Using QA403 to measure impedance of an inductor?

Do you have any tips for measuring impedance of an inductor with the QA403?

I think repurposing the speaker impedance plug-in seems a good option, with some additional offline processing to calculate and inductance or capacitance from the data.

In the example above, a low power audio amplifier is used to drive the speaker, so the L+ output from the QA403 connects to the amplifier.

Would it be possible to connect an inductor direct across the L+ output of the QA403 to eliminate the amplifier?

Or is the amplifier playing an important role by protecting the QA403’s output ICs from too much current draw?

Maybe there is an easier way for doing this measurement? A frequency response plug-in?

Interested to hear what you think.

I’m following the speaker measurement tutorial at the moment: creating my BNC board!

Hi @Dan, remember the QA403 output has a 100 ohm series R, so if you want to measure something like a 2.2uH inductor, the impedance of the output is huge compared to the inductor. Those types of things are why you usually want a bit more drive. You could also look at pairing the L with a known C and then looking at the resonant peak.

Hi Matt,

That makes sense. So connecting the speaker or DUT to a low output impedance amplifier is essential.

What kind of amplifier would you recommend for doing impedance measurements? An audio amplifier, chip amp? Or something a bit more special?

Thanks for the advice!

I have been using an Aiyma amplifier for making my speaker impedance measurements:

Really only need a couple of watts, plus I can use it for powering the speaker when doing speaker measurements or other things that may come up. It tests pretty good and my speaker impedance measurement results seem pretty good, though it takes a bit of doing to get the sweep levels set up correctly and of course you need to make the test cable.

Var, I’m not sure the TPA3255 has a flat frequency response when changing the load, as most class D amplifier (Hypex UCD and B&O icepower where the first amplifiers to have a flat response whatever the load is). Expecially when measuring driver/speakers, the amplifier to be used has to be flat as a rule, otherwise you are measuring the driver+the amplifier. I built a measuring amplifier for the purpose, using the LM3886.

@clane, this is a very good point about class D. In the link below, you can see a TPA3255 running into 4 and 8 ohm resistive, and the response changes a fair bit due to the LC output filters. TI has some filterless amps around 5W that might not be so sensitive.

The QA461 Transducer Driver uses an OP564 linear amp with up to 1.5A of drive for this reason. It has about 1 MHz of bandwidth and current sensing of output, including front panel indicators for Over Current and Over Temp. I will try to share some measurements this weekend for characterizing L and C so you can see if it helps. It would be nice to have a generic automated test that would sweep a component and show the impedance over frequency so that the speaker plug-in wasn’t doing double duty.

I am looking at responses that I measured for the Aiyma amp, and it is pretty flat to about 10khz and then is up .7dB at 20khz at 30w/8ohms. Totally different response at 4ohms/5w- down about a dB at 20khz… What was I thinking…? The channel balance is about .2dB- there is a mod you can do on it but way too much work. I don’t measure a lot of speaker impedances, but when I do I am comparing them to their data from 20-40yrs ago, and it compares reasonably. Next time I measure a speaker- which will be a JBL Paragon at my friend’s house :laughing:, I will bring by bryston class ab amp…