Here comes a Spice simulation of Null tester with 2x 1V input. Output was a 50k ohms resistor connected to ground as load.
A: The output resistance of the 1V input is NOT 50k at all. You even missed the most basic part of electronics here.
The output resistance seen from the opamp input is 1.1k.
The load the 0 Ohm output resistance of the 1V source sees is 50k
Back to electronics 101 for you here.
Probing shows after the opamps, inversed channel introduced a 0.001V (or -60dB difference against 1V input) voltage difference in absolute terms with input. This voltage bumped out between OA2 and OA4.
Assuming perfect circuit, this is 0.1% difference in voltage against input in absolute terms.
So you are saying there is merely 60dB signal noise ratio left in the inversed channel ?
Question: If that were the case and this would be random noise opposite the non inverting channel and that would be amplified 80dB then the noise level shown by the meters would be +20dB ?
Why is this not happening ?
Is it possible you got this wrong somehow, or did you mean the amplitude is 0.1% off compared to the non inverting opamp.
Did you simulate for possible tolerance difference extremes as well ?
In such case would the 22 Ohm nulling pot not be able to compensate for a level difference of the source signal ?
Which is it ?
If you centered the trimmer, you will get a -64dBv signal against 1V input in this circuit, which is "error" due to Ethan's design.
As Spice points out, the Volt difference across load is V10+V12, not V10-V12. Physically, there is nothing being "subtracted" by this circuit. But this tester is capable of hiding a 0.001V signal difference.
Why would anyone center the trimmer. It cannot possibly work this way. He could have left the trimmer out if that is the point of the circuit as a trimmer pot most likely would have a physical error in the mechanical centering anyway.
NO you do not understand this well.
The whole object of the game is to set the pot so that the input signal in level is exactly the same in amplitude, but reverse phase, to that of the non inverting output when looked at from the center tap of the pot.
You do that by ear or meter. THAT determines the null.
The only difference remaining is the added noise + distortion of the input circuits + distortion of the DUT (tested cable)
When nulling cable A and B and the result is the same there is no difference between cable A and B (in the audible range).
True proponents of the nulling process would moan over the sound quality would degrade so much using opamps (as they all sound bad compared to the heaven sent discrete amps or tubes) and because of this all cables null the same because the sound of the cable is changed due to the opamps used.
Again, based on NwAvGuy's claim, 0.1% difference is distinguishable. This shows under perfect condition, null tester would not show a minor audible difference.
0.1% = 0.00868dB level difference. Where did NwAvGuy claim you could hear such differences ?
When you speak of 0.1% distortion = -60dB then in certain cases you can actually hear that depending on many factors.
You seem to say the pot cannot null perfectly despite the small adjustment range (fine control of the diff signal)
If you claim you can detect -110dB difference in both channels, you are simply wrong.
Nope because you can even hear below the noise floor so even if it were your -100dB (where did the -60dB go to ?) you can still null below your calculated -100dB.
And where did the Sansa go that seemed to be the biggest issue in your gripe ?