I have recently received my QA403 and have started running some basic tests in order to get familiar with the unit. I have a pair of 6” RG174 loopback cables connected to the + inputs/outputs with the – inputs terminated into 75 Ohms.
When I run the automated AMP Frequency response test, I notice a sharp rolloff before reaching 20K. I have run the test at the following settings:
Also, when starting the analyzer, if I change FULL SCALE INPUT (dBV) to any value less than 24, the analyzer displays a yellow message “Connect the QA403, See File-> Device to change” in the lower left corner and appears to lock-up, with the only means of resetting being to disconnect the USB cable and reconnect it.
Looks pretty darn flat to me. I think you could use much less attenuation than 42dB- probably like 18dB with a 0dBV input. My guess is that here is something off in your setting for the AmpFreqResp plug in as far as your start and stop frequencies and step size…?
Giuseppe and VAR . . . thanks to both of you for the timely replies and excellent recommendations. Both problems have now been resolved. The early rolloff problem was resolved by changing the THD stop frequency to 24kHz per Giuseppe’s recommendation. The Automated AMP Frequency response test now plots correctly, and is identical to Guiseppe’s plot.
With regard to the QA403 disconnect problem (last paragraph of first post), this problem was resolved by removing the USB connection from my Dell docking station port and connecting it to an externally powered USB hub (also connected to the Dell docking station). If I was to wager a guess, it would be that the QA403 was drawing too much current from the docking station and it was disconnecting the QA403, thus requiring a USB disconnect/reconnect to work again.