Author Topic: Parasitic...house?  (Read 3099 times)

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Offline KungFuJosh

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Parasitic...house?
« on: September 14, 2018, 03:49:56 pm »
If you saw my scope thread you maybe know I've been troubleshooting a couple tube amp builds that were suffering from parasitic oscillations. Blaming myself, I went through them both thoroughly, and got rid of the oscillations. Basically. Today I was finishing up the second amp, when I noticed I was still getting a waveform with the amp off and unplugged. Messing around a bit, this lead me to put the probes next to my bench lamps and eventually power strips. My amps weren't causing the parasitic as I originally thought, they were simply amplifying one.

I placed my probe on two different power strips connected to two different outlets. The same cycling parasitic can be seen with the probe hovering over the power strip (not actually connected to anything).

What should I do? Do I need to start unplugging things? Test outlets on other breakers? How do I track this down? I disconnected everything from the power strip and the results were the same.

strip 1:


strip 2:


Thanks,
Josh


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Offline cvanc

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Re: Parasitic...house?
« Reply #1 on: September 14, 2018, 04:50:48 pm »
One thing to consider is RF junk coming from nearby light fixtures (both LED and even fluorescent if using electronic ballast).

I've got some el-cheapo LED lights over my bench and they radiate a *lot* of junk.  Easily picked up by a floating scope probe.
 

Offline KungFuJosh

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Re: Parasitic...house?
« Reply #2 on: September 14, 2018, 05:19:55 pm »
One thing to consider is RF junk coming from nearby light fixtures (both LED and even fluorescent if using electronic ballast).

I've got some el-cheapo LED lights over my bench and they radiate a *lot* of junk.  Easily picked up by a floating scope probe.

Thanks for the thought, but while correct (this was part of what lead to the power strips), I've eliminated that idea by unplugging them (and everything else plugged in) and going direct to two different power strips in two different outlets. This suggests something of a larger scale is going on.
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Offline drussell

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Re: Parasitic...house?
« Reply #3 on: September 14, 2018, 06:09:32 pm »
Let me get this straight, just so I'm not confused with what you're asking...

You're waving around an oscilloscope probe that isn't connected to anything at the probe end, just open, which runs to the (relatively) high impedance oscilloscope input and you're seeing 60 Hz hum/noise of a few hundred mV on your scope, correct?

Why would this be surprising?   :-//
 

Offline KungFuJosh

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Re: Parasitic...house?
« Reply #4 on: September 14, 2018, 06:16:08 pm »
Let me get this straight, just so I'm not confused with what you're asking...

You're waving around an oscilloscope probe that isn't connected to anything at the probe end, just open, which runs to the (relatively) high impedance oscilloscope input and you're seeing 60 Hz hum/noise of a few hundred mV on your scope, correct?

Why would this be surprising?   :-//

No. Look closer, you can see the funky parasitic oscillation as it pops through the 60Hz. I can post a video if that would help, as that's a little more obvious than these stills.
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Offline KungFuJosh

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Re: Parasitic...house?
« Reply #5 on: September 14, 2018, 06:17:46 pm »
Here's some more photos of different outlets in my house. The second floor is on a separate box, but still has issues. I think these are a little more obvious.

Other rooms on first floor:






Second floor:


« Last Edit: September 14, 2018, 06:22:11 pm by KungFuJosh »
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Offline Gyro

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Re: Parasitic...house?
« Reply #6 on: September 14, 2018, 07:02:45 pm »
It looks as if you have a good shot at identifying the noisier items in your house.

It's not quite as bad as it looks though... Because you have a loose capacitive coupling between the scope probe and the outlets, higher frequencies will be coupled a lot more efficiently than the fundamental 60Hz mains frequency, so will look much bigger in comparison. (Note that this is not a suggestion to start probing the socket contacts directly!).

What you are doing right now is what you want - emphasizing the noise (sources) vs the mains fundamental.
« Last Edit: September 14, 2018, 07:08:46 pm by Gyro »
Chris

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Offline KungFuJosh

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Re: Parasitic...house?
« Reply #7 on: September 14, 2018, 07:13:08 pm »
It looks as if you have a good shot at identifying your noisier items.

It's not quite as bad as it looks though... Because you have a loose capacitive coupling between the scope probe and the outlets, higher frequencies will be coupled a lot more efficiently than the fundamental 60Hz mains frequency, so will look much bigger in comparison. (Note that this is not a suggestion to start probing the socket contacts directly!).

What you are doing right now is what you want - emphasizing the noise (sources) vs the mains fundamental.

In most of those cases, there was nothing but the scope plugged into the outlets in question. The noisiest one from upstairs did have a wall-wart plugged into it, which you could probably guess by looking. I should probably test that one again with that removed.  Maybe I should plug the scope into a different outlet than the one under test, but the oscillation was still present in other places without the scope present, so I dunno if it matters? ...and no, I have no interest in connecting directly. ;)

Do you think it's more likely that there's a specific item causing this oscillation, or a problem with the wiring, or something else I need to keep hunting for?
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Offline drussell

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Re: Parasitic...house?
« Reply #8 on: September 14, 2018, 07:32:02 pm »
There will always be some high frequency noise imposed unless you're WAY out in the middle of nowhere, I'm not sure why you're surprised.  We shouldn't expect anything approaching a clean sine wave.  :)

If you're meaning those seemingly short bursts of high frequency riding on the side of single cycles, it could be anything.  The fact that it seems to be synchronized to the power line means it is probably coming from some kind of switching power supply, seemingly basically in a sleep-burst mode, rather than interference from, say, a cell phone, or something that would be more randomly placed on the 60 Hz wave.

I would have to zoom in more with my ancient DSO in order to see anything like that riding on my 60 Hz but I'm sure there is some cruft there.  :)

Just for kicks, this is what my living room looks like this away from any wiring, scope floating:


With the scope ground attached to power ground and the probe near a power bar:


With the scope grounded, probe laying away from anything and me not touching it, so no capacitive coupling from the big antenna (me):


Maybe later I will pull out a good torroid that has some HF capability or rig up a danger-divider and capture what's actually on the mains, just for fun.  For now I need to get back to some Real WorkTM  :)
 

Offline ogden

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Re: Parasitic...house?
« Reply #9 on: September 14, 2018, 07:34:09 pm »
Do you think it's more likely that there's a specific item causing this oscillation, or a problem with the wiring, or something else I need to keep hunting for?

Did you consider to blame switching PSU of the scope? Waveforms seems nearly identical no matter from where you reported them
 

Offline ogden

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Re: Parasitic...house?
« Reply #10 on: September 14, 2018, 07:36:02 pm »
The fact that it seems to be synchronized to the power line means it is probably coming from some kind of switching power supply

Triac dimmers are nastier than switching supplies BTW
 

Offline Gyro

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Re: Parasitic...house?
« Reply #11 on: September 14, 2018, 07:37:28 pm »
It looks as if you have a good shot at identifying your noisier items.

It's not quite as bad as it looks though... Because you have a loose capacitive coupling between the scope probe and the outlets, higher frequencies will be coupled a lot more efficiently than the fundamental 60Hz mains frequency, so will look much bigger in comparison. (Note that this is not a suggestion to start probing the socket contacts directly!).

What you are doing right now is what you want - emphasizing the noise (sources) vs the mains fundamental.

In most of those cases, there was nothing but the scope plugged into the outlets in question. The noisiest one from upstairs did have a wall-wart plugged into it, which you could probably guess by looking. I should probably test that one again with that removed.  Maybe I should plug the scope into a different outlet than the one under test, but the oscillation was still present in other places without the scope present, so I dunno if it matters? ...and no, I have no interest in connecting directly. ;)

Do you think it's more likely that there's a specific item causing this oscillation, or a problem with the wiring, or something else I need to keep hunting for?

There's always a possibility of a problem with the wiring I suppose, but that would be an unlikely option. The main problem is so many wall warts, smps's, LED bulbs etc putting crap into the mains. The inductance of the mains wiring and circuit that it's on (you're in the US, so radial circuits I guess) will tend to attenuate HF noise with distance. If you need a quiet supply for test equipment etc.  these days it's more or less required that they have a mains filtered input socket.

In terms of where you plug in the scope, its own PSU should be reasonably well filtered against direct noise pickup (although as ogden points out, the scope PSU itself could be a noise source), however it does set the point where the ground connection on your scope probe is referenced to. The noise you see will be the difference between the ground point and the insulated tip of your scope probe. You can use this as a tool - the scope plugged into the same strip for local noise sources, plugged into different circuits to look at noise between circuits.

You probably want to put some sleeving or tape around the probe hook/tip to prevent any chance of accidental direct contact. You're getting into the realms of EMC investigation here - try googling near-field probes / E-Field probes.
Chris

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Offline drussell

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Re: Parasitic...house?
« Reply #12 on: September 14, 2018, 07:40:35 pm »
Triac dimmers are nastier than switching supplies BTW

They are, indeed, however, they wouldn't normally be pulsing on and off only on some cycles of the wave, which is why would I suspect some sort of sleeping-bursting power supply.
 

Offline KungFuJosh

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Re: Parasitic...house?
« Reply #13 on: September 14, 2018, 07:54:15 pm »
Thank you all for the input. Sounds like I need to unplug all the crap in my walls and see if it makes a difference. Then if it actually does, plug stuff back one at a time until I find the culprit(s).

I'm not looking/expecting a clean waveform from the wall supply, but those parasitic spikes turn into helicopter sounds in tube amps, and they're no fun. On the bright side, they've forced me to build amps that are less susceptible to outside interference.

I did consider the scope as a possible source, but testing from the strip or outlet it was connected to, to another outlet across the room gave the same results. Also, this issue has existed since long before I got the scope (it's what lead me to finally buy it in the first place).

Thanks,
Josh
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Offline Bratster

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Re: Parasitic...house?
« Reply #14 on: September 14, 2018, 07:57:49 pm »
Keep in mind that there is more than one Outlet on each circuit in your house. So with the scope plugged into one side of the room and testing on the other side of the room you're probably still on the same circuit.

so that may be your scope noise that you're seeing on the same circuit thinking that because it's a different Outlet it's not.

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Offline ogden

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Re: Parasitic...house?
« Reply #15 on: September 14, 2018, 08:02:53 pm »
I did consider the scope as a possible source, but testing from the strip or outlet it was connected to, to another outlet across the room gave the same results.

Unfortunately "gave same results" is not proper baseline measurement. If scope is possible source, you can exclude it *only* when you see clean sine in some measurement. Do you have hefty isolation transformer? Maybe you can borrow such? Or with your scope go to place where you know - power is cleeeean
 

Offline KungFuJosh

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Re: Parasitic...house?
« Reply #16 on: September 14, 2018, 08:03:38 pm »
Keep in mind that there is more than one Outlet on each circuit in your house. So with the scope plugged into one side of the room and testing on the other side of the room you're probably still on the same circuit.

so that may be your scope noise that you're seeing on the same circuit thinking that because it's a different Outlet it's not.

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I considered that, but as I said, this noise issue predates the scope. I guess for the purpose of testing I should check across two known different circuits.
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Offline KungFuJosh

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Re: Parasitic...house?
« Reply #17 on: September 14, 2018, 08:05:13 pm »
I did consider the scope as a possible source, but testing from the strip or outlet it was connected to, to another outlet across the room gave the same results.

Unfortunately "gave same results" is not proper baseline measurement. If scope is possible source, you can exclude it *only* when you see clean sine in some measurement. Do you have hefty isolation transformer? Maybe you can borrow such? Or with your scope go to place where you know - power is cleeeean

My DUT amps were always on toroidal isolation transformers with floating neutral.
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Offline Bratster

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Re: Parasitic...house?
« Reply #18 on: September 14, 2018, 08:05:39 pm »
you could also set your scope up in one spot, and then go and turn off all of your circuit breakers except for the one powering the scope and see if you still have the noise or not.

If it goes away then turn your circuits on one by one until it comes back and there's your culprit.

If it doesn't go away try changing which circuit your scope is on and turn everything off again.

If it still doesn't go away then it's something to do with your scope or there's some other source and it just happens to be picked up

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Offline KungFuJosh

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Re: Parasitic...house?
« Reply #19 on: September 14, 2018, 08:56:28 pm »
you could also set your scope up in one spot, and then go and turn off all of your circuit breakers except for the one powering the scope and see if you still have the noise or not.

If it goes away then turn your circuits on one by one until it comes back and there's your culprit.

If it doesn't go away try changing which circuit your scope is on and turn everything off again.

If it still doesn't go away then it's something to do with your scope or there's some other source and it just happens to be picked up

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I was considering this, and I think it may have to be my next option.

I tested across two unique circuit breakers, same results.

I tested the back of the isolation transformer with no DUT, same results.

I tested the outlets in my office with no other devices plugged in, and my wireless networking and everything else unplugged, same results.

I think I need to try the breaker test next when my wife is ready to help since I'll need an extra set of hands/eyes. I still don't see how it could be the scope picking up itself since the issue predates the scope, but if it is picking up itself, it would make sense that the source electricity going to the scope is supplying the scope with the scope's issue maybe? I dunno.

If the circuit breaker test doesn't help I'll have to try bringing the scope to another house and test it there to see if the results are any different.
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Offline KungFuJosh

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Re: Parasitic...house?
« Reply #20 on: September 14, 2018, 08:58:54 pm »
There will always be some high frequency noise imposed unless you're WAY out in the middle of nowhere, I'm not sure why you're surprised.  We shouldn't expect anything approaching a clean sine wave.  :)

If you're meaning those seemingly short bursts of high frequency riding on the side of single cycles, it could be anything.  The fact that it seems to be synchronized to the power line means it is probably coming from some kind of switching power supply, seemingly basically in a sleep-burst mode, rather than interference from, say, a cell phone, or something that would be more randomly placed on the 60 Hz wave.

Side note- in my tests I switched my 10x probe for a 100x probe, and while it obviously didn't solve my issue, it did make the sine wave prettier. ;)
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Offline KungFuJosh

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Re: Parasitic...house?
« Reply #21 on: September 14, 2018, 09:18:20 pm »
I propped the probe on the scope's IEC power cord connection at the back of the scope. The first screenshot is noisier (partly) because the probe cable is touching my body.

This is the scope powered by the IT:


This is the scope powered by the wall, there were parasitic spikes, but the screenshot didn't catch them for some reason:


This is with the probe pressed against my tooth. Seriously.
« Last Edit: September 14, 2018, 09:21:06 pm by KungFuJosh »
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Online tautech

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Re: Parasitic...house?
« Reply #22 on: September 14, 2018, 09:32:43 pm »
Welcome to the world of EMI !

The biggest culprits are SMPS and any mains switching like dimmers that use phase control for both RF and mains induced muck but especially those tiny warts without input common mode filtering.
Live with it, learn to recognize EMI types and develop measurement techniques to minimize its influence on the fundamental frequencies of interest.
Spring connector ground probe connections jump to mind.  ;)
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Offline KungFuJosh

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Re: Parasitic...house?
« Reply #23 on: September 14, 2018, 10:17:15 pm »
Welcome to the world of EMI !

The biggest culprits are SMPS and any mains switching like dimmers that use phase control for both RF and mains induced muck but especially those tiny warts without input common mode filtering.
Live with it, learn to recognize EMI types and develop measurement techniques to minimize its influence on the fundamental frequencies of interest.
Spring connector ground probe connections jump to mind.  ;)

No, you're missing the point. The measuring technique isn't the issue. The inevitable issue is that the existence of the parasitic on my household electricity is coming through into tube guitar amplification. This is disturbing, annoying, and potentially dangerous. The scope seeing it is a good thing.

In a guitar amp it may sound like a helicopter. If the amp's wiring picks up and enhances the parasitic, then it becomes a bigger issue in the amp itself which may lead to a heap of other problems. It can overheat the amp and destroy tubes or transformers if it's bad enough. I have no problem correcting the amp wiring/lead dress to avoid these things, but it's quite a headache when I have no clean reference with which to work...and even with the amp wiring/lead dress perfect, I can never know if it's perfect without a clean power source. This isn't a headache I can ignore.
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Online tautech

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Re: Parasitic...house?
« Reply #24 on: September 14, 2018, 10:34:52 pm »
Welcome to the world of EMI !

The biggest culprits are SMPS and any mains switching like dimmers that use phase control for both RF and mains induced muck but especially those tiny warts without input common mode filtering.
Live with it, learn to recognize EMI types and develop measurement techniques to minimize its influence on the fundamental frequencies of interest.
Spring connector ground probe connections jump to mind.  ;)

No, you're missing the point. The measuring technique isn't the issue. The inevitable issue is that the existence of the parasitic on my household electricity is coming through into tube guitar amplification. This is disturbing, annoying, and potentially dangerous. The scope seeing it is a good thing.

I can never know if it's perfect without a clean power source. This isn't a headache I can ignore.
I better than 'quite' understand !
Good luck finding a 'clean' mains sine wave in this day and age !
All the modern mains powered 'anythings' that don't use a linear PSU are responsible, pure and simple.
There are efforts to find cleaner methods and what I heard some time back IIRC was resonance SMPS where current was better harvested instead of voltage.....but I might be wrong about that.

Anyways, suppression of the 'effects' of SMPS operation influencing the mains supply is the issue and hard to address properly to maintain a clean mains supply sine wave.
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