Power supply filters

[kepeb]

hey, merry xmas to you to, hope you did'nt eat as much as i have

little lost in some of that again. i find it a lot easier to understand when i can look at it.
my filter is a little elaboration on the one i posted earlier, thats all.. the diode is as suggested on beavis audio. as is the 100 ohm resistor.

i dont have any magnet wire
any chance you could alter this drawing i just made to example what you are telling me?
how much different is what you are suggesting?
anyway. here is what i have on there atm:



and that other bit was made more confusing because of a typo.
i have 3 ac adapters supplying 3 separate daisy chains, i intend to put a filter in each.

[Steve Conner]

Change the 100 ohm resistor to 10 ohms or even 4.7, this'll cure the problem of not being able to run several pedals.

As others have said, all those capacitors in parallel is overkill, you only really need the 1000uF.

[kepeb]

great, that sounds simple, i'll give it a go
cheers man

[kepeb]

just an additional...

is the resistance here directly related to voltage AND current not noise then?
say i wanted 150ma to come through... that would be a value somewhere between 100 ohm and 4.7 ohm?

if i am putting in such a small value resistor what purpose is this serving in the filter?
does that help with noise too, or is that just current/voltage limiting?


also, if i only need the 1000uf, does that mean theres little truth in the other forum post about different cap values clearing different hum frequencies?

[rockin roy]

Hi Kepeb,
It wsn't the food it was all the chocolate,

The resistor is just limiting current. I wouldn't have added it. There isn't any way to guess value and current because we don't know what the peddles draw.

The filter you are building is just to supply power when voltage drops off between ripples.
If you were building a cross over or equalizer you would be working with an ac signal not dc power and the value of the capacitors (and their composition) would be a subject of intense discussion.

The diode is also something I wouldn't have added. It is something you might see on a coil that has frequent on off cycles like a solenoid. In that case it would protect other circuits from transmitted interference.

I guess that leaves us with just the 1000 mfd cap.

[Mark Hammer]

The RC combo works like a treble-cut (lowpass) filter, except that the treble you're cutting is waaaaayyyyyyyy down low. A 10 ohm resistor and 1000uf capacitor form a lowpass filter that is 3db down at 16hz and shaves another 6db off every time you double the frequency, such that 60hz hum (the remaining ripple in the power coming from the wallwart) is about 12db lower with that "filter" in place.

If the value of the resistor is increased, the filter corner frequency moves even lower and provides even more ripple rejection, but it does so at the cost of limiting how much current can pass. So, ideally, you want to either a) know that your current requirements/needs are modest enough that you can afford a higher resistance, or b} adjust the capacitance and keep the resistor value as low as possible.

If it should be the case that you need to use every last ma of current from the supply, you probably want to use a 1/2w or even 1w resistor and not a 1/4w unit, since the current will generate heat.

[rockin roy]

low pass filter?

The capacitor works as a battery smoothing the 120 hz ripple (which can be thought of as an ac element) but adding resistance to make it work better defeats the purpose of the power supply; to supply power. Adding a choke to put the current out of phase with the voltage is more acceptable (and used in low pass filters too) but not usually seen in applications like this. If we can get a regulator to really fix things up proper then I might recommend small capacitors before and after it to dampen any harmonics it might produce. Again not really needed. Since regulators only cost a couple of bucks I can't imagine not using one if clean and accurate dc is what is wanted. In the mean time I would again just stick to one added component (the 1000 mfd capacitor) to mitigate any noise caused by ripple. I can't help but add, the best DC available always has and still does come from a battery.

[Mark Hammer]

In the mean time I would again just stick to one added component (the 1000 mfd capacitor) to mitigate any noise caused by ripple.

Which is what I have in the box I showed. It is not perfect, but it does work. It is the case that capacitance is generally correlated with size; higher = bigger. No small surprise then that wallwart manufacturers will skimp on caps and maybe stick in a 470 or even 220uf unit in the sealed black box, just to keep things compact, when really a 1000uf or 2200uf unit was called for. As such, simply adding on some capacitance, as shown, will take what is a reasonably regulated supply and bring it the extra lap to make it as smooth as it has the potential to be.

I can't help but add, the best DC available always has and still does come from a battery.

Here, here!

[rockin roy]

Sorry if I sounded testy Mark.
I got started on this thread looking for something else and have spent to much time selling simple, and bang for the buck. We should start another thread on why batteries suck but Prius' don't.

[Mark Hammer]

You didn't sound testy one little bit, buddy. I just wanted to note in my post that while I would advocate best regulation possible, it can happen that all one needs is juuuuuust a but more capacitance, and when that happens, what I showed is enough to do the trick.

I still wish that wallwarts came in clear sealed units, instead of black sealed units. If we could open them, or see what was inside them, then we wouldn't have to cross your fingers, bite our tongues, and hope that when it says 9vdc outside-pos, it was the "right kind" of 9vdc, which is what we are all faced with now. Right now, it's a bit like buying a guitar and not being allowed to see how many pickups, or what type, are on it. Sure, it's a guitar, and it works, but does it do what you need it to do - I guess you'll have to buy it and see, eh?

That is precisely why Roland and other companies say "Use our supply". It absolves them of the need to explain, and the consumer of the need to comprehend and measure.

[kepeb]

ha, too true.
i think a lot of things i have bought would of been better off with transparent plastic actually.. maybe if i saw how little effort went into whats inside some of this stuff i wouldn't even buy it.

still, the whole point of this learning curve was to avoid being a consumer and splashing out on a 'recommended' PS when i can get acceptable results with a few minutes to spare and some capacitors.(i even salvaged them from the bin)

although, i dont think that many 'consumers' hang around the Music Electronics Forum either

i really appreciate all your input and advice. i now have a pedal board that idle's at less than half the garbage noise volume! with £4 ac adapters result! whats next!?

also, prius' run off 9v batteries dont they?

[rockin roy]

I've got a bunch in the garage I'd be happy to smash if you want to see what they use.

If you really are curious try taping it on the corners with a hammer. You should be able to get the square back to separate from the flat part where the prongs are. The prongs are not glued to the plastic so you should end up with a transformer with two prongs held by some really thin wires (which will break). Mark is right about the low value cap (if it has any). Just look at the height of the unit and picture a square transformer with the prongs on the primary side and the secondary side equal distance towards the output. There will be a space about as wide as your finger for your slide switch and components. Wholesale these sell for around 50 cents, Surplus $4.50 and new $15. Yes. There is a huge difference between hobby grade and OEM components. And sadly you do pay some premium prices for equipment. However there isn't a lot of stuff out there that can survive a musician on stage blindly using his foot to change settings. So don't think twice about paying $35 bucks for a treble booster that just has a 45 cent FET in it.

[rockin roy]

OK, Just signed up for a photo bucket account for this, I hope it works.

http://i808.photobucket.com/albums/z.../wallwarts.jpg

The center one is the best, it actually uses a molded bridge rectifier and a 4700 mfd cap. The one on the left uses 1000mfd grounded center tap and two diodes, same for the one on the right but it has a 470 mfd cap and uses crimped connections instead of solder. The farthest left is just an AC adapter. I thought it was interesting how they were able to get the cap in the same size case.

[kepeb]

nice comparison. cheers guy.
hope you got some good glue to put them back together sti'll at 50 cents i guess its just as easy to use a new one.

thats me sorted till i decide to upgrade anyway.

[Mark Hammer]

Thanks for the pix. Instructive.