QA402 Expo Chirp FR and Windowing

QA40x
1

I wanted to check the FR of the MM Phono input of my mid 80’s preamp. It looked pretty crappy below 180Hz. (I had the RIAA playback user weighting selected- am glad that is working!) See picture below:

c1mm_hann
c1mm_hann1274×1013 151 KB

Then I decided to look at the FR of the tuner input, and saw the same ugly response below 180Hz or so. I am thinking maybe the power supply caps might be leaking a bit since there is a spur at 60hz. Recapping the preamp sounds like a fun project, anyway. The more I looked at it, I did not like how the response looked so good >200Hz. I then decided to just loop the QA402’s outputs to the inputs, and set up another FR using the Expo Chirp and got this ugly response, which looked similar to the one above:

hann window fr
hann window fr1218×1042 147 KB

Thinking there might be a a problem with the QA402, I followed the procedure in the manual to look at the THD/N of just the Left Channel and pretty much got what was shown in the example at 1kHz. I set the Frequency to 70Hz and got what I consider reasonable results. I then set up for the Expo Chirp FR and decided to change the Windowing, and with the Rectangular window I got this:

rect win fr
rect win fr1217×1040 140 KB

which looked really good- so I went back and measured the preamp’s MM phono input stage again with Rectangular Windowing, and got this:

c1mm_rect
c1mm_rect1277×1015 158 KB

Which looks much better! Of course, there is still the 60Hz spike that comes and goes, but considering the Phono preamp stage advertised +/-0.5dB FR, this is not too bad at all (IMHO).
Should I continue using the Rectangle Windowing for measuring older gear? Or Just cycle the them and see which gives me the best measurement. Previous measurements I had made on an integrated amp looked fine with the Hann windowing. Oh, the Hamming Window also gave a “good” response, though I did not capture it to compare the two.
Thanks for your inputs…

2

Should I continue using the Rectangle Windowing for measuring older gear?

Hi @VAR, yes, keep going on rectangular. Here’s why: When you apply windowing, you effectively apply a curve to the start and stop of the sweep. So, the very low frequencies and very high frequencies are greatly attenuated. And the noise you see is because of the degraded SNR at the lower frequencies.

3

Thanks for getting back with me so soon- I will sleep better tonight! Now I am wondering why the FR measurements I made on my integrated amp with the Hann windowing selected looked fine. The only configuration difference was that for the Preamp I measured the output from the Tape 1 Output and the integrated amp I took it directly the output directly from the preamp out that went into the power amp through jumpers… But I will continue with Rectangular.

4

Hi @VAR this was changed in 0.997 such that in ExpoChirp now has windowing disabled.

5

That is good to know. Is there a more detailed explanation as to how the windowing works ?

6

Hi @VAR, if you aren’t using a mic, you can skip windowing. It let’s you ignore room reflections. If you search on chirp and windowing you’ll have a lot of reading material.

7

I did a quick search like you mentioned and there was way too much math than I would like to see now. At this point in time I do not see doing anything with the QA402 and a microphone. Keep up the great work!