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2nd September 2005, 12:30 PM  #1 
diyAudio Member
Join Date: Aug 2005

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 
2nd September 2005, 01:47 PM  #3 
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? 
2nd September 2005, 01:54 PM  #4 
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.

2nd September 2005, 02:10 PM  #5 
diyAudio Moderator

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.
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2nd September 2005, 02:28 PM  #6 
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Join Date: Aug 2005

I know of this method but what was wrong witht he one I was using? How are these results explained?

3rd September 2005, 05:05 AM  #7 
diyAudio Member
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 ESL63 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 
3rd September 2005, 07:20 AM  #8 
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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. 
3rd September 2005, 10:32 AM  #9  
diyAudio Moderator

Quote:
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3rd September 2005, 02:33 PM  #10  
diyAudio Member
Join Date: Mar 2005
Location: Stockholm

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

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