Full Scale Input Setting Changes Automatically

I’m a new user (and not an engineer), so I may not be using the QA403 properly, but I’m having issues with the Full Scale Input (built-in attenuator) that automatically resets itself when a test begins at too low a level for the test.

For example, when I want to run the “AMP Freq. Resp.” test, I first make sure the “Full Scale Input” is set to 42. I then select the “AMP Freq. Resp.” test from the pull down menu, click “OK,” but it immediately resets the “Full Scale Input” from 42 to 12 then the system almost immediately terminates the test and gives me an “Acquisition Overload” error message.

I can’t find a way to keep the “Full Scale Input” at 42 because it always changes it to 12 as soon as I click “OK” to start the test. Or should it be automatically readjusting the input level as the test progresses and the wattage increases? What am I doing wrong?

It sounds like you are tripping the overload attenuator?

Bear in mind the start level in dBV. Whatever that is, will be amplified by your amplifier’s gain and may overload the range setting you selected.

Here is the scenario: I am measuring a old Altec 129A tube amp with a maximum output of 6 watts. Per the specs, it has a maximum gain of 40dB. Even if I have the volume control set to 25%, the QA403 still overloads because the attenuator automatically sets itself to 12 when the test begins (it immediately overloads). If I have the volume control barely up at about 10%, I don’t get the overload message and the test will run.

@ScottRichardson. Not to toot my channel, but the video I am releasing tomorrow morning is about using the QA40x to measure a power amp (solid state), and go through a lot of testing. I have rarely used the amp freq response test as the expochirp works great. I have found that frequency response does not vary a lot vs power, so I usually just measure at 5w into 4/8 ohms, and sometimes at another power level if the gear was spec’d there (sometimes 1w). I am surprised that you are having this issue but don’t run the test so can’t be of more help other than try using the expochirp…

Back to the question at hand.

What is causing the input overload is the issue.

Are you using the QA’s differential inputs? Is the OPT amplifier and transformer chassis grounded at the output, or floating?(not familiar with the Altec myself).

Can you just run the continuous frequency response utility (freq response button + run) with no problems?

  1. Yes, I’m using the differential inputs on the QA (the + speaker terminal goes to the L + QA terminal and the - speaker terminal goes to the L - QA terminal).
  2. The output transformer on the Altec is floating (not grounded).
  3. I see the Frequency Response Utility you mentioned, but I don’t see “Run” anywhere.

I did find that if I set the level to -20 dB, I don’t have the problem. However, if it is much higher than that and I have the volume control past 1/3, I get the error message. I guess I need to adjust the input level accordingly. I’m new at this and not an engineer, so I’m learning.

I’m looking forward to seeing your video. Please send me the link.

Your amp has a gain of 40dB QA403 can handle max 42dBV. So your input level + 40dB should not exceed 42dBV. Test your amp on the spectrogram page with volume at full. Set your max input to 42dBV and use an input level of say -60dBV, then slowly increase the level until you see clipping (tons of harmonics) that input level (on your generator) is then your maximum. Then set the frequency response etc levels for your automated tests.

Once I am done testing a preamp I will hook up and amp with some gain, probably 34dB if I remember correctly, and run this test and see what happens. Here is the link to the video- it is kinda long for my videos:



He should refer to the rated input sensitivity of the amplifier for full rated power.

Level controls should not be wound back as it affects the S/N numbers. All specifications are using wide open level controls. They are not amplifier gain controls, they are attenuators at the front end. The amplifier gain is fixed.

@var tested an MC-2300 in his linked video I see. Its rated sensitivity is 500mV for full rated power. So it’s a 39.8dB gain power amp. Fine in 1972. That is with the attenuators wide open (no attenuation). All testing should be done with reference to that 500mV when comparing numbers to spec.

As soon as you move the attenuators, comparisons with published specifications are out the door. They are there for level/stage matching in use, not on the bench for tests. Vary your input level, not the attenuators. Same with all integrated amplifiers and preamplifiers- the volume control is always at maximum (or 0dB for electronic volumes that go above 0dB) for all published specs.

@restorer-john. Thanks for the feedback and suggestions regarding the volume controls and testing. I am not sure how much of a difference it would make with THD, but certainly with SNR it would as those pots are typically in series with the input. For amps (integrated and receivers too), I followed what Amir does by setting the gain for 29dB (now 25dB) and taking measurements at 5w 4/8ohms and then at rated power. I have recently tried to test for the input sensitivity. I am not sure I did that with the MC2300 as I probably did the testing and watched the video last over a month ago…

That’s an excellent video. Thanks for making it!

Thanks. I try to contribute where I can within this “community”, and I have used the QA40x a lot over the past few years since I started my little channel, and thought I would show some of my thinking (right or wrong, I suppose) as to how I do things that I have learned- and I am still still learning.