Amplifier Input Impedance Measurement

I read the blog on input impedance and had a couple questions.

  1. Should the load on the amplifier be resistive rather than a speaker
  2. What is the lowest resistor ratio on input to amplifier impedance that you’d recommend
  3. Would the be any reason at all to involve the WA461 in this measurement
    Thx as usual your great instructive answers!

Hi Moto,

Can you link to the blog post. I might be able to help, I’ve been doing similar things.


Hi @Moto, you won’t need a QA461 to measure input Z because the most Zs are readily driven by the QA40x (eg > 100 ohms). But you will need a higher value series R like it shows at the blog post.

What is the approx input Z you are looking to measure?

@Dan, please feel free to post any measurements here as I think most just ignore input Z while it’s actually a pretty interesting area given some inputs (like mic pre). Have you built any special hardware to help with your measurements?

@matt about 10k on some buffers and active filters, 40 k on amplifier and 2k on adc’s.

Hi Matt, Moto,

@matt Yeah I will post some updates that might be of interest to people. I just submitted a board to JLCPCB today and the parts are arriving from Mouser next week!

I had also designed a current sense board with BNCs (I think I posted once before, a screenshot). I plan on uploading that to GitHub so anyone here can get in made easily. But with the other project “done” I will have a bit of time to return to it.

@Moto I’m not sure what you mean with those values?

I was experimenting building some buffers and active filters and trying to check the input impedance of them which should be in the area of 8-10k ohms.