2. tech-talk
  3. 193011-20253

Impedance - The most ignored and useful measurement tool

I’m constantly reading about audiophiles diagnosing their speakers or attempting to mod their crossovers with expensive new parts. The one tool I wish they’d all get and rarely do is an impedance graphing tool. These are either nearly free or affordable.

They let you produce impedance charts like Stereophile does, as well as measure capacitors and inductors with ESR/DCR respectively.

In the nearly free variety you can build one and use Room EQ Wizard. In the affordable is the Dayton Audio V3. Either one does an excellent job of measuring a driver, crossover parts and the entire speaker as well. Completely irreplaceable tools.

Diagnosing your speakers with the help of others on the internet is made so much easier when you have one of these. Even if your speakers are fine, measure them and keep the charts handy for when they do go bad, it will make it much easier to understand what is and is not working. Replacing a cap/coil? Measure them and the speaker before and after when you are done to make sure everything came back together properly.

You’d be surprised how many speakers have a woofer or tweeter that has stopped working but the listeners don’t even realize it.  These are immediately visible problems in the impedance plot.

Of course, it's just a tool, but when a driver goes bad, or a solder joint fails the impedance charts will go wildly off track.  It's up to you to dig in and diagnose further.

erik_squires
lanx0003
923 posts
 

So I emailed Mr. John Atkinson regarding what exactly the fomula they have used in Stereophile for reporting the EPDR. He kindly repsond promptly as follows:

I use a formula created by editor Jim Austin. after importing the Impedance magnitude and phase data into an Excel spreadsheet.

Data = R (magnitude) ohms & R (phase) degrees

Calculate Vdiss = 1+4.2 x ABS(R Phase)/90

Then EPDR = R(Magnitude)/Vdiss

For example:

If R = 4.0 ohms and Phase = +45 degrees

VDiss= 3.1 and EPDR = 1.2903 ohms

Here Vdiss represents voltage dissipation. As a further analysis, this linear equation for Vdiss provides two exact boundary values: one at 0 degrees (which is obvious) and the other at 90 (-90) degrees, as derived by Mr. Jack Oclee-Brown, shown below. However, the linear equation overestimates Vdiss values in between, which in turn results in even lower reported effective impedance.

One possible remedy is to fit the data range with an exponential term, as illustrated in the attached figure. Instead of using 1 + 4.2 * abs(phase angle)/90, nonlinear fitting yields 1 + 4.196 * (abs(phase angle)/90)¹.¹⁷⁸, which provides a much closer approximation to Jack’s formulation.

 

westcoastaudiophile
381 posts
 

@erik_squires +1 (again) “I was only posting this to show how an important tool can help audiophiles diagnose their problems..

thanks for pointing to the simple tool to check speakers! 

my intension was to emphasize on PA system (amp+cable+spkr) importance, and methods to test it. 
 

unsound
7,336 posts
 

@re-lar-kvothe , “ 2 Ohm stable” while better than nothing is really more market speak than of much value. It only suggests that the amp won’t go into oscillation when presented with a 2 Ohm load. It does not suggest how much power and/or at how much distortion will be provided at 2 Ohms. Much more useable would be an actual spec. of  ? Watts into 2 Ohms +/- dB. And in the case of stereo amplifiers with both channels driven.

More to discover