Power! ?

[ashcatlt]

For a while now I’ve been thinking about building a high quality power supply for my pedals. I looked over schematics involving transformer isolated outputs, regulators of various sorts, current limiting…

…and then I realized that I’ve been using the same powered pedal board for over 10 years now. I’ve used it in both recording and live situations. I’ve never had any reason to complain about the power supply. Maybe I’m just used to it, but then again it must be good enough for me.

Unfortunately, it’s a soft-sided case. It’s falling apart at the seams, and must be replaced. It’s also not particularly convenient when I’m trying to power pedals for several different instruments. And then there’s this…

Notice the nasty discoloration of the resistor and its attached wiring? I have no idea when, how, or why this happened. I guess that’s a good thing, in that it didn’t cause a big enough fire to get my attention.

So I’m going to re-house it inside a tuna can. Actually, I guess it was crabmeat. Our friends in the Real World might call this a tin, but it seems to be made of steel. At least, it sticks to magnets.

I’m also going to replace that resistor. Which leads to the big question I have:

What is a good value for this resistor?

I just can’t get my head around the color codes. I’m never sure what color they think I’m looking at… Anyway, I’m pretty sure that - in this case - the colors I’m seeing are not the ones that were there when it left the factory. I snipped it out of circuit and it metered out at 10 Ohms. Yes, no multiplier.

The BAC you can see in the picture says it’s 6800uF 16V. My meter only reads up to 4000uF, so it is not helping me. I thought I might devise a test where I put it in Ohm mode, time the thing as it rises, and use the slope of that line to determine the capacitance… It’s a big slow cap and I don’t have the time or patience for that.

I couldn’t get well-focused pics of the rest of the layout, but there are 6 x 1/8” TS phone jacks. Close to each of these is a tiny little cap between the + and – busses (the top and bottom of the PCB). My wife said she thought they said “104”, and my meter says 100nF. I’ll have to tell her she was right.

The basic wiring is like so:

I’m ignoring the LED and it’s current limiting resistor, since they’re not going to make the trip.

And then there’s the component - labeled C6 - in the place where one would expect to find a capacitor beside one of the output jacks. It looks exactly like a diode to me. Also seems to act like one as it presents infinite resistance in one direction, and relatively little in the other. The print on it is too small to read. I tried to shoot it with my digital camera and apply forensic image enhancement techniques. Failed.

BTW – I have no idea what kind of load will eventually be applied to this circuit. I use mostly Boss pedals, but it could be anywhere from 1 to 5 at any given time, and they could range from basic boost to digital pitch shift and who knows what else. I likewise have no way of predicting the minimum operating voltage for any of these. Digital pedals seem to be pickier. I’d like to get somewhere close to 9V at the outputs. The power supply is rated at 9V DC and somewhere up to 1A (don’t have it close right now, but it’s plenty, or easily replaced).

[newey]

Ash-

I'm assuming that this item is of a vintage where one would not expect to find a schematic anywhere.

If the power supply was/is working fine and you're just rehousing it, why worry about the value of the capacitor? Just reuse it.

That resistor does look sketchy and probably should be replaced, but otherwise, why rewire the rest of it?

As far as the value of the resistor, if you measured 10Ω that's probably what it is. It would seem unlikely that, if it was bad, it would fail to a different resistance value- more probably you'd get either no resistance or infinite resistance if it was fried.

I can't make much of the color code from your photo, either, and I'm certainly no expert on this stuff. But it looks to me as if it could be brown/black/black/gold, which would indicate 10Ω. But, again, don't take my word for it.

[sumgai]

ash,

That 10Ω resistor is known as an 'in-rush current limiter". You'll find that most power supply filters have one of these. More or less, it's to protect the capacitors from the large current surge as they initially draw the full amount of voltage/current being sent down the line. Even in low voltage and low current supplies, this initial surge can be damaging to components, if care isn't taken to limit it. Of course, the trick is to not limit the available current too much during normal operation (after the initial surge), thus the seemingly low resistance value.

I'd be happy to discuss this more fully, should you so desire. But at the moment, I think you're on the right track, and therefore I don't wanna cloud you up with a bunch of rigamorale, unless you want it. ;D

HTH

[JohnH]

Ash - you described the set up for smoothing and distribution to the outlets, but what actually provides the dc input. Is it a regulated or unregulated supply?

[ashcatlt]

John, I don't have an answer to that question. It's a wallwart. I know it's 9VDC, with a Boss type positive barrel adapter. It's currently in the scary basement where my band is practicing, powering my HyperFuzz (which I use exclusively in Boost mode). I was there today, but we jammed acoustic so it was out of sight and out of mind.

Anyway, I'm not really looking to "improve" this power supply, except maybe a little shielding by putting it in a metal box*. Just want to replicate it.

So, can it really be this easy? It's just a 10 Ohm resistor, and is working fine? I'm a little suspicious of it. It's burned at least once before, from the looks of things. If I was to replace it, what would be a safe power rating? I figure if the power supply is rated for 1A x 9V = 9 W right? Something a bit bigger just to be safe, I guess?


*It has occurred to me that the positive barrel input jack will want to be isolated from the metal box. I am open to suggestions on how to accomplish this. I was thinking of perusing the "grommet section" of a few hardware stores till I find something that fits...

[ashcatlt]

Oh! And what about the "diode"? Could it be providing some sort of regulation, or is it just a reverse polarity protection? Is it necessary? If so, what would be an appropriate replacement.

Things are coming off this board with leads far too short for me to work with.

[ChrisK]

The diode is likely for reverse voltage protection. Use a 1N4002 or higher (I don't think much of the 1N4001's specs).

One of the ways that you can read caps with values larger than your meter is to measure two in series (a concept that is a parallel to reading two resistors in parallel to read one with a value higher than one's meter ranges).

This works best with a cap that is similar to the one over range. If you have a 1,000 uF cap, read it directly. Then read the value of both in series. A 3,300 uF cap is ideal since it's within range and close to the unknown, but use what you have.

If you read 1,000 uF for the reference cap and 872 uF for both in series, the unknown one would be 6,800 uF.

1/series - 1/reference = 1/unknown.


However, since the resistor is tired, the cap likely is too. Remember, the best (most expensive) electrolytic caps rated for 105C have a life rating of only 5,000 hours. At lower temps these can last a much longer time, but the one in your power supply is not likely one of this quality level.

The weakest link in a power supply is the electrolytic caps. They dry out.

[newey]

*It has occurred to me that the positive barrel input jack will want to be isolated from the metal box. I am open to suggestions on how to accomplish this.

My suggestion- mind you, I haven't tried this- would be to drill the hole for the barrel slightly bigger than it needs to be, and wrap a thin strip of electrical tape around the barrel right where it would hit the edge of the hole. Then make up a pair of plastic washers, cut out of an empty milk jug, one for inside and one for the outside of the can. I am assuming this mounts with a hex nut of some sort, true? But, then again, these things are pretty tiny and I'm not sure there's enough length to accomodate 2 washers.

You might also consider masking off the area around the jack, inside and out, and spraying it with some of that plastic coating stuff they sell at craft stores.

There is some precedent for using seafood containers . .



;D ;D

[ashcatlt]

So it works!

It was a fairly simple project, but I'm very proud that I actually finished it. Once I tore the pedalboard itself apart it became kind of time critical. Still have to whip up some sort of bolt/nut mechanism to hold the thing together, but I think I'll have that done over the weekend.

I replaced all of the parts except the input jack and the 10 Ohm resistor. Sort of a funny story on that resistor. Based on my idea above that it would have to handle somewhere around 9W max, I ordered a pair (I like to have spares on hand) of 10 Ohm 15 Watt wirewond power resistors. These were delivered wrapped in bubbles and affixed with a sticker proclaiming "FRAGILE - Handle with Care". This, of course, makes them unacceptable for this application. They're also HUGE, which doesn't help either.

I was thinking for a while that I'd ought to strap a large resistor across the BAC, to help drain it down after things are disconnected. It occured to me, though, that I could accomplish the same thing by simply disconnecting the wallwart input a minute or two before I disconnect any pedals. They should suck up that current pretty quick.

Also, I think I've got an answer for the question JohnH asked re: the wart itself. Now that I'm looking at it, it says 9VDC 500mA. It says nothing about being regulated. I suspect it's not much more than a transformer. With nothing but my meter as load it was reading ~13.5VDC output last night, and something like 12.8VDC today. I think this is a fluctuation in my home's AC line.

Since my studio is all torn apart right now, I did a very unscientific test here in the living room today. Plugged up my SansAmp and my Big Muff (to which I just added a filtered and polarity protected DC input jack) and they sounded fine. There wasn't enough noise or anything to make me think there was a problem.

Thanks guys. I'll post pictures here shortly.

[D2o]

Cool, ash.

With nothing but my meter as load it was reading ~13.5VDC output last night, and something like 12.8VDC today. I suspect this is a fluctuation in my home's AC line.

Sounds reasonable.

I am assuming you mean fluctuation in the power grid, as a whole - not just your home.

[ashcatlt]

Just noticed I never actually posted pictures of the Tuna Can.



Sorry for the wait!

[jcgss77]

LOVE it!!!!!