Impedance Measurement Comparison – Part 2

by Langston Holland

In Part 1 I gave an overview of the impedance measurement methods we would be comparing. More importantly, we learned that we should expect equivalent results from each when driving the DUT at low enough levels to stay within its linear region. Exceeding the linear region is easiest to determine by increasing levels until the peaks at resonance begin to compress. In Part 2 we begin by describing the measurement setup and conclude with a thorough look at each of the methods including actual results into standard resistances as well as a loudspeaker.

All images can be enlarged for clarity by clicking on them.

Setup

Reference resistances were provided by a GenRad Type 1432M decade resistor. This eBay sourced golden oldie is a work of art. It has extremely low reactance and measures as a perfect resistor per the indicated value within the tolerances of my Fluke 8060A multimeter with the test lead resistances subtracted out with the “REL” function.

GenRad Type 1432M – External View

GenRad Type 1432M – Internal View

The reference loudspeaker used was a fairly new Danley SM100 known to be in good working order. In regard to the loudspeaker, tests were made both without and with an obstruction.

Without Obstruction

With Tool Box as Obstruction

This obstruction may seem severe, but little effect was visible with smaller items due to the fact that there is very little horn-loading in this loudspeaker. You’ll also notice on the plots that the traces showing the obstruction are somewhat different between measurement methods. Given that the non-obstructed measurements are virtually identical, we must conclude that I placed the obstructing box slightly differently between measurements. Next time I’ll go to the trouble of leaving the obstruction in place and running through the different methods before moving anything. Bad scientist! 🙂

An overview of the mess:

Light Bulbs?!

DATS

Great engineering is the art of using the least resources to get the job done with the necessary accuracy, reliability, etc. DATS is a home run in this dept. I’ve used DATS for several years – my software was upgraded from when it was called WT3 while the hardware continues to work perfectly.

Though the manufacturer doesn’t include this information, measurements show that the small epoxy encapsulated USB based hardware interface uses a 100 ohm generator output impedance to drive and measure the DUT. The unit boots up with a default indicated +5dBu output into no load regardless of the setting you may have selected in a previous session. The 1.40V no load voltage drops down to 125.54mV when loaded with a 10 ohm resistor. The math to calculate the output impedance of a circuit measured in this way is:

Rout

The circuit schematic probably looks like this:

DATS Schematic

This 100 ohm resistance allows a higher drive level than typical current source type methods. I reduced it to an indicated -5dBu (616mV peak) during testing to make it equivalent to CLIO and the other methods, but normally I’d leave it alone for improved S/N.

Resistive loads of 1, 5, 10, 50, 100 ohms measured 1.02, 4.99, 9.96, 49.81, 99.63 ohms at 1kHz with flat traces. Measurements up to 3,000 ohms were good to within 1% at 1kHz with a slight HF rolloff. Above that wasn’t reliable.

SM100:

DATS SM100

SM100 with obstruction:

DATS SM100 with Obstruction

CLIO

CLIO also is a great work of engineering and more than justifies its cost for those who’s work includes a broad cross section of loudspeaker design and testing, acoustic work and electrical domain measurement. CLIO offers four impedance testing methods, each of which can use four very different stimulus types and associated calculations. Though they all yield the same answers when used correctly, the non-FFT sine sweep and gated sine sweep methods have slightly lower frequency resolution than the two FFT based methods of sine sweep and MLS. I chose FFT sine sweep for loudspeaker measurements to keep apples to apples. CLIO’s next version due out very soon will add a 5th method with a simultaneous division of voltage by current sense like SysTune Pro offers.

CLIO’s “internal mode” impedance method was used in this test, which simply involves connecting a XL cable to one of its outputs and attaching the DUT between pins 2 and 3.

The circuit schematic looks like this:

CLIO Schematic

This 660 ohm resistance provides a typical current source method low level drive to the DUT. You do need a quiet environment during the test for adequate S/N.

Resistive loads of 1, 5, 10, 50, 100 ohms measured 1.01, 5.01, 10.01, 50.01, 99.98 ohms at 1kHz with flat traces using the non-FFT based sine sweep method. Measurements up to 50,000 ohms were good to within 1% at 1kHz with a slight HF rolloff. Above that wasn’t reliable.

SM100:

CLIO SM100

SM100 with obstruction:

CLIO SM100 with Obstruction

APx515 with ZBox

This may sound crazy, but even at the 40dB price the APx515 is a very good deal in my opinion and a stunning piece of engineering. Apart from all the fancy things it can do and its extreme accuracy, what I really like is that you don’t have to be careful to get dead-on repeatability.

The thing I call a ZBox is the result of an effort to get a highly accurate impedance measurement method that didn’t need an external amplifier and could be used for both loudspeakers and electronic testing up to at least 50,000 ohms. It uses the V/I differential method nicely documented by Paul Henderson and Chris Strahm. One of the lovely things about the V/I method is that the division cancels any like values in the numerator and denominator, thus transfer function source imperfections are removed.

The circuit schematic looks like this:

ZBox Schematic

The 100 ohm current sense resistor attached to circuit ground proved to be the best overall compromise for the “one size fits all” design goal.

ZBox – External View

ZBox – Internal View

The 50 ohm generator output impedance added to the 100 ohm current sense resistor provides a strong output level if desired given the APx515’s capabilities.

Resistive loads of 1, 5, 10, 50, 100 ohms measured 1.03, 5.02, 10.01, 49.96, 99.85 ohms at 1kHz with flat traces using a 5 second swept sine FFT with no averaging. Measurements up to 50,000 ohms were extremely flat. Up to 100,000 ohms was within 1% at 1kHz with HF rolloff beginning above 2kHz.

SM100:

ZBox SM100

SM100 with obstruction:

ZBox SM100 with Obstruction

The nominally 8 ohm loudspeaker results in more voltage swing than current given the 150 ohm source:

ZBox SM100 – Voltage and Current

APx515 with VI-Box at 1 Ohm

The circuit schematic can be seen in the VI-Box manual linked to in the ZBox section.

The 1 ohm current sense resistor is attached to circuit ground given that the majority of voltage across this resistor will be the same at either end, thus common, thus stressing the CMR ability of the analyzer hardware. It is far better to place the loudspeaker load across the floating differential circuit given that it will rarely get below 2 ohms and will mostly be much higher, resulting in a much greater voltage drop across the differential input relative to the common voltage.

This will be 100 times as true in the next section that covers the VI-Box using the 0.01 sense resistor.

LinearX VI-Box with Joe Nickell Modification

LinearX VI-Box – Internal View

We now must use an amplifier to drive the 1 ohm resistor into the load and this amplifier must have it’s negative terminal in common with the circuit. A bridged type amplifier will overwhelm the CMR requirements of many analyzers, and switching to the 0.01 ohm sense resistor with a bridged amp will overwhelm the CMR of any analyzer.

The amplifier used in each of the following constant voltage type measurements was a Stewart Audio PA-50B in common ground mode. Although the amp is not rated for use below 4 ohms, the low levels used in these tests allowed operation into 1 ohm for the duration of the sweep without problems. Again, the V/I division is our friend as it removes transfer function imperfections from the measured results.

Resistive loads of 1, 5, 10, 50, 100 ohms measured 1.04, 5.03, 10.01, 49.89, 99.78 ohms at 1kHz with flat traces using a 5 second swept sine FFT with no averaging. Measurements above 100 ohms are somewhat impractical due to the diminishing output from the current sense resistor. With greater current capability in my resistors, I could have measured above 100 ohms – but this isn’t the right hammer for high Z measurements – it’s designed for higher power, in-use type measurements which it does beautifully.

SM100:

LinearX VI-Box SM100 – 1 Ohm Sense

SM100 with obstruction:

LinearX VI-Box SM100 – 1 Ohm Sense with Obstruction

The nominally 8 ohm loudspeaker results in much less voltage swing than current given the 1 ohm source:

LinearX VI-Box SM100 – 1 Ohm Sense Voltage and Current

APx515 with VI-Box at 0.01 Ohm

We again use an amplifier to drive the 0.01 ohm sense resistor into the load and it’s more important than ever that this amplifier have it’s negative terminal in common with the circuit. Given the 100 fold decrease in the current sense voltage, I was forced to increase the DUT drive voltage to about 300mV to get adequate electrical S/N. Fortunately, the nearly direct connection between amp and loudspeaker kept it within its linear region, thus the measurements are again virtually identical to all the other methods.

Resistive loads of 1, 5, 10, 50, 100 ohms measured 0.96, 4.98, 9.96, 49.93, 99.30 ohms at 1kHz with flat traces using a 5 second swept sine FFT with no averaging. Measurements above 100 ohms are completely impractical due to the diminishing output from the current sense resistor at the chosen drive level.

SM100:

LinearX VI-Box SM100 – 0.01 Ohm Sense

SM100 with obstruction:

LinearX VI-Box SM100 – 0.01 Ohm Sense with Obstruction

The nominally 8 ohm loudspeaker results in nearly zero voltage swing. Almost all the action is in the current realm with the amp’s 0.04 ohm source impedance rating combined with the 0.01 current sense resistor:

LinearX VI-Box SM100 – 0.01 Ohm Sense Voltage and Current

APx515 with IBox

I call this an IBox to highlight the fact that it uses a Pearson 411 current monitor to directly measure the current going through an 8″ piece of 12 awg wire. The goal of this project was to eliminate the need to use common ground amplifiers and to increase the maximum voltage and current the device could handle beyond the capabilities of any bridged audio amplifier for the foreseeable future. Gene Brandt sent me one of his two model 110 CM’s which are identical to the 411 excepting size and somewhat higher RMS current handling capability. After using Gene’s for about an hour I wanted one really bad and got the 411 because it’s smaller, yet still rated at 50 amps continuous and 5,000 amps peak. I think it’ll handle audio amplifiers into the next century.

I called Pearson and talked to a guy that I could tell knew his stuff. I asked him if the wire needed to be run through the middle of the eye, if a metal box would affect it, etc., and he said no – all the action is in the eye and it doesn’t matter where you route the wire through it. That sounded too good to be true, so I took Gene’s 110 with 2″ eye and placed the current wire in the middle, flush against the inside wall, even diagonally from one side of the eye wall to the other and got the same measurements within 10nV! (0.00001V) Then I laid it on top of steel and aluminum boxes – again the measurement was unchanged and virtually identical to what Ohm’s Law said it should be with the 4 ohm power resistor I was driving. After installing the 411 inside the final IBox, measurements again were perfect.

The circuit schematic looks like this:

IBox Schematic

The physical unit:

IBox – External View

IBox – Internal View

We have to use an amp of course, but this time it doesn’t matter which type as long as it can drive the loudspeaker successfully to the level of interest. An interesting observation with the CM is that both the voltage and current outputs of the IBox show identical THD on the AP analyzer no matter what the amp’s output is. This simply means the CM is tracking flawlessly. It also means that the V/I division will again remove any transfer function source imperfections.

Resistive loads of 1, 5, 10, 50, 100 ohms measured 1.04, 5.01, 9.98, 49.71, 99.34 ohms at 1kHz with flat traces using a 5 second swept sine FFT with no averaging. Measurements above 100 ohms at low drive levels are again impractical due to the diminishing output from the CM.

SM100:

IBox SM100

SM100 with obstruction:

IBox SM100 with Obstruction

The nominally 8 ohm loudspeaker results in the least possible voltage swing from this setup given the 0.001 ohm resistance added by the 8″ piece of 12 awg cable:

IBox SM100 – Voltage and Current

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