universal ac adapter questions

[strangerock]

hello,
i have decided to add dc jacks to some of the pedals i have made.
so i bought a universal ac adapter 500 ma deluxe, 3, 4.5, 6, 7.5, 9, and 12v dc.
after adding the jacks to my green ringer and kay tremolo, neither worked.
after checking the wiring and doing a bit of research i tested them with my
multimeter.on the 9v setting the adapter is giving 14. something volts to my
pedals. i tried a few resistors in each to get them to 9 volts and now they
work.
my questions are:
1 is this a crappy/wrong adapter for my pedals and if i get a new one
will i have to remove the resistors?
2 why when i added a certain resistor to the jack it reads 9 volts but
when i hooked up the jack and resistor to the circuit board it changed the
voltage reading?
thanks alot
chad

[GibsonGM]

Sounds like "loading", chad...a voltage source will read let's say "9V" (or higher) when nothing's on it, but when you add it to a ckt, the ckt pulls current and drops the voltage.  Maybe your circuit pulls so much current that the adapter can't keep up...it then will trade voltage for current, the way a transformer does (in fact, that's what's happening).  Adding the resistor(s), creating a voltage divider, will get you the voltage you want, but there may not be enough "circulating current" there to power up your pedal.   The designer anticipated a load around 500mA, this is why you read 14V with nothing on it...it will go around 9V with a 500mA load. 

1)  I don't add adapters to my pedals, I'm a battery guy, but in my humble knowledge I'd say try a higher current-rated adapter...circuits pull current, you're not pushing it in there, so it's safe (to a point).  My idea on the subject would be to install a 9V voltage regulator circuit; then you could use any adapter you want above 9V and below the max rating for the regulator with the correct polarity, provided is puts out enough amperage (say 1A). 

2)  something in your ckt is probably loading down the power supply, so it's more than just the single resistor you're testing with.

Hope this helps, I'm no guru, just my idea on the subject and what I'd try next.   It's neat stuff, isn't it?? Nothing is as stable as we think      Try adding lower value resistors to the PS and measuring the voltage, see what adding a load does to it. 

[aron]

Welcome to the world of unregulated power supplies. That's why most of us either pay more for a regulated power supply or we make our own regulated supplies. Basically an unregulated power supply will usually put out more voltage than you expect. Typically 14-16 volts when set at 9V for example. The good news is that you can connect this to a voltage regulator like the LM317 and have a regulated output.

See:

http://www.national.com/pf/LM/LM317.html

[strangerock]

thank you so much for the help.
i got to say, this forum is absolutely
amazing.
chad

[Mark Hammer]

That it is.

I regularly recommend to folks to make themselves a "power distribution box".  By this, I mean a small plastic box that has one input jack, multiple output jacks, additional regulation inside, and a status LED so they can tell when stuff is actually getting power.  The wallwart plugs into the box, and the user runs male-to-male patch cords from the distribution box to the individual pedals on their pedal board, or multiple daisy-chain boxes.  One reason I recommend this is because unless you are using a single pedal format exclusively (e.g., ALL Boss), it is usually awkard or difficult to make a single daisy-chain cord that will serve all pedals conveniently without slack.  Heck, all it takes is one wah pedal to screw up the neatness of your wiring/power-patching.  Using tailored power-patch cables to run to individual clusters of pedals can make things far more flexible with respect to how you physically arrange your pedals, and neater as well.  The additional regulation for the overall output and/or isolation between output jacks is also helpful for avoiding problems and improves power "quality", even when you DO start with a decent power supply.  You can also use downregulation and feed 12v into the box, providing 12v and 9v outputs.  Another thing I like about the distribution-box idea is that the distribution box can remain on the pedal-board, while the wallwart is situated elsewhere.  Pack-up is as easy as unplugging the wallwart from the distribution box.  Finally, precious few wallwarts have power LEDs.  You always want to be able to tell where the breakdown in power occurs.  Knowing that the main power is out (or not) helps.

As noted, wallwarts, whether single output voltage or "universal", can vary in the amount of regulation they provide onboard.  Heck, a wallwart could probably use two diodes, a 10uf cap and still be labelled "regulated", even though the quasi-DC it provided would yield nothing but hum in any audio circuit.

Unless you are using it to power 20 analog pedals, or a handful of digital pedals (which usually want at least 60ma per pedal), a 500ma supply should not pose any current limitation problems.  That doesn't mean it would remain your be-all and end-all supply, but from the information you've provided, it should be able to meet your needs.