Author Topic: Hiss with AC-DC buck supply: how shielding ?  (Read 1207 times)

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Online mauros

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Hiss with AC-DC buck supply: how shielding ?
« on: November 26, 2017, 10:37:42 PM »
Hi everybody,

i bought two bucks 220V to 9V like this

eBay auction: #https://www.ebay.com/itm/NEW-High-G...hash=item3aedf56b33:m:mU9l2J_hWAfiMR8zs66GTXw]https://www.ebay.com/itm/NEW-High-G...hash=item3aedf56b33:m:mU9l2J_hWAfiMR8zs66GTXw

to power two sets of pedals effects for my guitar. It works all but I hear from the speaker a hiss that changes the frequency depending on the changes I make to pedal effects. A linear power supply with a transformer I have already realized but it interferes with the inductance of the wah wah pedal and i feel hum. In this video, a mu-metal sheet solves the problem of noise, but it costs too much.




Make some changes low cost to improve shielding ?

Cheers !
« Last Edit: November 26, 2017, 10:41:40 PM by mauros »
 

Offline PChi

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #1 on: November 27, 2017, 02:03:45 AM »
The noise may be due to them electrical noise on the output (most likely) or from radiated noise. Before plugging in please watch some of the videos by Bigclivedotcom on YouTube.
 

Online mauros

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #2 on: November 27, 2017, 03:19:51 AM »
Thanks for replying but I did not understand if you want to tell me that in Bigclivedotcom's videos I find the solution or whether it is dangerous what I'm doing.
 

Offline PChi

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #3 on: November 27, 2017, 04:27:37 AM »
I can't see the Ebay listing.
Sorry I wasn't clear,  some of the power supplies bought by Bigclivedotcom from Ebay have been dangerous. 2 bucks is suspiciously cheap.
The video appears to show the effect of shielding the magnetic field from I presume a conventional 50 Hz transformer. Steel will also work at screening magnetic fileds (if it is the problem) but not as well as mumetal.
 

Offline ogden

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #4 on: November 27, 2017, 05:30:09 AM »
i bought two bucks 220V to 9V like this

Make some changes low cost to improve shielding ?

Simply don't use switching supplies for audio applications :) Video is not related to hiss problem, it's more or less "snake oil" scam :D

Indeed you can (try to) make EMI filters for high frequency noise filtering, but easiest fix for you is to get Wall Warts. They do not "hiss" but "hum" instead - because are AC tranformer-based. Linear regulator inside pedal most likely will be able filter low frequency hum w/o problems.



Quote
A linear power supply with a transformer I have already realized but it interferes with the inductance of the wah wah pedal and i feel hum.

Just don't put those transformer-based supplies close to pedal!! You need to plan your layout of supplies and audio gear - put them as far away as possible. Then you will not need to build shields. Well, if you really have no way to somehow split gear/supplies - put all the supplies in the solid aluminium box. Box made out of ferrite would be even better.
« Last Edit: November 27, 2017, 05:42:41 AM by ogden »
 

Online mauros

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #5 on: November 27, 2017, 06:04:13 AM »
Linear regulator inside pedal most likely will be able filter low frequency hum w/o problems.

What do you mean ?

Edit:

At the moment I did not think audio switching is noisy, I did the purchase and I realized this
https://i.imgur.com/DTeJNgL.jpg
although it would be very convenient especially for the small size
« Last Edit: November 27, 2017, 06:12:33 AM by mauros »
 

Offline ogden

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #6 on: November 27, 2017, 06:33:21 AM »
Linear regulator inside pedal most likely will be able filter low frequency hum w/o problems.

What do you mean ?

I mean PSRR of linear regulator. The lower frequency of the noise coming into regulator - the better chances it will be filtered out.

For mains tranformer-based DC supply ripple frequency is 100/120 Hz after rectification. Switching supplies usually operate in 50KHz and up frequencies, with harmonics going into MegaHertz range. This noise is able not only propagate through cables, leak through linear regulator unfiltered, but also radiate all around as radio waves.

https://en.wikipedia.org/wiki/Power_supply_rejection_ratio


Quote
At the moment I did not think audio switching is noisy, I did the purchase and I realized this
https://i.imgur.com/DTeJNgL.jpg
although it would be very convenient especially for the small size

Those indeed are switching supplies. - Crap which MUST be avoided on stage by all means.

In your place I would test every component one by one in minimal setup: guitar+pedal+amp. If you hear hum when tranformer based DC supply is located >= 1m away from pedal - try to change supply. Maybe it's isolation is faulty. Try swapping pedal as well. Why don't you rent equipment which is known to not hum and by swapping check which part of your setup causes problems?
« Last Edit: November 27, 2017, 06:35:20 AM by ogden »
 

Online mauros

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #7 on: November 27, 2017, 07:03:38 AM »
Guitar, cables, amps and all pedals are ok but one thing I have neglected to say is that the problem is generated by the inductance that is inside the wah wah that captures the magnetic fields in its vicinity, and indeed I hear the hum which changes in intensity with the movement of the power supply.

Edit:
If I set the power supply to 50 cm, the noise disappears, but I would like to fix it under the pedalboard so I would avoid carrying pedalboard + power supply separately, even with the risk of forgetting somewhere  ;)
« Last Edit: November 27, 2017, 07:08:06 AM by mauros »
 

Offline ogden

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #8 on: November 27, 2017, 07:47:23 AM »
the problem is generated by the inductance that is inside the wah wah that captures the magnetic fields in its vicinity

Then don't put magnetic fields in it's vicinity or get rid of this wah wah and get another one which does not capture magnetic fields  :popcorn:

Quote
If I set the power supply to 50 cm, the noise disappears, but I would like to fix it under the pedalboard so I would avoid carrying pedalboard + power supply separately, even with the risk of forgetting somewhere  ;)

So you want to fix power supply under pedalboard and whats worse - route AC mains there, only because you are afraid to forget it somewhere? I would just buy couple of spare Wall Warts... Maybe instead of consulting electronics forum, you shall consult doctor and address your memory problems? :)

You can try to use 1/2 inch steel plate as a magnetic shield  :-DD
 

Online mauros

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #9 on: November 27, 2017, 09:33:22 AM »
Do you are making a fool of me ?  ;) When you are over 50, memory can betray you but that's not the main problem. I want to optimize by assembling everything you need in the pedal and if possible, I would like to eliminate the hum due to the interference of the magnetic fields
 

Offline ogden

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #10 on: November 27, 2017, 10:51:44 AM »
Do you are making a fool of me ?  ;) When you are over 50, memory can betray you but that's not the main problem. I want to optimize by assembling everything you need in the pedal and if possible, I would like to eliminate the hum due to the interference of the magnetic fields

Just kidding, never mind. To sum up your case - personally I would not risk to bring AC mains wires close to sensitive pedal you have. What's worth convenience if quality can suffer in result? Who knows what noise will ride in the AC mains wires of next stage? After all I do not see huge difference between low power Wall Wart with DC cable compared to AC mains cable. Even length of cable is close. One just have bigger plug but thinner cable, that's it. Both does not cost much, both can be lost.

In case you are putting supply into pedalboard - 1/2-inch plate indeed was overstretch ;) You shall test with lighter plates first, see results. Just do tests - what can I say. Also consider wah wah pedal upgrade. NEVER ever use switching supplies to power audio gear.

Good luck!
 

Online mauros

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #11 on: November 28, 2017, 02:47:42 AM »
Regardless of my musical need, let's suppose the problem is just to figure out how to reduce EMIs from a switching power supply like an economical AC-DC buck. My question is, can I use such a filter ?
http://ka7oei.blogspot.it/2014/08/completely-containing-switching-power.html

How should the components be sized ?

 

Offline ogden

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #12 on: November 28, 2017, 06:11:26 AM »
Regardless of my musical need, let's suppose the problem is just to figure out how to reduce EMIs from a switching power supply like an economical AC-DC buck. My question is, can I use such a filter ?
http://ka7oei.blogspot.it/2014/08/completely-containing-switching-power.html

Indeed you can use filters. Your supplies needs filters on both - AC side and DC. Also EMI shielding cage will not hurt. It is not feasible to buy crap 2$ supply, then invest your money and time building filters, test results, fix errors and so on. In result it will cost more than proper, clean switching supply with filters already built-in.

Quote
How should the components be sized ?

Article you mention provides quite enough information. Anyway you need to learn so you can size components yourself and comprehend what you do. To do it safely you need AC mains isolation transformer. Also oscilloscope will not hurt.
 

Online mauros

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #13 on: November 28, 2017, 07:55:25 AM »
wow, for a beginner I would say that the oscilloscope is the best choice  :clap:

cheers !
 

Offline ogden

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #14 on: November 28, 2017, 08:22:51 AM »
wow, for a beginner I would say that the oscilloscope is the best choice  :clap:

Best choice for beginner - avoid choosing switched mode supply projects. Do you really think that you can pick any area of electronics and just by asking question in the forum - succeed?
 

Offline janoc

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #15 on: November 28, 2017, 08:50:42 AM »
wow, for a beginner I would say that the oscilloscope is the best choice  :clap:

cheers !

No he did say that "it will not hurt" - i.e. will be useful. That's not quite the same thing. And I agree with that - checking noise levels at the output of the PSU without a scope is difficult, not everything can be heard!

You can have e.g. 100kHz noise from one such switching supply superimposed on the power rail, another such supply for another gizmo will produce 115kHz and these get parasitically mixed together somewhere in an amplifier or the console - producing a nasty 15kHz squeal that will be very hard to debug.  If you are unlucky, the superimposed noise could get amplified and blow out your tweeters as well, depending on the setup.

So don't assume that when the output is silent there is no noise there - you can easily have high frequency noise or even oscillations there that you can't hear. And without a scope you will never know.

I also concur with the idea that buying a $2 supply and then trying to improve it by adding filters and what not is a fool's errand. Even if that $2 supply isn't explicitly a deathtrap (most of these are - that's why the link to BigClive's videos - he is an electrician and shows what to look for when checking such gadgets for basic safety), just the components needed for the filter will cost you at least 10x as much. Plus work, time and the risk that something blows up catching fire or electrocuting someone because you are dealing with mains. Is that worth saving the 30-40 bucks you would pay for a proper and safe wallwart? You could probably even get a free one if you scavenge one from an old laptop, printer or a similar device - they are still switching supplies but likely of much better quality.

Re your pedal - the most logical thing to do would be to shield the coil in the pedal (build a metal can around it) instead of trying to shield everything else around. What if your colleague puts his big guitar combo (or whatever) next to your pedal and you start picking up noise from his power supply/amplifier? If you can't shield it, you can try to replace the inductor(s) with toroidal ones, those are "self-shielding" because the magnetic field is confined inside the core.

« Last Edit: November 28, 2017, 08:53:25 AM by janoc »
 

Online mauros

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #16 on: November 28, 2017, 09:15:40 AM »

Re your pedal - the most logical thing to do would be to shield the coil in the pedal (build a metal can around it) instead of trying to shield everything else around.

The last thing you've written is just what could help me, that is shield the coil inside my pedal. Do you say I have to connect the can of metal to the ground of the pedal ?
 

Offline janoc

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #17 on: November 28, 2017, 12:46:39 PM »

Re your pedal - the most logical thing to do would be to shield the coil in the pedal (build a metal can around it) instead of trying to shield everything else around.

The last thing you've written is just what could help me, that is shield the coil inside my pedal. Do you say I have to connect the can of metal to the ground of the pedal ?


Well, that's difficult to say without knowing how your pedal is constructed, but yes, normally you would ground the can to the signal ground of the circuit (not necessarily the metal chassis if there is one).
 

Offline floobydust

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #18 on: November 28, 2017, 05:53:07 PM »
mauros, use an old-style adapter with power transformer. I know they are bigger and heavier.

Don't use the SMPS in the photo - no fuse, no safety ground on cheap chinese SMPS that make so much EMI your entire pedal array and audio chain will forever be noisy.
Nobody likes to get electrocuted holding a guitar and grabbing a mic stand.
 

Offline vealmike

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #19 on: November 28, 2017, 06:46:56 PM »
Switch mode PSUs can be used with audio, but only with a great deal of care and some expertise.
A ordinary transformer could also be used, but will hum.

By far the simplest solution for powering the Wah-Wah pedal is also the one that will yield the best quality audio for our novice.
[spoiler] [/spoiler]
 

Offline IanMacdonald

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #20 on: November 28, 2017, 11:11:26 PM »
I could suggest winding both output wires of the Buck through a  ferrite ring. That often works wonders for induced hiss.

Alternatively if hum is a problem from a linear supply, a small toroidal transformer may be a better option than a rectangular core type.

BTW I would echo the concerns about using any mains PSU from a dodgy manufacturer on music gear.  Stage power is not always reliably earthed, and the combination of sweaty hands on metal strings (or a mike) and a live effect unit case, is a deadly one.  >:D Therefore only use reputable stuff if you want to stay alive.  :-+
 

Online mauros

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #21 on: November 29, 2017, 12:42:37 AM »
I do not think to use that chinese object on stage but the curiosity to understand how to eliminate those noises remains.
 

Online mauros

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #22 on: November 29, 2017, 07:52:32 AM »
I tried an adapter found in a drawer. It works fine but when i activate the wah wah, a pulsating noise comes out of the speaker which in my opinion is always generated by the internal inductance of the wah wah pedal. I tried to surround the inductance with copper tape but the space is narrow and there is a risk of creating a short circuit. The adaptator is this http://truetone.com/1-spot/ , designed exclusively for guitar effects.
 

Offline ogden

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #23 on: November 29, 2017, 09:16:31 AM »
I tried an adapter found in a drawer. It works fine but when i activate the wah wah, a pulsating noise comes out of the speaker which in my opinion is always generated by the internal inductance of the wah wah pedal.

Don't you find it strange that any power you throw at your wah wah, results in noise? I would try to swap pedal and upstream audio cables/amp as well. Could be so that power supplies actually are not actual cause of your problems.
 

Online mauros

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Re: Hiss with AC-DC buck supply: how shielding ?
« Reply #24 on: November 29, 2017, 10:18:17 AM »
Unfortunately it is a problem that afflicts all the guitarists https://tinyurl.com/y9wot8zf and that's why I think the chances are 3, or wah wah away from pedalboard (I prefer not) or the power supply away from wah wah (i prefer not but i could do it) or shield the power supply.
 


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