also for pointing out that the horn needs to be attached to the LE85 for an accurate measurement.

Simplist way I can think of is to put a suitable measured resistor in series with the transducer and hook up to an audio signal generator & amp. Then measure the rms voltage across it and the transducer as the frequency is changed. Then use Ohm's Law to determine the actual impedance based on the rms voltage measurements.

Of course, being careful to use frequencies and amplitudes that won't cause damage!

Any better, simpler, easier way or suggestions?

duaneage
02-21-2005, 11:57 AM
Thanks, Giskard!

Now that you say it, having the impedance curve is obvious - if you don't know the impedance at the crossover frequency then it's just a guess. Thanks also for pointing out that the horn needs to be attached to the LE85 for an accurate measurement.

Simplist way I can think of is to put a suitable measured resistor in series with the transducer and hook up to an audio signal generator & amp. Then measure the rms voltage across it and the transducer as the frequency is changed. Then use Ohm's Law to determine the actual impedance based on the rms voltage measurements.

Of course, being careful to use frequencies and amplitudes that won't cause damage!

Any better, simpler, easier way or suggestions?

I use a HP signal generator with a 600 ohm output for testing drivers. I use small signals below 1 volt. Go with a 100 ohm resistor between the driver and the signal generator. Then get a small value resistor, between 4 and 8 ohms, to use as a calibrator.

Insert the 8 ohm resistor in place of the speaker. Adjust the output of the signal generator until you read 80mv on your meter. Replace the test resistor with the speaker. Now you can read impedance directly. 45 mv is 4.5 ohms, 200 mv is 20 ohms, etc with a .1 ohm resolution.

My fluke meters can read frequency as well, al I do is hit a switch. But they need at least 20 mv to do so reliably. I would also warm up woofers for a bit by feeding a sine wave into them amplified a bit to get some cone movement. Fs and other values change after they are warmed up.

Uncle Paul
02-26-2005, 03:14 PM
Wow! Better, simpler AND easier! Thanks for the suggestion :applaud:

duaneage
02-27-2005, 12:01 PM
Wow! Better, simpler AND easier! Thanks for the suggestion :applaud:

That is known as an impedance bridge. The Voltmeter uses a similar method to measure resistors ( Whetstone )
The best device there is is an HP 4800A vector impedance meter. I almost had one from ePay but I got snipped at the last moment. That machine is really the cat's meow.

4313B
02-28-2005, 10:40 AM
HP 4800A

Ah yes, I wouldn't mind having one of those myself.