QA401 input stage question

Hey there Matt, hope all is well. Question about the analyzer input impedance. 100K @ AC and an open circuit at DC, correct? Thanks!

I’m not Matt, but we did a bit of deep dive on this recently in another thread.

100K @ AC and an open circuit at DC

That is what we assumed. It’s actually documented in the User Manual (BNC Inputs section, pg 8).

Here is a input impedance in Spice. In the audio band it’s basically 100k ( a little higher) within 1% accuracy and yes approaches infinity as you approach DC.


You did not model the shunt capacitance at the input. A guess would be 20 pF. Not a lot but it can be significant at higher frequencies or higher impedance sources.
A way to accurately check this would be to set the generator to 1V them measure loopback. Then insert 100K resistor in series at the BNC (needs some clever kludging) and replot the response curve. It should be 6 dB lower. Where it deviates indicates the impedance shifts. This stuff is important when using divider probes and the process above is similar to the one Tek used for its input amps.

@1audio I tried your experiment on my qa402.

At 10 KHz the level drops by 1 dB. You can calculate the parallel capacitance needed to get that drop or (being lazy) pull up the simulation and try parallel caps at the input to get the same 1 dB additional drop at 10 KHz. A wag would be around 15 pF. (1 MegOhm at 10 KHz)