Speaker Impedance with QA403 + QA461

Hi Matt, everyone,

Well I’ve designed a simple PCB for the current sense resistor

following your tutorial. In another thread there was some discussion about benefits of class AB over class D amplifier for this setup.

Well now I’m actually considering getting QA461. It will save me loads of hassle with getting this board made and buying a chip amp.

I see the differential to single ended conversion for the current sense resistors is done internally with QA461.

Are the following connections correct for an impedance measurement?

QA403 Output L+ to QA461 Input
QA403 Output L- terminated with 50r BNC
QA403 Input R+ to QA461 Output current sense
QA403 Input R- terminated with 50r BNC.

QA461 Output- to Speaker-
QA461 Output+ to Speaker+

QA461 Output monitor to QA403 Input L+
QA403 Input L- terminated with 50r BNC

So voltage sense and current sense are single ended?

I guess there is a way of telling the software the output is attenuator by -20dB from the output monitor port of the QA461.

Hope that is close!


Hi @Dan, the QA403 unused outputs should never be terminated. It will cause the output opamp to run hot, and eventually it will go into self-protection.

On the input side, it is correct to terminate unused inputs.

Aside from the output shorted, the rest is correct!

The speaker plug-ins allow you specify a voltage gain (-20 dB for the QA461) and sense resistor value. So, if you want to roll your own you have a lot of flexibility. But I do think the QA461 use of an INA199 was a good change, because the part has outstanding CMRR. At 1 kHz, it’s still well above 100 dB. The INA199 bandwidth is about 80 kHz. I think a future QA46x would aim to buffer the the output divider at the output port (so the output divider current isn’t counted in the output current). And probably reduce the output current sense resistor to 0.01 ohms and use the gain of 100 INA199 to keep the effective output sense resistor at 1 ohm. The OPA564 power opamp used in the QA461 has about 0.1 of output Z around 1 kHz, but it climbs quickly after that. However, the way the impedance measurements are made with the QA461 means the output Z of the power opamp doesn’t matter.

The INA199 CMRR plot is below.



Hi Matt,

Thank’s for the correction - will make sure not short my output :woozy_face:

That’s great to learn a little how it works.


Note to future self (and others), here are the connections

QA403 Output L+ to QA461 Input
QA403 Output L- no connection.
QA403 Input R+ to QA461 Output current sense
QA403 Input R- terminated with 50r BNC.
QA403 Input L+ to QA461 Output monitor
QA403 Input L- terminated with 50r BNC
QA461 Output- to Speaker-
QA461 Output+ to Speaker+

Very happy with QA461 and QA403.

The first thing I did was measure a 1 Ohm resistor I think the results are great, the output level used for the measurement was -20dBV.

I’m curious about the high-end noise and slight rise and I do just mean curious - what could cause that?

I want to measure some speakers that have resonance peaks in at around 100Hz that could be as a high as 80 Ohms. The nearest resistor I had to hand was a 100 Ohm, so I swapped that in, and used an output level of -10dBV.

I think might be pushing the QA461 a bit beyond it’s design with this. I remember in your tutorial it said the current sense resistor should be no more than 100x smaller than the largest resistance you want to measure.

Again I’m just curious at what is going on. I can hear an audible chirp which surprised me, when the output level is set high. Would you expect this measurement to fall outside of scope of the driver?

I played around with different output levels and what is shown are approximate optimum values for each case.

Other settings:

192k Sample Rate
Full scale 0
Attenuator Off
Sys. FFT 512k

Hi @Dan, the thing to keep in mind is that the impedance is computed from the left and right frequency response plots. The left is the voltage across the load, and the right is the current. It’s done as a complex calculation, which means phase matters. And so, as the signal-to-noise of the sweeps degrades at the band edges, the small changes in phase and mag will translate to larger changes in impedance. So, you can often look at the frequency domain plots and see what is the cause.

There is a new automated test coming that will you sweep inductors and capacitors at increasing current levels (assuming you are driving from the QA461). And at higher current levels the parts really squeal. The phenom is is microphonics. That is, if you mechanically distort a ceramic cap, you will change its value. And similarly, as you stimulate a R or C or L, you’ll hear it as the applied voltage/current will cause the part to slightly deform and make a noise. There are lots of app notes written on how to get loud caps quieted down in circuits. It’s a very interesting problem. I first encountered it in the 90’s when rubbing a plastic case on a cell phone would induce a change in a VCO capacitor value, changing the tank frequency slightly before the PLL loop corrected it. But as the phone was FM, it would manifest as as “shhhhhh” noise in the received (and transmitted) audio.

That makes sense, so might just be at the limits. I have not yet checked in more detail as you suggest, but I will investigate those regions for noise bearing in mind the complex division.

Oh cool! Once you have the phase between voltage and current you pretty much have all info that characterises capacitors and Inductors, including all the parasitics! As you already know ;0) I will definitely be using that when it is released, let me know if you need a beta tester!