Impedance Measurement Comparison – Part 1

by Langston Holland

Background

It took almost a month to pull off, but predictably several of my assumptions went down in flames and the guys with the physics backgrounds proved right. Imagine that. I am very thankful for the significant time and effort put into the SAC list impedance thread by the usual suspects. What a gift you guys are. 🙂

My goal was to find the most accurate way to measure audio related electrical impedance that was as simple as possible to execute. I also wanted to understand it. I do have some weird applications in mind, but that’s for another day.

The Players

DATS, CLIO, Audio Precision APx515 with the LinearX VI-Box and a couple of other boxes I made.

Summary

The DATS tester very accurately covers 99% of loudspeaker impedance measurement needs at $100, about 40dB less than the Audio Precision based methods I tried. You should buy this thing.

CLIO is almost as convenient, extremely accurate, wildly more powerful since it’s a full audio measurement system and reasonably costs about 30dB more than DATS. As you’ve probably noticed in my prior posts, CLIO is my primary measurement platform.

I bought the Audio Precision APx515 about a year ago because I wanted to make higher resolution electrical measurements in both analog and digital domains. The hardware performance of this thing baffles me – on one hand it can show low level detail I didn’t know was possible outside of a simulator. On the other hand the philosophy behind the software interface is very restrictive for someone used to EASERA or CLIO for example. Accuracy with the AP is state of the art if you setup the test correctly. For impedance measurement I used two-channel setups that divided voltage by current represented by a sense voltage. Z = V/I.

Note

Anyone interested in using one of the two-channel methods that I used with the AP can do so easily with either the old version of Smaart v5.4 or the current version of SysTune Pro v1.2. Both achieved very high accuracy in my tests using the high impedance TRS inputs of the Smaart IO hardware interface. Smaart has by far the easiest calibration setup, but SysTune Pro obviously offers much more power and has the slight advantage in that you can buy it. 🙂

Myth Buster 1

On the SAC list I speculated that voltage and current source methods would yield different results due to: “Effectively zero electrical damping provided to the reactive loudspeaker load, which exaggerates resonance peaks and general sensitivity relative to an actual connection to an amplifier.” WRONG. I found no difference between methods that had direct connections between amplifier and loudspeaker vs. methods with up to 660 ohm generator source impedances. Jay Mitchell wrote “Measured correctly, the impedance of a loudspeaker will indeed be the same when measured with different methods.” I didn’t believe this until I saw it. The key appears to be keeping drive voltages low enough that everything that moves remains linear.

Dale Shirk’s contribution to the thread clarified the reason that we see the same result from either type of measurement. We are dividing voltage by current to calculate impedance. The lower source resistance constant voltage type methods will show a decrease in voltage variation and an increase in current variation, while the opposite occurs with the higher source resistance constant current type methods. The V/I quotient remains the same in either case. This will be demonstrated in Part 2 where differing V and I plots are shown generating virtually identical Z curves.

Take away (Pat™): Measure impedance any way you want to, just do it correctly with proper cabling and connections. It’s only when you want to measure at higher levels that you’ll need to minimize source (amplifier) to loudspeaker resistances.

Myth Buster 2

In reference to the constant current method, I stated “Very low voltage delivered to the loudspeaker, thus high sensitivity to environmental noise induced errors.” DECEPTIVE. The fact is you have to keep drive levels low regardless of method in order to stay within the linear region, you can’t just choose a high drive level with S/N as your only consideration. Drive voltages at the loudspeaker terminals begin to skew the impedance measurement results when you exceed very low average voltages (approx. 50mV) with constant current type methods. With constant voltage type methods, you can get away with higher drive voltages prior to skewing the results (approx. 300mV). Methods that land in the middle of this range, such as in DATS, show good S/N in normal indoor environments.

I adjusted each measurement method to about 40mV across a 10 ohm resistance to keep the results from being biased due to drive voltage differences. The one exception to this was the VI-Box with the 0.01 ohm current sense resistor switched in which required nearly 300mV to get reasonable S/N.

For sake of discussion, I now define “constant voltage type” and “constant current type” methods as those having resistances between source and loudspeaker of 1 ohm or less and 500 ohms or more, respectively. Those landing in the middle of this range were called “voltage divider” methods by Bill Whitlock and that is exactly what is going on.

Finally, as mentioned by both Bill and Jay, loudspeaker temperature makes a difference. Heating from use can cause significant changes, but I also saw minor variations due to the temperature changes throughout the day in my home (sorry – I mean laboratory).

lh