# Measure HV Power Supply Ripple & Noise **URL:** \ **Category:** QA401\ **Created:** [October 26, 2020, 6:01pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186 "2020-10-26T18:01:55Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![dbotich](https://avatars.discourse-cdn.com/v4/letter/d/b2d939/32.png) [@dbotich](https://forum.quantasylum.com/u/dbotich)\ **Post date:** [October 26, 2020, 6:01pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186/1 "2020-10-26T18:01:55Z")
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? ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [October 26, 2020, 6:24pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186/2 "2020-10-26T18:24:06Z")
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! > [@My new QA401 has a problem](https://forum.quantasylum.com/t/my-new-qa401-has-a-problem/78/11): > > Yes, you can use the QA401 to drive single-ended or differentially into a transformer, you just need to limit the levels depending on your load impedance. This is true on all test equipment. Remember the output opamp max output is around 65 mA (round number) and you can find that on the TI data sheet for the OPA1612. TI doesn’t go into great detail on the specs versus the output current, but a reasonable assumption is that the opamp isn’t meeting normal specs when you are asking it to drive tha… ---
**Author:** ![dbotich](https://avatars.discourse-cdn.com/v4/letter/d/b2d939/32.png) [@dbotich](https://forum.quantasylum.com/u/dbotich)\ **Post date:** [October 26, 2020, 6:39pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186/3 "2020-10-26T18:39:52Z")
Matt, which thread link? ---
**Author:** ![ilya](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ilya](https://forum.quantasylum.com/u/ilya)\ **Post date:** [October 28, 2020, 12:41pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186/4 "2020-10-28T12:41:52Z")
I think Matt is referring to this thread: [https://forum.quantasylum.com/t/my-new-qa401-has-a-problem/](https://forum.quantasylum.com/t/my-new-qa401-has-a-problem/) ---
**Author:** ![dbotich](https://avatars.discourse-cdn.com/v4/letter/d/b2d939/32.png) [@dbotich](https://forum.quantasylum.com/u/dbotich)\ **Post date:** [October 28, 2020, 3:59pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186/5 "2020-10-28T15:59:33Z")
Ilya, thanks. Did you ever build your setup for measuring HV power supply noise? I would be interested in your results. ---
**Author:** ![ilya](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ilya](https://forum.quantasylum.com/u/ilya)\ **Post date:** [October 28, 2020, 4:05pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186/6 "2020-10-28T16:05:50Z")
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… ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [October 29, 2020, 4:41pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186/7 "2020-10-29T16:41:57Z")
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? ---
**Author:** ![ilya](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ilya](https://forum.quantasylum.com/u/ilya)\ **Post date:** [October 29, 2020, 4:51pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186/8 "2020-10-29T16:51:14Z")
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. ---
**Author:** ![ilya](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ilya](https://forum.quantasylum.com/u/ilya)\ **Post date:** [May 22, 2022, 9:27pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186/9 "2022-05-22T21:27:57Z")
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: ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/b/b9be2ea72e09e8d6819d36b090ef788d823b8fe5.png) 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: ![image](https://canada1.discourse-cdn.com/flex032/uploads/quantasylum/original/2X/e/ed76d056c3207e956df47a89d7383dcb48088ec4.png) ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://forum.quantasylum.com/u/matt)\ **Post date:** [May 23, 2022, 7:51pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186/10 "2022-05-23T19:51:59Z")
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. ---
**Author:** ![ilya](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ilya](https://forum.quantasylum.com/u/ilya)\ **Post date:** [May 23, 2022, 10:05pm UTC](https://forum.quantasylum.com/t/measure-hv-power-supply-ripple-noise/186/11 "2022-05-23T22:05:11Z")
@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.