Measuring JBL impedance - diyAudio
Go Back   Home > Forums > Live Sound > PA Systems

PA Systems A forum for discussion of all parts of a sound reinforcement or DJ system: loudspeakers, mixers (desks) etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 13th August 2012, 11:08 PM   #1
diyAudio Member
 
Join Date: Aug 2012
Default Measuring JBL impedance

I have a couple of gutted JBL EON series speakers I got for $10. The horns are gone, but the woofers and cases are still good. I would like to know the impeadence of the woofers so I can convert them to run off an external amplifier (in college, so I can't afford anything really good). I have a 200W Carvin amp I was given, and would like to drive them with. I was trying to use a 1ohm resistor in series with the speaker to get the current in the circuit, and a second meter across the speaker coil, and then Z=V/I when I realized my I was not changing. Since I am sending sine waves, my DC meter is useless for measuring them. The AC functions don't go low enough to get a good reading. Does anyone know the impeadence of these or know a better way of finding out?

Thanks in advance
  Reply With Quote
Old 14th August 2012, 12:18 AM   #2
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
Measure the resistance of the voice coil. I can;t speak to the Eons, but in general, the DC resistance is typically a little lower than the nominal impedance. AN 8 ohm speaker will usually read about 6-7 ohms, a 4 ohm speaker about 3 ohms, a 16 ohm speaker about 12-14 ohms. This is just a very crude rule of thumb.


Another way is to ask JBL.

Just looking I find spec sheets for their products. I don;t know which Eon you have, but for instance this one has a 130 watt amplifier for hte woofer.

http://www.jblpro.com/pub/mi/eon15p-1.pdf
  Reply With Quote
Old 14th August 2012, 12:36 AM   #3
diyAudio Member
 
Join Date: Aug 2012
Not sure what model they are. I have 1 EON 15, but the good two are earlier models, but both are missing their name plates. I was hoping for something more exact than guessing based in the real (nominal DC) resistance. I'll be going back to school soon, so I hopefully I can use a LCR to measure the imaginary part as well. Thanks for the rule, though. It should at least tell me if I need to be scared of overloading my amplifier or not.
  Reply With Quote
Old 14th August 2012, 02:19 AM   #4
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
Look at this impedance curve on an Eminence speaker:
http://www.eminence.com/pdf/Legend_1218.pdf

Note the impedance shoots up to about 100 ohms at the resonant peak about 100Hz. Drops down to 8 ohms and then back up to about 30 ohms by the time the response falls off at 5kHz.

So when using equipment to measure impedance, be careful what freqs you use, it matters.
  Reply With Quote
Old 14th August 2012, 02:50 AM   #5
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
There is nothing "more exact".

Check the DCR and then you know the nominal impedance.

You can be pretty darn sure that your Carver will drive any JBL speaker. Only car speakers are at 2 ohms or so...

Unless ur planning on playing it VERY loud, you won't do squat to the amplifier... even if it is 2ohms nominal. It's not, so don't worry.

_-_-bear
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 14th August 2012, 06:09 PM   #6
diyAudio Member
 
Join Date: Aug 2012
The DC resistance of each is about 2.4 ohms, which makes me think the coils are probably rated at 4 ohms. My amp is rated to drive two 8 ohm loads or one 4 ohm in bridge. I know I can still drive the speakers with it at low volume, but they're going in a common area, and I'm afraid someone will come along and crank it up at some point, and then I'll have an overheated amp. I got this amp from someone who used it as a monitor in his home studio. He said he replaced it because one transistor overheats when driving heavy loads. I have yet to push it that far, but with a lower load, I don't know. Do you think I will have any issues with this? Should I get some high wattage 4 ohm resistors and throw them in series with the speaker? I would just run them in series, but I want stereo, so that won't work.
  Reply With Quote
Old 14th August 2012, 06:58 PM   #7
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
If your meter won't resolve low AC volts then knock up an opamp gain stage of *10 or *100

Drive the speaker via a 600 ohm resistor from an amp. Set the amp to give 6 volts RMS output. Connect the AC voltmeter across the speaker and the value in millivolts/10 equals the impedance.

So 80 mv is 8 ohm. 30mv 3 ohm and so on.

Just make sure the DVM is accurate at the frequencies you want.

(credit for the test method to Doug Self)
__________________
-------------------------------------------------------
A simulation free zone. Design it, build it, test it.
  Reply With Quote
Old 14th August 2012, 10:46 PM   #8
diyAudio Member
 
bear's Avatar
 
Join Date: Apr 2002
Location: New York State USA
CarVIN amp or CarVER amp??

Yeah looks like a lowish nominal 4 ohm speaker...

You could get away with a 2 ohm resistor in series, but you'd be best off paralleling a set of 4 x 20watt resistors, 8 ohms each to get the required 2 ohms... and put air space between the resistors...

You'll lose a few dB, but not enough to talk about, and also reduce the damping factor, so you will get fatter bass... should be sufficient to keep the not so good amp from getting too hot...

_-_-bear
__________________
_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
  Reply With Quote
Old 14th August 2012, 11:29 PM   #9
diyAudio Moderator
 
pinkmouse's Avatar
 
Join Date: Apr 2002
Location: Chatham, England
The Eons are 4 Ohm drivers.

Quote:
Originally Posted by dpeterson3 View Post
My amp is rated to drive two 8 ohm loads or one 4 ohm in bridge.
Are you sure? That's completely the opposite of any amp I've ever come across
__________________
Rick: Oh Cliff / Sometimes it must be difficult not to feel as if / You really are a cliff / when fascists keep trying to push you over it! / Are they the lemmings / Or are you, Cliff? / Or are you Cliff?
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Measuring impedance with ltspice Telstar Software Tools 1 27th January 2011 02:50 PM
Measuring impedance of Pot/attenuator 6L6 Analog Line Level 14 7th January 2011 07:00 PM
measuring speaker impedance dallaire Multi-Way 4 11th December 2007 10:44 AM
Need help measuring impedance. ch83575 Multi-Way 4 24th November 2005 08:31 AM
Measuring speaker impedance gary f Multi-Way 12 5th June 2004 12:18 AM


New To Site? Need Help?

All times are GMT. The time now is 06:06 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2