Measure HV Power Supply Ripple & Noise

Can the QA401 be used to measure ripple and noise of a HV DC power supply as used in a typical tube power amplifier? In the 300-600 VDC range. If yes, how would this be set up?

Hi @dbotich, there’s some discussion of a circuit for measuring high voltages in the thread linked below, including schematics and sims. A lot off folks think that a just a cap will block the DC, and it will after a few seconds. But when you initially connect the 600V, the cap will appear as a short and pass the full voltage through. So, you need a clamp after the cap. And you probably should have input/output bleed down resistors, etc. All very dangerous stuff!

Matt, which thread link?

I think Matt is referring to this thread:

Ilya, thanks. Did you ever build your setup for measuring HV power supply noise? I would be interested in your results.

Yes, I did built the device. But I didn’t get the reliable reading out of it. The noise after this DC iso box is much lower than when fed directly to QA401.
I haven’t done any extensive measurements on this because I’ve got other urgent projects that I’m busy with right now. I’m planning to post a thread and discuss these issues there.
Sorry, not that much info. But that’s all that I have…

Hi Ilya, this is very interesting and I hope you are able to post your findings at some point. Does a tone that is pushed through the iso box measure correctly?

Matt, I can’t remember if I tested the tone or not. My main test was for clamping voltage and speed. This test was passed just fine - the box clamped at approximately 6V IIRC. I’ll do the tone test and see how it works out.

I’ve finally returned to my DC isolation box. I think I know what wrecks the gain and frequency response. It’s the input stage of my LN amplifier. The circuit looks like this:
First of all, my input CR arrangement interferes with the LN input CR filter. And given a rather low impedance input of the LN amp, it seems to load the passive DC filter and I loose around 40 dB of gain.
Apart from sticking an op-amp buffer between the two I don’t see any solution to this…

P. S. As a reminder, here’s a quick mock-up of the DC iso box schematic:

Hi @Ilya, I think you have correctly diagnosed the problem. Why not do a non-inverting amp? That way, your protection stage will be unloaded, and you can still get just about any gain you need from the first stage. Usually, non-inverting will have slightly higher distortion, but for looking at noise, that shouldn’t matter much.

@matt, thanks for your reply. This LN amp is a finished design and built box. I’d rather not take it apart and make modifications.
I think I can take in account additional attenuation and live with it for a time being. Adding a battery powered buffer amp is always an option as well.
I’m glad I sorted this out finally.