regulated power supplies that hum

[mac]

Tired of replacing batteries, I built my own regulated power supply using a lm317 variable regulator with a pot to adjust the output voltage.
I place the transformer and the circuit in a metal box, plugged it into my mxr+ and... hum!
Why? The lm317 is supposed to reject ripple up to 80db.
The answer is simple.
If you place the regulator circuit AND the transformer in the same box, the strong magnetic field H generated by the transformer will induce a variable 50/60hz current in any closed loop of the circuit and cables that´s not parallel to the H field (Ampere´s law).
So the circuit must be placed in a different box. For example, inside the pedal or a spider array.
Want to feel the H field of a transformer? hold a screwdriver loosely and place it near any wall wart in diferent positions.

mac

[toneman]

If that's the case,
how does EH get away with putting the little AC transformer inside the box

U got (pure)DC going into the reg?
What's the ripple on the INput DC 2 the reg?
What's the circuit U built?  (sch link)
Got a jpg of your construction techniques?
MayB U just forgot the common connection on the IN & OUT jacks? 

My 317based pwrsupplies are ripplefree,
but I use a WallWart for the "raw" pwr.   

stayregulated
tone

[mac]

I have no doubt that a well regulated hum free power supply can be made placing all componentes inside a metal box.

What I want to say is that if you are not careful or know little about EM, and dont know much about circuit design, the circuit can pick up some magnetic field the lm317 was clever enough to reject. The problematic loops are those after the lm317.

I used the Ultra Clean PS schem from GGG and a big transformer. I unfortunately placed the circuit perpendicular to the field. And had a big loop between the 240 Omh, the 10uf output cap and gnd.
I guess that the guys at EH placed the circuit where the H field is mostly parallel to the circuit and minimized loops areas (never saw one). In this case induced current is minimal. That is what I finally did... and a rectangular piece of iron dividing the box acting as curtain, just in case... Some of the H field is lost generating currents in the iron. It can be showed that the intensity of the H wave drops exponentially inside the iron.

mac

[Paul Perry (Frostwave)]

It's suprisingly difficult to get a REALLY clean power supply...... you would think it was easy, but nooo...
Here is where I had trouble once: if you don't use 'star' earth connection (that is, running all the earth connections in the power supply DIRECTLY to the one point) then, you might find that as the rectified current pulses in and out of the main cap, there is a small AC voltage developed along some of your 'earth' wiring (or PC trace), because all conductors have some resistance in them. And, depending what the wiring is like, you might find this 'tiny' bit of AC hum crap (and its harmonics) getting into the system.

[RDV]

Sounds like you need more filtering to me, I've got no such problems with a 300ma tranny and the 317 in the same box.

RDV