Noise from a 9VAC adapter

[Esppse]

I recently assembled a rack system and located the source of hum. My Prostage Wah module is the only unit that uses 9VAC. When I shut power to that adapter the hum stops. I made sure when designing this. there was no ground loops, and nothing is physically touching any unit.

All the units in the rack are plugged into the same switched power strip.

I am using this power supply:
https://www.amazon.com/dp/B00B886DEA?psc=1&ref=ppx_yo2ov_dt_b_product_details

Is it this specific brand adapter that is causing the issue?
 
Also, does it matter if the adapter is plugged in upside down? It is AC after all, that shouldn't matter right?

Tomorrow I will try an EBTech Hum X on the adapter. I actually have had success before using one of those HUM X things on 2 prong units, even though it takes 3 Prong units.

[antonis]

does it matter if the adapter is plugged in upside down?

In theory, NO..

In practice, Yes (sometimes..)

[Rob Strand]

Tomorrow I will try an EBTech Hum X on the adapter. I actually have had success before using one of those HUM X things on 2 prong units, even though it takes 3 Prong units.

Unlikely to work.  As far as I'm aware these work by breaking the ground cable and inserting a network into the ground to stop loops, similar to what is done inside of equipment.   An AC wall-wart is already isolated and the 3rd pin, if it has one, usually goes nowhere.

You could check if there's any connection between the ground pin and the output.

I agree with Antonis.   You could try flipping the low voltage AC.
It certainly wouldn't hurt to try another power supply.

Do you get the problem if you use that pedal alone?

[Esppse]

I tested all the units separately before installing in the rack. There definitely was no hum on the wah unit, but I forgot if I used that 9VAC adapter currently installed, or a different one I had laying around which was actually 12VAC, yeah risky heh but it worked well during the isolated test.

[Esppse]

I just tried reversing the polarity at the 9VAC end of the adapter, also did not help unfortunately.

[Rob Strand]

I tested all the units separately before installing in the rack. There definitely was no hum on the wah unit, but I forgot if I used that 9VAC adapter currently installed, or a different one I had laying around which was actually 12VAC, yeah risky heh but it worked well during the isolated test.

Maybe the problem is the metal wha chassis being bolted onto the rack?
Try isolating the metal wha enclosure from the rack.

[Esppse]

I tested all the units separately before installing in the rack. There definitely was no hum on the wah unit, but I forgot if I used that 9VAC adapter currently installed, or a different one I had laying around which was actually 12VAC, yeah risky heh but it worked well during the isolated test.

Maybe the problem is the metal wha chassis being bolted onto the rack?
Try isolating the metal wha enclosure from the rack.

I actually took that into consideration when building. It's velcroed on for isolation currently. I bought this adapter new a couple weeks ago, could it be the way this adapter is constructed or maybe I got a dud? I currently don't have a different 9VAC adapter, since I bought 2 of the Jameco ones from Amazon.

[Slowpoke101]

Most likely the transformer's magnetic field is being picked up by a sensitive circuit in one of the other pedals on the board,
I have similar problems with some of my single coil pickup guitars when I sit down to practice. I keep finding magnetic fields in unexpected places.

[Esppse]

Hmm, yeah that may be what is happening. Every other pedal has either an isolated power supply or a switching style power adapter.

Does anyone know where I can get a 9VAC adapter that is not Class II transformer?

[Rob Strand]

Hmm, yeah that may be what is happening. Every other pedal has either an isolated power supply or a switching style power adapter.

Does anyone know where I can get a 9VAC adapter that is not Class II transformer?

Stray field is a possibility.

The way to prove it is to put some distance between the rack and the adaptor (or more precisely the transformer inside the adaptor).  Take the adaptor out of the rack and run the low voltage AC into the rack.

The problem here is the field comes from the transformer itself.   This has nothing to do with grounding or Class II circuits.  The field comes from the way a transformer works.   Some constructions of transformers have less external field than others but most wall-warts use EI core transformer and will be similar.   More expensive EI transformers have flux shorting bands but I can tell you now it would be a miracle if you found a wall-wart made like that.

The simplest practical solution is to put distance between the transformer and the sensitive circuits/cables.   You can put thick metal between the transformer and the cables and an eddy shield but is not really practical for mains frequencies it's a fair amount of effort with little guarantee it will do anything.

You can also get capacitive coupled noise from the mains wiring itself in the wall-wart to the sensitive circuits.  Usually the generall shielding on audio equipment will fend that off.

Yet another form of hum is from the cable between the adaptor and the adaptor plug.  If that cable passes over sensitive audio cables or circuits humm/buzz can couple into the audio.

[Esppse]

Hmm, yeah that may be what is happening. Every other pedal has either an isolated power supply or a switching style power adapter.

Does anyone know where I can get a 9VAC adapter that is not Class II transformer?

Stray field is a possibility.

The way to prove it is to put some distance between the rack and the adaptor (or more precisely the transformer inside the adaptor).  Take the adaptor out of the rack and run the low voltage AC into the rack.

The problem here is the field comes from the transformer itself.   This has nothing to do with grounding or Class II circuits.  The field comes from the way a transformer works.   Some constructions of transformers have less external field than others but most wall-warts use EI core transformer and will be similar.   More expensive EI transformers have flux shorting bands but I can tell you now it would be a miracle if you found a wall-wart made like that.

The simplest practical solution is to put distance between the transformer and the sensitive circuits/cables.   You can put thick metal between the transformer and the cables and an eddy shield but is not really practical for mains frequencies it's a fair amount of effort with little guarantee it will do anything.

You can also get capacitive coupled noise from the mains wiring itself in the wall-wart to the sensitive circuits.  Usually the generall shielding on audio equipment will fend that off.

Yet another form of hum is from the cable between the adaptor and the adaptor plug.  If that cable passes over sensitive audio cables or circuits humm/buzz can couple into the audio.

OK Hum X verified not to work as well as flipping the adapter...

I followed your advice and plugged the adapter into an external strip outside of the rack... the hum magically disappears!

I then used an extension cord and slowly moved the adapter into the rack towards the other units... the hum comes back! So it definitely is a proximity issue. This is crazy! Since that adapter was not even touching the other units, the field of effect the sensitivity of that 9VAC adapter is really bad! Can I expect better performance from a different brand adapter?

I had never had such a problem putting adapters close to each other on a power strip, you see that all the time on professionally built pedalboards sometimes with pedals requiring specific voltages or currents that pedal power supplies with 8 or more outputs cannot take care of.

Can anyone recommend me an adapter that won't be susceptible to this hum?

[Rob Strand]

I then used an extension cord and slowly moved the adapter into the rack towards the other units... the hum comes back! So it definitely is a proximity issue. This is crazy! Since that adapter was not even touching the other units, the field of effect the sensitivity of that 9VAC adapter is really bad! Can I expect better performance from a different brand adapter?

Better performance maybe but it's more likely to be similar.  For example a factor of 2 when you really want a factor of 10.

I had never had such a problem putting adapters close to each other on a power strip, you see that all the time on professionally built pedalboards sometimes with pedals requiring specific voltages or currents that pedal power supplies with 8 or more outputs cannot take care of.

I've got doubts it's because the adaptor is close to the other adaptors.   You might be able to see a pattern but it's usually when the transformer is close to something related to the audio.  So maybe when you bring it close to the other adaptor units it's actually close to something audio?

If the hum really does depend on the proximity of the transformer to the other adaptors and not something audio I'd be trying to identify which one of the other adaptors is doing it.    It's possible one of the adaptors has a fault or design issue which is making it susceptible to the field.   Even if you fix this problem a sensitive unit like that is likely to show up some other weird-ass behaviour down the track, it might even fail if it has a problem.

So lets suppose there is something related to audio in the vicinity of the adaptor.   The best thing is to put as much space between the audio and the adaptor.   You can use the AC adaptor as to tool to identify where the actual problem is located.   Put the adaptor on an extension lead and move it closer to the items in the rack.  (You could use to same trick to see which of the other adaptors is sensitive.)  Maybe one of the devices is sensitive to the field,  it might even have a fault or design issue.    If you can fix that problem the whole rack will be less sensitive to noise/magnetic fields.

Can anyone recommend me an adapter that won't be susceptible to this hum?

It's pretty much impossible to know from outside of the adaptor.     You probably should dig a little deeper into the problem or try to solve it another way. 

Assuming that is done and you still have the problem.  Some of the commercial isolate output PSU's had AC outputs and *some* of those had R-core or toroid transformers inside.  Not a cheap solution.  An R-core or toroid transformer has a much lower leakage field.   You could also build your own AC supply using a toroid transformer.

[Esppse]

OK thanks, I moved the adapter via extension cable to a corner of the rack. It still exhibits hum, but it is at a manageable level now. Basically the hum doubles when I use 2 pedals in a row, but I think I am fine with it.

[Elektrojänis]

If the equipment in the rack is isolated from the rack itself, are the cases still grounded somehow? This especially for walwart powered units.

The metal enclosures might not shield the circuitry properly if the enclosure is not properly grounded.

And if they are grounded through signal wires only and the transformer manages to induce hum current to the case, the only place the hum current can go is your signal ground.