Hello, my name is Bartosz and I purchased the QA403 and QA451B to test a Class A amplifier I am building. I am new to taking measurements so this is a learning experience for me, so I will be asking some silly and newbie questions.
This is the setup I am using to test my amplifier:
I am struggling because I am finding I am getting two very different results when measuring the SNR and THD+N at 5 watts depending on which Full Scale Input (dBV) setting I use.
For example, I am using the QA451B which has an attenuation of -12dB. So I set up my simulation and ensure the external gain is set to -12dBV. Looking at the results when I use this setting, I have a RMS dBV of 16.09 dBV so I choose the Full Scale Input to be 24dBV. Because of this it will attenuate the input signal. Doing this, I get a SNR ~89dB and THD+N around -88dB. This is shown in the screenshot below:
However, I know the QA451 attenuates the output. So I zeroed the input gain to see what the QA403 was received. When I did this, I found it would have input of 4.09 dBV. This is shown below:
As this is much lower now, I thought I could adjust the Full Scale Input to 18 dBV and this would no longer attenuate the input on the QA403. Doing this, I found the the SNR now was 107 dB and the THD+N was -105.41dB. This is shown below:
So, the performance has gone up significantly! The SNR and THD+N match the simulation perfectly. But it seems far too good to be true as I would imagine the real world SNR would be much worse than the simulation.
I wanted to ask this forum; how do I know if the results are too good to be true? Which set of results should I believe? Is there anything I can do further to validate these?
Please note I am new to this so you would need to explain in simple terms haha.
Thanks for your help and patience!