Help with power supply noise.

[compuwade]

I recently designed this power supply for use with my pedal board. It's very clean as long as I keep any high gain pedal at least 12 inches away, and in some cases further separation is needed. Otherwise I get AC hum when the pedals are too close. My question is, what is normally used to block the noise from exiting the supply enclosure? I figured since all my pedals are built in hammond boxes, that alone should be enough to block the noise. But I figured wrong. Any help would be greatly appreciated. Here are some picks of my build. I used an old printer switch box for my enclosure:





Thanks!!

[GibsonGM]

Hmmm, you are right...any completely enclosed, grounded metal box should be shielding most of the noise radiation.  BUT, no shielding is complete unless it takes into account ALL pathways for RFI. 

You might need a ferrite bead on the output of the PSU, with decoupling caps as well (something from radio power supply days...).   Orienting (rotating) transformers 90 degrees from each other helps, too, so they won't couple as strongly, but I doubt that's having much to do with the interaction here.  The long and short of it is - no shielding is perfect - you could try more capacitance, like a 1000 uF to ground followed by a 100 ohm resistor on the output.   Experiment and maybe you can get the noise down, or keep pedals more than 12" away, LOL.

Nice looking project, BTW!

[ilcaccillo]

You Should replace the 4 transformers you used for 2 toroid transformers.

Toroidal transformers create a smaller magnetic field, so they are much cleaner.They're also more efficient than standard transformers.

check this link:

http://uk.farnell.com/multicomp/mcfm70-09/transformer-mini-7va-2-x-9v/dp/9531580

You can use 2 of these, each has 2 secondaries , so you would have in total 4 isolated outputs of 389mA each.

[merlinb]

Your metal enclosures are great for blocking electric fields, but generally rubbish at blocking magnetic fields, for which you need good permeability and no air gaps or cracks in the enclosure. With a string of guitar pedals it is also easy to get ground loops, which will pick up magnetic interference easily. This is avoided by supplying each pedal from its own floating power supply.

Your supply appears to be capable of giving four floating supplies (I see four transformers and four regulators), although I only see three banks of DC jacks??

[R.G.]

Nice build.

The cheapest thing to do is to buy that 12" spacing and use it. 

Mother Nature is giving you a lesson in magnetic fields. I personally have several scars to remind me of this same lesson back when She taught it to me. I suspect that some of the other repliers do too.

Merlin has the right general comment. It is very difficult to shield out magnetic fields. Very. And the lower the frequency, the harder it is to shield it. It is almost impossible to keep a static, nonchanging magnetic field out of a volume. Even very thick layers of high permeability materials tend to just move it around or attenuate it a bit. Microphone input transformers use several alternating layers of solid copper and high permeability special magnetic shielding alloys to do the best they can.

What's really going on here is that one or more of your transformers is leaking a lot of the magnetic field from its core. Unfortunately, that's almost a luck thing. Small transformers are run right up to the limits of core saturation, which makes them leak more, and cheap, small transformers are not carefully assembled, both of which make for lots of magnetic radiation. ilcaccillo is correct about toroids having a more contained magnetic field, less of the internal field is radiated. The caution is that this also depends on whether the windings are designed properly to not run the core into saturation. Again, small and cheap toroids are worse than big, expensive toroids in general, but that's not much help to you. The semi-toroidal "flat pack" transformers I mentioned in the first "Spyder" article are intermediate.

It is always better to spit out less radiation to deal with than to try to shield it out, if you can. When you can't, you deal with what you can do.

Current flowing in loops of wire creates a magnetic field too. This was merlin's point about ground loops. For the explanation, see the Spyder power supply article at GEO. Large loops of wire broadcast a magnetic field that other loops pick up. One way around this is to minimize the area of the loop by not having the send and return wires separated. That's one reason why AC power wiring always has at least two wires - one going and one returning. The area between them is limited to the fraction of an inch between the wires. It's also the reason for twisted pair wires. Twisting a send and return wire together ensures that the loop area between them is only as big as two thicknesses of insulation, and the twisting continuously reorients the axis of the "loop" so not all the loop is pointed in the direction of maximum pickup. Works the same for pickup wires - minimizing the loop area also minimizes the area for external M-field flux to generate voltages.

Even with twisted pairs, there is the possibility of the "ground" signal wire forming a large loop with the power supply ground wire and any AC wires connecting power supplies. This is what Merlin was referring to in the comment about floating power supplies, and what I was describing in the "Spyder" article at GEO.

With more experience, the issue on pedalboards may be more with poor transformers leaking and poor wire routing picking up more. I've seen setups with dozens of pedals that "ought" to hum, all daisy chained together from a single power supply, and they're quiet as a crypt. Sometimes it depends on whether you've recently offended Maxwell's Demon. 

As for guidelines:
- space is cheap; if you can simply move it further away, good.
- toroids leak less than EI/shell type transformers if you can find and afford them.
- semi-toroidals leak more than toroids, less than EI **if they're carefully made**
- EIs leak in proportion to how sloppily they're put together and designed.
- twisted send/return lines both send and receive less field induced voltage.
- there are many ways hum gets in; think of onions, not potatos.
- shielding low frequency M-fields is very much harder than shielding E-fields, or high frequency anything. Radio Frequency Interference is much easier than power line hum to shield out. Thick, seamless sections of iron and copper help, but are not a definitive answer.

As for what you can do here:

I would try installing copper "belly bands" on each transformer. If you can get some solid copper strip of some thickness, not just copper tape, and install a shorted turn around the outside of the transformer, windings and all, it may help force some of the radiation back into the core. This is expensive, because you need as thick a copper layer as you can, and a PITA because you have to do it well, including soldering thick copper bands on the outside of the transformer without nicking the insulation and introducing a shock hazard.  The multi-secondary transformer that Weber sells is a good choice here: https://taweber.powweb.com/store/magnetic.htm down near the bottom of the page.