Parasitic...house? - Page 1
Forum section: electronics / repair

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 03:49:56 pm
content: 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

author: cvanc
author_rank: Frequent Contributor
timestamp: September 14, 2018, 04:50:48 pm
content: 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.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 05:19:55 pm
content: Quote from: cvanc on September 14, 2018, 04:50:48 pmThanks 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.

author: drussell
author_rank: Super Contributor
timestamp: September 14, 2018, 06:09:32 pm
content: 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?

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 06:16:08 pm
content: Quote from: drussell on September 14, 2018, 06:09:32 pmNo. 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.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 06:17:46 pm
content: 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:

author: Gyro
author_rank: Super Contributor
timestamp: September 14, 2018, 07:02:45 pm
content: 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 isnota 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.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 07:13:08 pm
content: Quote from: Gyro on September 14, 2018, 07:02:45 pmIn 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?

author: drussell
author_rank: Super Contributor
timestamp: September 14, 2018, 07:32:02 pm
content: 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 someReal WorkTM

author: ogden
author_rank: Super Contributor
timestamp: September 14, 2018, 07:34:09 pm
content: Quote from: KungFuJosh on September 14, 2018, 07:13:08 pmDid you consider to blame switching PSU of the scope? Waveforms seems nearly identical no matter from where you reported them

author: ogden
author_rank: Super Contributor
timestamp: September 14, 2018, 07:36:02 pm
content: Quote from: drussell on September 14, 2018, 07:32:02 pmTriac dimmers are nastier than switching supplies BTW

author: Gyro
author_rank: Super Contributor
timestamp: September 14, 2018, 07:37:28 pm
content: Quote from: KungFuJosh on September 14, 2018, 07:13:08 pmThere'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.

author: drussell
author_rank: Super Contributor
timestamp: September 14, 2018, 07:40:35 pm
content: Quote from: ogden on September 14, 2018, 07:36:02 pmThey 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.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 07:54:15 pm
content: 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

author: Bratster
author_rank: Frequent Contributor
timestamp: September 14, 2018, 07:57:49 pm
content: 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.Sent from my Fi Moto x4 using Tapatalk

author: ogden
author_rank: Super Contributor
timestamp: September 14, 2018, 08:02:53 pm
content: Quote from: KungFuJosh on September 14, 2018, 07:54:15 pmUnfortunately "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 iscleeeean

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 08:03:38 pm
content: Quote from: Bratster on September 14, 2018, 07:57:49 pmI 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.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 08:05:13 pm
content: Quote from: ogden on September 14, 2018, 08:02:53 pmMy DUT amps were always on toroidal isolation transformers with floating neutral.

author: Bratster
author_rank: Frequent Contributor
timestamp: September 14, 2018, 08:05:39 pm
content: 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 upSent from my Fi Moto x4 using Tapatalk

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 08:56:28 pm
content: Quote from: Bratster on September 14, 2018, 08:05:39 pmI 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.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 08:58:54 pm
content: Quote from: drussell on September 14, 2018, 07:32:02 pmSide 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.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 09:18:20 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2018, 09:32:43 pm
content: 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.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 10:17:15 pm
content: Quote from: tautech on September 14, 2018, 09:32:43 pmNo, 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.

author: tautech
author_rank: Super Contributor
timestamp: September 14, 2018, 10:34:52 pm
content: Quote from: KungFuJosh on September 14, 2018, 10:17:15 pmI 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.

author: drussell
author_rank: Super Contributor
timestamp: September 14, 2018, 10:40:58 pm
content: Quote from: KungFuJosh on September 14, 2018, 10:17:15 pmWell, that's the thing. If it really is coming into your amp via the power line, then you need better power supply filtering. If the amp is picking it up because the electromagnetic fields in your house are huge and wonky (whether or not they are actually being radiated by your power lines or not) and guitar amplifiers are very high gain devices, then you need better shielding and added filtering from RF on your inputs, etc. Using your scope you should be able to see where in the amplifier it is being induced.Are these home-built amplifiers and are they designed by you from scratch? Are they in shielded chassis, or have you tried putting tube shields on any high gain pre-amp stages? (If it is ending up on signal lines but not power, etc. etc.) Does shorting the input have any effect, for example? You definitely want to design for electrically noisy environments since the power at many venues where an amp might be used are oftenindustriallynoisy environments. Refrigeration loads, stage lighting, multiple supplies and panels and cables running all over the place spewing all manner of EM garbage around.QuoteIt seems strange to me that you're picking up enough of that power line noise to account for the amplifier noise. Do the blips on the scope actually coincide with the noises you hear from the amplifier, or are you chasing the wrong problem?

author: DaJMasta
author_rank: Super Contributor
timestamp: September 14, 2018, 10:56:30 pm
content: This is about normal from what I've seen/heard of, lots of stuff out there and it doesn't take a whole lot of power to be picked up on a 10M scope probe. The cleaner 60Hz sine with a 100X probe may just be that the probe has a lower bandwidth.You've got radiated noise and conducted noise to every device connected to a socket (you can get rid of conducted noise by being on a battery supply, many very low noise preamps for mics use this). If you have particularly bad offenders in your house, then you can track them down and replace them (or slap some ferrite cores on the power lead or something), but it's ALWAYS going to be present in some quantity. If you want maximum isolation, you filter the hell out of your input line and the point where it enters the shielded chassis, since any length of cable is an antenna and can just pick up more. Then you have to be sure your chassis is grounded and makes a good shield to minimize the radiated emissions picked up by connections in your system.Ferrite cores, even snap on ones, can offer tangible results on high frequency noise immediately. Inline filters where the cable enter the chassis are a good start to reduce the effect of external noise on the power line and to prevent noise from your system getting out. Then ultra low noise systems generally have an extensive filter network between the filter at the power plug and the actual power supply - you look at the power supply of a low noise scope or spectrum analyzer and there is TONS of filtering before you get to the transformer.You can then regulate and filter after the transformer to further reduce noise, you can shield high impedance, low signal level portions of your amp internally as well, just to be sure, and can use additional local regulation or LC filters to make sure the power supply to those sections is as clean as possible. This is less a parasitic coupling issue and more of a general shielding and conducted noise isolation issue, it seems, to me, so looking into EMC stuff and even low noise RF design could give you some good ideas of approaches to solve your problem.Also worth mentioning that at least some of that HF content will make almost no difference to an audio amp, since it's outside of the audio band. If you build in some basic 20kHz or so low pass filters along your amplification stages, you can keep a lot of high frequency noise out of the signal entirely, whether or not it would be audible or even playback on audio equipment.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 11:05:07 pm
content: ...aaand problem solved. It turns out it was my sleep apnea machine and/or its power supply causing the issue. Here's my bench power strip with that machine disconnected. Spikes are gone, and the sine is much more consistent/cleaner.

author: drussell
author_rank: Super Contributor
timestamp: September 14, 2018, 11:10:50 pm
content: Quote from: KungFuJosh on September 14, 2018, 11:05:07 pmThe real question is, does the amp still make noise with the machine unplugged?If it doesn't, then you still need to seriously work on your power supply filtering regimen in the amplifier. Small amounts of this type of noise coming in via the power line shouldn't be audible in your amplifier.

author: BrianHG
author_rank: Super Contributor
timestamp: September 14, 2018, 11:21:23 pm
content: Without an amp schematic and design layout, transformer design spec, one cannot address the amp's ability to properly reject AC line and local EMI. Mr Carlson's Lab has some good illustrations on tube amps and guitar amps where just the placement of the secondary side AC-DC line filtering inductor makes a world of difference in hum, or, how placing an additional small cap across the DC rectifier diodes prevents them from injecting high frequency bounce into the DC rail side of the amp due to low quality AC power sources. It is all to easy to slap together an off the shelf transformer, 4 big ass diodes and 2 huge caps and say you are in the clear for everywhere you sell your amp.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 11:43:03 pm
content: Quote from: drussell on September 14, 2018, 11:10:50 pmThe one that I didn't finish yet, yes. But what's important is that I can distinguish now what's actually the amp on the oscilloscope. Before I was trying to troubleshoot, usually via the amp lead dress (in these tube amps simply shifting a wire half an inch is often the answer), but I was really attempting to correct for the power supply in the other room without realizing it, and obviously nothing was working. Now I can work on the amp and I know I'm actually working on the amp.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 14, 2018, 11:48:57 pm
content: Quote from: BrianHG on September 14, 2018, 11:21:23 pmI mostly do vintage design stuff. I draw my own layouts and schematics, but the circuits are classic-ish (with some modification or additions). For example, the noisier one is an 18 Watt Lite 2b. The transformers are Heyboer 18 Watt vintage spec transformers, except the reverb portion which uses a Hammond 1750A reverb driver. This particular amp is a smaller build, and the transformers are closer together than I would prefer, but the power transformer and output transformer are exactly how they should be. I will check out Mr Carlson's Lab though, that sounds interesting. Thanks!

author: BrianHG
author_rank: Super Contributor
timestamp: September 15, 2018, 12:01:36 am
content: Diagnosing hum and buzz:https://youtu.be/6P4C9at5rV8?t=4592Removing diode ring oscillations:https://youtu.be/JPm5eQdJm38?t=531Watch his other videos on the directionallity of AC caps (yes, they have a direction and will leak additional EMI interference into your amp if backwards) and build a tester to orient your caps correctly.https://youtu.be/BnR_DLd1PDI?t=2Another video of his to look for, build his super-probe buzz-circuit fault finder to diagnose your amps.https://youtu.be/uVkJqqZroN0?t=2

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 15, 2018, 01:02:18 am
content: Quote from: BrianHG on September 15, 2018, 12:01:36 amThanks for all of that, his capacitor leakage tester project looks cool too.EDIT: LMAO, looks like that leakage tester got a little controversial.

author: BrianHG
author_rank: Super Contributor
timestamp: September 15, 2018, 01:56:48 am
content: Quote from: KungFuJosh on September 15, 2018, 01:02:18 amIf it were anyone else... Believe me, he knows his stuff....

author: drussell
author_rank: Super Contributor
timestamp: September 15, 2018, 02:08:08 am
content: Quote from: BrianHG on September 15, 2018, 01:56:48 amIn high sensitivity, high impedance tube circuits, it absolutely does matter in certain places.I have seen it personally, multiple times. Calling it "leakage" though, is a bit misleading compared to what we usually call capacitor leakage, which is essentially a shunt-resistance effect. What you are actually doing is checking for the outside foil on capacitors types that are wound that way in a spiral, so it is more like a "capacitor shielding detector".And yes, Mr. Carlson definitely knows his stuff.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 15, 2018, 11:36:43 am
content: Would you guys like him as much if he wasn't also Canadian, eh?

author: drussell
author_rank: Super Contributor
timestamp: September 15, 2018, 12:32:27 pm
content: Quote from: KungFuJosh on September 15, 2018, 11:36:43 amYes. Absolutely! He could be from Mars and still know his stuff.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 15, 2018, 08:24:32 pm
content: Quote from: drussell on September 15, 2018, 12:32:27 pmNow that you mention it, his lab does look like a spaceship.