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