I was curious if there are plans to add the Speaker Impedance plugin to the QA402’s toolset, and if so, when that may occur ? Thanks!
Hi @Var, release 0.997 has the SPKR Impedance automated test. Inside that plug-in is a Help menu and if you click on it, it will take you to a wiki page. That page will be uploaded in the next 24 hours. But in short, the connection diagram is same as the QA401: Your left channel measured across the speaker, your right channel measures across a sense resistor. I have confirmed the same speaker gives the same plot on the QA401 and QA402. But more QC will be forthcoming.
That is good news, and I can’t wait to try it!
Matt- I followed the setup pretty much as described above and in the HELP section, as well as double checked my wiring. I am using a 0.1ohm sense resistor as shown in the diagram above. The amp I am using is actually and integrated on and I have it set such that with a -12dBV input I have~ 1w/8ohms. Here are the settings used for the plugin:
Here is what the impedance plot looked like:
and here is what the EXP FR looked like:
I did add a bit for gain to the amp, and changed the input attenuators, but nothing really changed. Any suggestions ? Oh, the speakers are “8 ohms” and I tried a different speaker with about the same results. Also, it just dawned on that that I am not hearing a “chirp” from the speaker during its sweep- should I expect to hear a brief one during this test ? That might be why the FR level is so low…
thanks
Hi @VAR, did you hear anything out of the speaker? The yellow trace is the measured voltage across the speaker, and you are showing -60 dBV = 1mV measure across the speaker which is nanowatts. So, with -12 dBV out, if you are measuring -60 dBV that’s 48 dB of loss someplace…
Matt- I was checking the set-up again and figured that I would start ohming out each cable that I made and realized that one cable going to the LEFT + Output to the amp Input, was just a single, 14guage speaker wire soldered into a BNC Male adapter connector- there was no ground connection from the 402 to the amp (none of the other connectors have a ground connection as well as I just made a single wire connector). The fastest solution was just to connect my “normal” bnc cable to the Right + 402 Output, to the Right Channel input on the integrated amp (using a bnc to rca adapter). Things seem to be working better now- and I hear the FR chirp from the speaker, BUT the plots are telling me something is off still. I kept the same setting as before and boosted the gain of the amp a few dB to see if it helped, which it did, but not drastically:
Hi @VAR, OK, this is looking better! Note that you are in +12 dBV full scale input range, but your max input (speaker voltage = yellow) is around -40 dBV. So, switch to 0 dBV full scale input and that will help your SNR. The noise in the plot is because the numbers are very, very small, and a little noise gets magnified by a lot. Your measured voltage across the speaker
Next, take a look at your red trace = current sense. You have specified 0.1 ohms for the current sense, and your value at 1khz on the plot is -80 dBV = 100uVrms. So, you are measuring 100 uV across a 0.1 resistor, so you are sensing 1mA of current into the speaker. that’s not much.
You really want to drive a speaker (4 or 8 ohms) with about 0 dBV = 1Vrms, and with a 0.1ohm sense resistor, you’d see 1^2/4 = 250 mA * 0.1 = 25 mV = -32 dBV for the red trace, and 0 dBV for the yellow trace.
Can you look again at the block diagram of the setup and verify you are connecting as shown? You must be using an amp, and that amp probably has 25 dB of gain or so. So, the QA401 might output -25 dBV, the amp gains that up to 0 dBV and drives the speaker. The left + and - measures across the speaker, and the right channel measures across the sense resistor. You’ll be pushing about 1/4W into the speaker and it’s pretty loud.
So I hooked up my 8ohm dummy load instead of the speaker, and put my scope across the resistor and cranked up the volume on the integ. amp and ran the impedance test and found that with about 360mv rms across the resistor life looked pretty good:
I did get better results with the 32k FFT instead of the 64k FFT. I then hooked up an “8ohm” speaker and got these:
I think it still needs more power applied and will try a different amp at some point, or will “push” this one some more. What do you think ? I have not looked at speaker impedances a great deal…
Also, with the speaker impedance module when you save the bitmap it does not remember the last directory a file was saved too- I think it defaults to PICTURES each time NOTE- this appears to only be if you start the test again. It may be helpful to a ground connection between the QA402 Input ground and the amp input ground as I missed it the way I built may cables…Thanks as always
Update- I couldn’t resist running another speaker I had nearby. For the same setting of the intg amp, this one was louder and an error message came up when I tried to run the test, suggesting that the QA 402 was possibly being over-driven, so I turned down the gain of the amp until it went away and this is what I got:
This too is as small, cheap 8ohm speaker.