Power filtering with a wall wart...why not IN the wall wart?

[triskadecaepyon]

I have a question about wall warts and daisy chains -

If you can add power filtering to a wah to take out the "ripple" effect of AC to DC, then why don't you stick it at the beginning of the daisy chain (i.e. before the daisy chain links)?  Has someone already done this? Or will it not work?

i.e.

Power -> DC filter ---------->daisy chains

it's been bugging me for quite a while

[ninoman123]

Some wall warts have the filter cap right inside of them. In fact many that I have seen have them.  Some schematics for pedals have them some dont. They are like...pull down resistors, or reverse polarity protection. Some have it some dont. You can always add them to a circuit.

[Mike Burgundy]

it will work, putting a cap in the wallwart. Even better is putting in a cap, regulator, and one more cap to get very good ripple-free power. Even better than that, would be to also include caps in the pedals, so there's less risk of ticking or crosstalk if anything pulls a lot of current.

[niftydog]

it's rare to see a system with TOO MUCH power supply filtering. Most devices I work on have filtered supplys, as well as on board filtering and sometimes dual regulation. Go crazy!

[toneman]

filtering is one thing...
*regulation* is another.
regs are better.
the cleaner the unreg V, the easier it is 4 the reg.
stayregged
tone

[robbiemcm]

I've got a supposed 9v 300mA power supply. I used my DMM on it and it read it to be 14v. I was told this would fix the problem...

http://www.altronics.com.au/index.asp?area=item&id=Z0509

Will it fix the problem completely or at all, lol. And how do i wire it. I think the middle connects to ground, but the other two.. non-polarised?

Oh and if all that does end up working... if i want a -9v power supply, can i just reverse the leads? Or do I need a seperate power supply to do -9v aswell?

[niftydog]

Will it fix the problem

keep in mind the issue of regulation. Sounds to me as if that supply is working exactly as designed, it's just an unregulated supply. Many pedals will happily work using that supply. Some may even REQUIRE an unregulated supply in order to work properly.

However, correct use of that regulator will sort of "fix" the "problem". It will keep the voltage fixed at 9VDC (for a 7809 regulator) provided the input voltage is approximately 11VDC or greater. Be warned, should the voltage drop below that 11VDC or so, the regulator ceases to work properly and problems may arise.

How much the voltage drops depends on how much current you intend to draw from the supply.

Regulators are "polarised" for want of a better word. They have an input and an output. I suggest you read "the basics" on power supplys at geofex before continuing.

While you can effectively acheive a -9VDC supply by swapping the leads, it will not work in most cases due to the fact that just about everything electronic is referenced to ground. By swapping the leads, what you are doing is moving the ground connection from it's normal position and placing it where the input power would normally be. Should you then connect another device with it's ground in the proper place, you will short circuit the -9VDC supply and it will stop working.

Far better to buy/build a power supply that is intended for the task.

[robbiemcm]

Thanks for that, I think I know mostly what I'm doing with this, just wasn't sure how to put the 7809 in. So I am right in thinking i'll connect one to ground, and the input to the supply and then the output ... well... yeah... ok I think I got that part down. And 10uF capacitor across the rails? If so, it should be easy.

And I build a seperate power supply for -9v. And if it needs to stay at above 11v, it's 9v at 300mA and something around 13v - 14v at almost no current draw (if I'm using my DMM is that true?) then is voltage drop for current draw linear? Like... I could use up to 150mA? And how much current would the average circuit draw?

[dpresley58]

http://www.kpsec.freeuk.com/powersup.htm

This is a good link for power supply info, as well. I just broke open one of the cheaper wall warts from Mouser and did the rectification, smoothing and regulation on it. Inexpensive fix, and it works fine.

[Paul Marossy]

It also depends on what the wall wart was designed for. Some 2nd hand wallwarts have horrible filtering, if any at all, because they were designed for an electronic gizmo that had the power supply filtering built into it. And some of them only do half-wave rectification, too, which will give you a very nasty hum problem! But, I have fixed almost every humming wall wart I have ever had by placing a 1000uF cap across the power supply and a 100 ohm resistor in series with one side of the power power supply - a little trick I learned about from Craig Anderton.  :wink:

[dpresley58]

Paul, would you incorporate that snippet into the circuit the humming wart was feeding or would you directly modify the wart itself?

[Paul Marossy]

I've done it both ways. I prefer to modify the wall wart before I would a circuit, especially if it's a commercial pedal. I usually add some power supply filtering in most things I build - usually a minimum of 100uF.

Anyhow, I buy oddball wall warts from the thrift store all the time, so I never know what I am going to get!

[niftydog]

most of your questions can be answered by having a look at this pdf. Most datasheets will have a diagram of basic operational circuits. This one has that, plus split rail and how to get a NEGATIVE supply using a positive voltage regulator.

As for full circuits, use the one at geo named "single regulated +9V supply". Usually the cap at the left is around 1000µF to 3300µF. You could if you wished also include the caps shown on the datasheet in the above link.

no current draw (if I'm using my DMM is that true?)

virtually true, yes.

then is voltage drop for current draw linear?

not always, only one way to tell, try it!

how much current would the average circuit draw?

No such thing as average circuits! Pedals are usually far less than 100mA even for the power hungry pedals. Mostly 10-30mA I suppose.

[robbiemcm]

Ok I think I have everything sorted out, but I really need the following two things answered, I havn't been able to find any info on the first question, which is why I think maybe it doesn't matter which way round it goes.

http://www.altronics.com.au/index.asp?area=item&id=Z0509
1. Does it matter whether the left or right leg (from the way it is currently facing us) is in relation to which one is on supply side, which is on stompbox side?

2. Should I put a 10uF capactior across the rails? If so, I don't have one handy, would a 4.7uF electrolytic do the job?

[RDV]

I've got a supposed 9v 300mA power supply. I used my DMM on it and it read it to be 14v. I was told this would fix the problem...

http://www.altronics.com.au/index.asp?area=item&id=Z0509

Will it fix the problem completely or at all, lol. And how do i wire it. I think the middle connects to ground, but the other two.. non-polarised?

Oh and if all that does end up working... if i want a -9v power supply, can i just reverse the leads? Or do I need a seperate power supply to do -9v aswell?

This will damn sure fix the problem.

RDV

[niftydog]

I havn't been able to find any info on the first question, which is why I think maybe it doesn't matter which way round it goes.

Have you not looked at the datasheet I linked to? The answer is on the first page. Here's the link again.

Should I put a 10uF capactior across the rails?

Yes, do it exactly as is shown on the geofex article I linked to and you cannot go wrong. (well, you can actually, but that's another topic!)

Use two 4.7µF caps in parallel and you've got pretty close to a 10µF cap.

[robbiemcm]

Thanks for that nifty, considering I don't have a 1000uF capactior, I'll just go and buy one of those and a 10uF one next time I go to the store.

[jasonober]

But, I have fixed almost every humming wall wart I have ever had by placing a 1000uF cap across the power supply and a 100 ohm resistor in series with one side of the power power supply - a little trick I learned about from Craig Anderton

Can this be used on AC wall warts as well?

[Paul Marossy]

Can this be used on AC wall warts as well?

I believe the answer would be no. The filter cap on a DC wall wart is to smooth out power supply ripple after rectification, which is what causes the hum in the first place. Electrolytic caps, to the best of my knowledge, should only be used in a DC power supply.