Go Back   Home > Forums > Loudspeakers > Planars & Exotics

Planars & Exotics ESL's, planars, and alternative technologies

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 2nd September 2005, 12:30 PM   #1
Pwoida is offline Pwoida  Australia
diyAudio Member
 
Join Date: Aug 2005
Default Impdeance of ESL varying with freq?

Hi
I have been trying to measure the input impdeance of my transformer's primary with my DIY ESL connected at the secondary. I am using a signal generator and a digital true RMS multimeter to measure the input voltage(Vrms) and then the input current(Irms). I then calculate Zin=Vrms/Irms. As expected the impedance has fluctuated wildy with frequency from 120 down to 0.3ohms.

What wasn't expected is the variation of the input impedance with the input voltage and power level. At low frequencies the input impedance appears to decrease quadratically with increasing input voltage. I have gotten similar results at several low frequencies. At high frequencies the input voltage and power doesn't seem to have an effect on the input impedance.

Anyone have any explanations on this behaviour?

Thanks
Pete
  Reply With Quote
Old 2nd September 2005, 12:59 PM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Could you sketch out a schematic of your test setup? I've got a suspicion...
__________________
"The way to deal with superstition is not to be polite to it, but to tackle it with all arms, and so rout it, cripple it, and make it forever infamous and ridiculous."- H. L. Mencken
  Reply With Quote
Old 2nd September 2005, 01:47 PM   #3
Pwoida is offline Pwoida  Australia
diyAudio Member
 
Join Date: Aug 2005
I've never attached a picture before but hope this works.


I'm a little suspicious of internal resistance or something the multimeter may change?
Attached Images
File Type: jpg measuring technique.jpg (40.4 KB, 248 views)
  Reply With Quote
Old 2nd September 2005, 01:54 PM   #4
Pwoida is offline Pwoida  Australia
diyAudio Member
 
Join Date: Aug 2005
Here is another sketch showing the trend in impedance as a function of input voltage and as a function of input power. I also included a sketch of input power as a function of Vin since it wasn't immediately obvious to me that the two graphs agreed.
Attached Images
File Type: jpg impedance trend.jpg (16.6 KB, 208 views)
  Reply With Quote
Old 2nd September 2005, 02:10 PM   #5
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
OK, what you want to do is to use a series precision resistor between the amp and the speaker. Derive the current from the voltage drop across the resistor.
__________________
"The way to deal with superstition is not to be polite to it, but to tackle it with all arms, and so rout it, cripple it, and make it forever infamous and ridiculous."- H. L. Mencken
  Reply With Quote
Old 2nd September 2005, 02:28 PM   #6
Pwoida is offline Pwoida  Australia
diyAudio Member
 
Join Date: Aug 2005
I know of this method but what was wrong witht he one I was using? How are these results explained?
  Reply With Quote
Old 3rd September 2005, 05:05 AM   #7
diyAudio Member
 
I_Forgot's Avatar
 
Join Date: Jan 2005
Location: Phoenix, Az.
As you have discovered the impedance at any given frequency is a function of many factors. I THINK the variation with power that occurs at low frequencies is because the amount of power coupled to the air is a larger (smaller?) fraction of the total power dissipated. You are not the first person to have seen this behavior. The attached image is from the Quad ESL-63 service manual.

Are you doing these measurements with DC bias applied to the diaphragms? I think you will find the impedance changes if you turn the bias on and off.

The meter has a precision resistor inside it (called a shunt, used when measuring current), so the only reason to use an external resistor is if your meter is more accurate at reading voltage than current. I would not expect the resistance of the shunt to change during your tests.

I_F
Attached Images
File Type: jpg esl63_manual06.jpg (40.2 KB, 187 views)
  Reply With Quote
Old 3rd September 2005, 07:20 AM   #8
Pwoida is offline Pwoida  Australia
diyAudio Member
 
Join Date: Aug 2005
I have been testing with the bias supply on so the ESL makes noise. I will test with it off and after long enough for most of the charge to leak away. I will also compare the two testing methods.

I was not confident with my results because the amp I use has two 32Vrms secondary windings giving rails of about +/- 45VDC which should mean the maximum Vrms measured at its output was 45/sqrt(2)=32Vrms(unsurprisingly!) but I got a reading of 44Vrms??? Any ideas?

This variation with power level makes it very difficult to design an amplifier for low frequencies since I don't know the impedance it will drive. If the impedance trend continues as voltage increases the impedance will be very very low with the voltages I am planning to apply.
  Reply With Quote
Old 3rd September 2005, 10:32 AM   #9
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally posted by Pwoida
I know of this method but what was wrong witht he one I was using? How are these results explained?
Much of this difference may be something simple like your meter changing ranges- that causes the internal resistance of the meter to change. The quasi constant current method (using a constant series resistor) avoids those errors and is much more accurate.
__________________
"The way to deal with superstition is not to be polite to it, but to tackle it with all arms, and so rout it, cripple it, and make it forever infamous and ridiculous."- H. L. Mencken
  Reply With Quote
Old 3rd September 2005, 02:33 PM   #10
orjan is offline orjan  Sweden
diyAudio Member
 
Join Date: Mar 2005
Location: Stockholm
Quote:
Post #8
I have been testing with the bias supply on so the ESL makes noise. I will test with it off and after long enough for most of the charge to leak away.
Hi,

Measuring with bias off will change the impedance because of the negative capacitance that comes from the membrane being charged.

örjan
  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
Varying Bass With Different Class-A Amps shaan Solid State 30 14th December 2009 08:18 AM
Varying 2 tube amp characteristics: SE treble and richness otto88 Tubes / Valves 5 14th October 2008 12:54 PM
Impedance rate VS impedance stability TomatoBangBang Tubes / Valves 6 4th September 2008 10:46 PM
varying values in crossover from specified pjpoes Multi-Way 3 28th December 2006 02:30 PM
Need help understanding transformer impedance ratios and impedance matching percy Tubes / Valves 5 28th February 2005 08:35 PM


New To Site? Need Help?

All times are GMT. The time now is 02:48 AM.


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