Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Effects => Topic started by: dude on November 05, 2019, 11:23:03 am
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Does it make much difference if the DC voltage out is higher than the normal 9 vdc? Almost all of the Wall-Warts I have say 9 vdc out @500ma but the actual voltage is 12 to 14 vdc out. Pedals seem to work fine at the higher voltage, is this higher voltage anything to worry about down the road. I've lower the dc out voltage with a resistor in some warts, but most the case is molded closed.
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In my experience the wall warts using an actual transformer inside are almost always producing more than their rating. The switchmode units regulate very stiffly, the transformers rely on just the low resistance of the windings. Chances are your pedals and the majority of the electronics these things are made for are not that picky about supply voltage and are happy from 9 to maybe 15 volts.
You may find that the output voltage drops to 10 or so once the pedal is connected and the supply has some load.
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Wallwarts contain a transformer. Transformers have poor voltage regulation. That means that when a load is attached, voltage drops. Physically smaller transformers have worse voltage regulation. So a wallwart tranny may need to output 14V unloaded to yield 9V under load. 9V @ 500mA means the wallwart is rated to output 9V into a 500mA load.
Many 9V guitar pedals draw only 20mA or less. Hence, if attached to a 500mA wallwart, it will be receiving over-voltage. Some pedals are oversensitive to this.
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Never thought about the transformer inside the wall-wart, an unloaded amplifier PT will be put out much higher voltage then when loaded. So, it makes a lot of sense an unloaded wall-wart would do the same, thanks jj. As far as the mA rating giving over-voltage to some pedals, when you say over-voltage do you mean more than 9vdc?. I thought the mA rating on wall-warts was what it's transformer can put out at max. draw. School me on what the mA rating is all about, please excuse my ignorance on this subject.
EDIT: The more I think about it, the mA rating is simply the the draw of current of the wall-wart, 500mA's is half an amp. So the transformer is capable of up to current draw of half an amp. A fuse box circuit breaker is say 15 amps, 14 gauge wire is capable of 15 amps current draw before the wire would start to melt, hence protected by that 15 amp circuit breaker. That wall wart is only going to allow a current draw of 500mA's, I can't see any guitar pedal drawing that much current, watts divide by volts = amps. So amps X volts = watts.
1/2 (amp) x 15 (volts) = 7.5 watts...?
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The wallwart does not draw current from the wall outlet, except for the small "vampire" current flowing through its primary coil. And, if it's a DC wallwart, AC ripple bleeding to ground through its internal filter caps (if any).
Rather, it's the load (here a pedal) that draws current from the wall outlet, through the wallwart. A wallwart that's rated for 9V @ 500mA means these things:
1. It can safely handle up to 500mA of current being drawn through it. If you exceed that, it may fail.
2. At 500mA it will deliver 9V. Unstated: a) at less than 500 mA it will deliver more than 9V; b) at more than 500mA it will deliver less than 9V.
These specs apply for all trannies. However a DC wallwart may have more components, such as diodes, caps, & internal leads which will have their own current limitations.
If you feed a 9V device 10V, that's over-voltage.
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Adding to my Reply #4.
"That wall wart is only going to allow a current draw of 500mA's"
No. The wallwart has no control over current draw. Many 9V pedals draw about 20mA. So, the 9V 500mA wallwart can power 25 of them.
But some pedals draw 500mA, so that wallwart can power only one of them. If you hookup 2X 500mA pedals to that wallwart, you will burn-out the wallwart.
There is the Law of Conservation of Energy. The energy In must = the energy Out; and vice-versa, minus real world losses. But let's ignore losses. For electricity, energy or power is stated in Watts. 9V @ 500mA = 4.5W, on the secondary (output) side of the wallwart. So there must be 4.5W on the primary side. 4.5W = 120VAC X I (current in Amps). I = 4.5 / 120 = .0375A or 37.5mA. So, the primary side of the wallwart draws 37.5mA; but the secondary side can tolerate up to a 500mA draw at the stepped down voltage of 9V, due to the said Law.
Remember current kills and voltage drives the current! For ease of math, let's say a 9V pedal draws 9mA, or .009A. Per Ohm's Law 9V = R X .009A; R= 1000 Ohms. IOW, the pedal is the equivalent of a 1000 Ohm resistor. What happens if we put 18V into the pedal: 18 = 1000 X I. Due to the over-voltage the pedal is now drawing 18mA. That will likely burn-out the pedal or some part(s) of it.
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I like pedals because you can build 8 to 10 of them for the money you would spend building an amplifier. I am not a fan of perfboard and the thousands of perfboard effects layouts available. I don't have a high success rate with them and I have half a drawer full of rejects waiting to be scavenged for needed parts. I prefer using a proven design offered in a PCB. Not getting paid to plug but I like General Guitar Gadgets and Aion Electronics.
I don't doubt that some less expensive, 'foreign born' pedals may not handle 15 volts from a Wart, but here is my base of knowledge after studying and building dozens of effects boards;
1. Warts other than Boss or some other copy cats are way to noisy. Buy one of the multi-outlet pedal power blocks and ease your mind about hurting your pedals.
2. The major component in most all pedals that is sensitive to 15 v rather than 9 v are the filter capacitors. If the pedal is inexpensive open it up and look for under-rated voltage on the caps. i.e. you don't want a 20uf cap rated at 15 v if your power pod is making 18 v. Nuff said
3. Many of the seemingly most experienced pedal designers/builders suggest 18-24 volts for some of the effects they put up on the pedal blogs. The only warning I have seen them suggest is make sure you use filter caps with high enough voltage ratings.
cheers amigos
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Not all wall warts and DC transformers are made the same. And not all are made to be used for amplifying the signal therefore why some are plain noisy. Just because you see one rated 9Vdc and a positive tip doesn't mean you're okay and will have no issues if you decide to use it. Many are used to power kids' toys, etc. Boss and others have regulation, isolation, noise reduction, proper grounding, etc. technical aspects which makes for a quiet and reliable unit.
Many pedals' circuit power demands are much less than the LED's that are lit up in them as indicator lights.
OJ, we should get you a small tipped iron, small core solder wire, and magnification glasses - then you'll be perf pro (as long as you can keep your hands steady)?! :)