Author Topic: Parasitic...house?  (Read 3069 times)

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Offline drussell

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Re: Parasitic...house?
« Reply #25 on: September 14, 2018, 10:40:58 pm »
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.

Well, 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 often industrially noisy environments.  Refrigeration loads, stage lighting, multiple supplies and panels and cables running all over the place spewing all manner of EM garbage around.  :)

Quote
In a guitar amp it may sound like a helicopter.

It 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?

 
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Offline DaJMasta

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Re: Parasitic...house?
« Reply #26 on: September 14, 2018, 10:56:30 pm »
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.
 

Offline KungFuJosh

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Re: Parasitic...house?
« Reply #27 on: September 14, 2018, 11:05:07 pm »
...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.

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Offline drussell

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Re: Parasitic...house?
« Reply #28 on: September 14, 2018, 11:10:50 pm »
...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.

The 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.
 

Offline BrianHG

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Re: Parasitic...house?
« Reply #29 on: September 14, 2018, 11:21:23 pm »
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.
« Last Edit: September 14, 2018, 11:24:10 pm by BrianHG »
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Offline KungFuJosh

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Re: Parasitic...house?
« Reply #30 on: September 14, 2018, 11:43:03 pm »
The 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.

The 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.
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Offline KungFuJosh

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Re: Parasitic...house?
« Reply #31 on: September 14, 2018, 11:48:57 pm »
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.

I 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!
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Offline BrianHG

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Re: Parasitic...house?
« Reply #32 on: September 15, 2018, 12:01:36 am »
Diagnosing hum and buzz: https://youtu.be/6P4C9at5rV8?t=4592
Removing diode ring oscillations: https://youtu.be/JPm5eQdJm38?t=531

Watch 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=2

Another video of his to look for, build his super-probe buzz-circuit fault finder to diagnose your amps.
https://youtu.be/uVkJqqZroN0?t=2

« Last Edit: September 15, 2018, 12:15:14 am by BrianHG »
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Offline KungFuJosh

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Re: Parasitic...house?
« Reply #33 on: September 15, 2018, 01:02:18 am »
Diagnosing hum and buzz: https://youtu.be/6P4C9at5rV8?t=4592
Removing diode ring oscillations: https://youtu.be/JPm5eQdJm38?t=531

Watch 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=2

Another video of his to look for, build his super-probe buzz-circuit fault finder to diagnose your amps.
https://youtu.be/uVkJqqZroN0?t=2

Thanks for all of that, his capacitor leakage tester project looks cool too.

EDIT: LMAO, looks like that leakage tester got a little controversial. :-DD
« Last Edit: September 15, 2018, 01:37:48 am by KungFuJosh »
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Offline BrianHG

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Re: Parasitic...house?
« Reply #34 on: September 15, 2018, 01:56:48 am »
EDIT: LMAO, looks like that leakage tester got a little controversial. :-DD
If it were anyone else...  Believe me, he knows his stuff....
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Offline drussell

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Re: Parasitic...house?
« Reply #35 on: September 15, 2018, 02:08:08 am »
EDIT: LMAO, looks like that leakage tester got a little controversial. :-DD
If it were anyone else...  Believe me, he knows his stuff....

In 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.  :)
 
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Offline KungFuJosh

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Re: Parasitic...house?
« Reply #36 on: September 15, 2018, 11:36:43 am »
Would you guys like him as much if he wasn't also Canadian, eh? ;)
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Offline drussell

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Re: Parasitic...house?
« Reply #37 on: September 15, 2018, 12:32:27 pm »
Would you guys like him as much if he wasn't also Canadian, eh? ;)

Yes.  Absolutely!  He could be from Mars and still know his stuff.  :)
 

Offline KungFuJosh

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Re: Parasitic...house?
« Reply #38 on: September 15, 2018, 08:24:32 pm »
Yes.  Absolutely!  He could be from Mars and still know his stuff.  :)

Now that you mention it, his lab does look like a spaceship.
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