Ground loop problem (9vac and 9dc from the same xformer)

[SISKO]

Im building a new PSU to power up all my pedals. Some of them work with 9ac, others with 9dc.
My building technique is to carry all ground thru the patch cables. In this way, there is no ground loop problem.
I send the positive and negative lead only to one pedal. Then on the power plugs I just connect the postive lead. the negative is carried by the patch cable as I said.
It was all good (been a few yeas since Im going with this techinque) but sudenly a problem ocurred.

This is what I got



This is my power supply arrenge and the whammy xp100 schematic wich is giving me the headaches. The problem is that whener I made connection trough the patch cable I receive a LOUD HUM, Im preety positive that this is a ground loop problem.
All I can see as a problem is L5 (forming a ground loop,a resistive one). I tried bypassing it with no succes, just made it quieter (not that much).

The "stompbox" is a regular pedal, I tried several of them and Im preety sure that there is no resistive path beetween their porwer supplys ground and their signal ground.
Anyone has any thought or had a simmilar problem?

Edit: I should say that the pedals power up nicely! They work as they should, but with a loud hum.

[Cliff Schecht]

I had this problem recently when I was running both unregulated (i.e. bridge rectifier-->cap) and regulated 9V from the same 12VAC transformer. A similar circuit would look like this: http://www.generalguitargadgets.com/diagrams/ultra_clean_ps_sc.gif

I was powering my 18V flanger from the unregulated power (taken right from the bridge rectifier) and 9V for all of my other effects (taken from the LM317T output). What I found was that if I ran both DC voltage and ground to both the flanger and the rest of my pedals the flanger would hum like a mofo. When I clipped the ground wire from the cord (so that I only had 18V DC on the outside of the 2.1mm barrel jack, pedal gets ground through signal ground) all of the hum was gone.

IME 99% of hum issues like these are a ground issue. Whether the issue is a missing ground or a ground loop or whatever is up to you to figure out. Start by pulling one pedal at a time out of the chain and see if it is a pedal that is causing issues. I typically start at the last pedal in my chain and continue down, pulling the input plugs until the hum stops. Then I figure out why it's humming.

[SISKO]

Thanks Cliff!

After a bit of thinking I got this solved. I connectd the ground wire trough the whammy (the one that need AC) and then it distribute all ground trough the patch cables.
Heres a diagram:

[Cliff Schecht]

Very cool, exactly opposite of what I did but just as effective. Only reason I didn't go this route myself is because my board layout relies on a premade power daisy chain and I didn't want to hack it up. Glad you got the problem worked out!