Converting 9VAC to 9VDC

[Cortexturizer]

Yeah, they've done the Super Ego as well which expands on the Freeze, now THAT's tempting.
I've expanded my board a lot lately, will show it in the appropriate thread as soon as the Freeze arrives, then it will be complete. I managed to get some more boutique stuff, like Randy's Revenge ring mod, Pigtronix envelope phaser EP-2, Zvex Loop Gate, I tend to go for those that not many players choose. Really happy with them all.

[Cortexturizer]

I wanna thank Thomas for being a wonderful sport and helping me get this thing done. Here it is, in a small-ish black plastic box with two 9VDC outs



The weird thing about it is this - my 3-in-1 build won't make a sound with this power supply, the only thing i get is ugly oscillations. Whereas my '77 EHX bass balls works perfectly. The new SpaceHarp by Haberdasher works great, and is silent as well.
I gathered the strength and courage to plug in the Freeze as well and this is what I get - https://docs.google.com/file/d/0B--6F_4JB7SYUmZ4ZTFKZG5rSnM/edit
So it works as it should, but there's oscillations happening. What to do?
Once again, kudos to Thomas for getting me this close.

[RobA]

If the output voltage level is high enough and that side of things looks to be OK. It kinda sounds like it might not be getting enough current. Is the regulator that's in there able to source the ~200mA the Freeze needs?

Some regulators are touchy about the output caps. Which regulator and caps do you have in there?

[Cortexturizer]

Well I followed Thomas' schematic from page 2 of this thread line by line. My 3-in-1 behaves even worse, there's oscillation only, no sound happening at all. I'll try some other pedal later, some of the digital ones and see what happens. I have one that draws 40ma and one that draws 100ma so that could be a nice test.

[RobA]

I didn't see the LDO package right when I looked in the photo the first time. Now I see that it's a TO-220 so that should be OK.

Here is one thing to look at from the LM2940 doc,

**COUT must be at least 22 μF to maintain stability. May be increased without bound to maintain regulation during transients. Locate as close as possible to the regulator. This capacitor must be rated over the same operating temperature range as the regulator and the ESR is critical; see curve.

The curve in question indicates that for the current draw you are looking at, the cap needs to have an ESR in the 0.5 to 2.5Ω range. You might be able to meet those specs by paralleling a decent 22µF electro across the 100nF cap or if that doesn't work, try using a 22µF ceramic cap (X7R or X5R type).

[Cortexturizer]

Hey Rob I tried your idea with paralleling the 100n cap with a 22uF electro but that only made things worse, I could hear the oscillation even on the pedals that haven't had it before.
Oh, and I can add MXR Micro Flanger to the list of the ones who work okay with this supply. I could hear less oscillation with the Boss RV5 than with the Freeze. Boss is also digital and draws around 50ma.

[Cortexturizer]

Also, I took a reading from the supply while connected to the Freeze, and there was the output voltage under load at the same 9VDC.

[RobA]

Since the total capacitance increase didn't fix the noise, it could be the ESR of the cap that's driving the oscillation. You are using the LM2940? Check out page 14 of this doc, http://www.mouser.com/ds/2/405/snvs769i-285393.pdf

You are using a film cap for the output cap? If so, I'd guess the ESR is too low. Adding the 22µF cap in parallel would actually reduce the total ESR of the combined caps. So, that would explain the oscillations getting worse after adding the 22µF even if you need the increased capacitance for stability.

To meet the spec sheet requirements, I think you'll need to replace the 100n cap with a tantalum cap or an electrolytic that's got the right ESR range. I'd guess an electro like a Panasonic FR or Nichicon PS would work, but I don't think I have an LM2940 around to breadboard and test it.

[Cortexturizer]

I tried a tantalum  but no progress. I tried another pedal though, the micro POG and it works brilliantly. So in the end I used the power supply to power the pog, small stone and micro flanger, and that made it possible to use the out that I used for the pog so far to power the freeze and I'm all set, all the pedals on the board are getting juice now. Pog draws around 170ma, worka perfectly silently and all. This supply hasn't turned out to be perfect but I got the job done in the end so all is good.
Thanks gents.

[RobA]

That's seriously weird. The docs make it seem like the Micro POG is more sensitive than the Freeze, although it looks like they both use the same power supply. I'm really curious as to what's going on. The POG draws more current? Maybe that's it -- the additional current draw puts the regulator into a stable area.

It's good you've got a working solution though, even if does seem weird.