Measuring high voltages using a scope probe. Safely

QA401
1

I have built an “oscilloscope emulator” front end for my QA401 that:

  1. Provides the AC line 3rd prong ground connection that all scopes provide
  2. Properly terminates oscilloscope probes at both AC & DC (one Meg + the usual few pF)
  3. Provides a DC block
  4. Provides a unity gain buffer after the DC block

With this box I can use any oscilloscope probe with my 401. However I have a set of 4KV scope probes this interface was built for.

I built it out of available parts and it surprised me by measuring pretty good on a loopback test. And it can be seen measuring THD of a signal that sits on 1650VDC. Works great ":^)

Bev amp measurement rig a
Bev amp measurement rig a1920×2560 512 KB

Bev amp measurement rig
Bev amp measurement rig1920×1440 306 KB

3 Likes
2

Very nice! Can you share more details of your design with us? Also some of your use cases?

1 Like
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Use case? Every vacuum tube product ever! ":^)

…but in my instance I built this to measure one specific thing, a weird amp from an electrostatic speaker that is “all kilovolts all the time” under the hood. It’s the silver unit being measured in the photo.

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Got it. Thx. Would you share your design with us?

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My priority was safety before precision. The most important design aspect is the ground wire.

I used a FET opamp as it seemed like a good idea, the only one I had in stock was OPA2134. And the DC block is a 1uF film chosen simply because I had some. It’s not a calculated value. My input cap is adjustable - I wanted to calibrate this new box to my probes, not the other way around. And I used a nice beefy metal film for the 1Meg termination.

There’s one more thing I forgot to mention in there:

  1. There are back to back 3.3V Zeners at the output “just in case” to make sure my 401 never gets hurt. They see a 1K resistor from the OPA2134 output to limit fault current.
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Thx @Chicago . Do you have a schematic?

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4KV front end schematic
4KV front end schematic1920×2488 156 KB

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Thx much Carl! I’ll be putting one of these together.

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You can do better than I did. My termination is spot on at DC but slightly off at AC, I need to reverse that by nudging up the 1 Meg resistor.

Also there are 22uF dipped tantalums as rail bypasses in mine. I really did an available parts thing and plenty of improvement is possible.

… and the REAL most important part is the scope probe you use! I have the GE3425 from Cal Test and can recommend it. Not expensive.

I’ll post my loopback tests, like I said it surprised me by performing pretty well ":^)

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4KV front end frequency response
4KV front end frequency response880×603 71.1 KB

4KV front end THD & noise floor
4KV front end THD & noise floor880×603 106 KB

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Thx Carl. What are the specific changes you’d make? I was thinking of going with an opa1656 and a couple 12v batteries.

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The OPA2134 worked out better than expected so I’ve no real complaints.

But you could do a discrete FET buffer instead, maybe with those new super quiet TI FETs? I don’t know the OPA16xx series well, sorry, can’t comment re: 2134.

The little box I found did not allow me to place a higher voltage DC block. Mine is 63V which isn’t awful, but this is a box were safety counts, I 'd like to have higher. And there’s no need for 1uF, that’s bigger than required.

My probes are 100:1 which means my kit should have an overall gain of exactly -40dB, you can see in my photos I’m more like -41.

Fine tune the 1M input R to calibrate that specific buffer channel & that specific scope probe to 100:1 overall ratio. Keep them together as a calibrated pair.

Final thought - be damn sure you know the voltage rating of your scope probes! Many 10:1 probes are only rated to 300 Volts. For vacuum tube audio stuff that is not enough.

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Ok. I’ll do some experimenting. Thx for your help!

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Hi @Chicago, where and how do you connect the output of the QA403 generator? After the rectifier with a blocking cap?