Attenuator for high RMS signal tests

Hello everybody, this is my first post on the QA forum about my great QA401 analyzer.
I’m an italian audio tube DIYer and, although I had been told that it was not created to be used for high voltage devices, I bought it and carefully used it… cap coupled. No problems so far. I’ve been careful enough not to fry it. Yet.
I need to test signals over 100V RMS (an 845 amp will soon follow). What’s the best path to follow?
Thank you in advance!

Hi @Fralippo, take a look at the link below. it walks through building a load and including an attenuator. And then feel free to post your design if you want some eyes looking at it!

Hi, finally the time for this test has come… So I need to analyze a signal up to 110V RMS = 41dbV with the QA401 which can accept 6dbV unattenuated. A 35db attenuator will then be required and it’d be a 100K+2K ohm voltage divider. The analyzer will be connected to the 2K resistor.
This multistage tube preamplifier has a sub 100 ohm output impedance and has a 0.22uF coupling capacitor at its output, followed by the 102K resistors to gnd.
Is this correct? Or would a much shorter, say 10K, attenuator be better?
Thank you!

Hi @Fralippo, I think your plan is correct. You might want a bit more attenuation because ADCs are usually starting to degrade a bit as you hit the max input. So, maybe aim for 0 dBV input at your max input. If you use the 2K shunt to sense across, you’ll have some noise from the 2K. If you sense across a 200 ohm, the noise will be less.

You can determine if the 2K will be excessive or not by just sticking a 2K across the QA401 inputs and see if the noise rise is OK for your application. So, first do a shorted input measurement. Then put a 2K across the inputs and note the rise in noise floor. If this is OK for your app, you are set.