Once you build the Stella amp, you have to power it. I've been told this page was long winded, so here is the summary:
Just use 3 or 4 AA batteries. Can be rechargables or non-rechargables.
I sell a great battery pack in the store. But if you don't want to use AA batteries, read on...
Consider a polarity diode
The Stella amp has no polarity diode in the power path. This was an intentional decision, since I wanted an amplifier that I could run at 3.3v for a synth project I am working on, and at low voltages like that, one wants maximum headroom.
Yet at the same time it can be easy to reverse the battery terminals by accident while you are changing the battery. A solution is to wire in your own diode, or put in a power switch and make sure you cut the power before changing batteries. Whatever you do, do not just solder a 9 volt battery connector to the board, if you do that you are asking for trouble. Speaking of 9 volt batteries....
A 9 volt battery
Using a 9 volt battery will work but the battery won't last very long at loud volumes. At maximum volume you'll be pulling ~ 800ma, and a 9v only has a capacity of about 450mah. Of course it's not a linear relationship, as you draw the battery down the supply voltage will be less, so you'll be pulling less current, it won't be pulling 800ma the whole time. But as a result the amp will keep getting quieter and quieter as the voltage draws down. And the distortion characteristics will change.
If you use the Stella amp as a headphone amplifier or stompbox then you'll get more battery life.
Anything at 12 volts or over
If your power source is above 12 volts or even gets near 12 volts you want to replace your op amp! The op amp that comes with the kit is only good up to 12 volts, at an absolute maximum. Go to Radio Shack and get a TL-082 and replace the op amp that comes with the kit. (You can also use any other dual op amp that comes in a standard pdip pinout.)
Once you've done that, you're good up to 16 volts.
Sealed lead-acid batteries
Rule 1: Never leave the Stella amp connected to your battery while it is charging.
That said, if you have a 6 volt SLA battery, you're probably fine with using the kit as is. A 12 volt sealed lead acid battery is something else entirely. A fully charged 12 volt SLA might actually measure 13 volts -- too much! Replace the op amp with something that can take a higher voltage range, like a TL-082 from Radio Shack. With a TL-082 you're good until 16 volts.
A 12 volt car system
Again, replace the op amp with one that can tolerate the higher voltages. The electrical system of a car is very complex and can produce transient voltages at hundreds of volts for a few microseconds, among other problems. I HIGHLY recommend some type of voltage regulation if you're going to plug it directly into a car cigarette lighter.
If I was building an amp that I wanted to power from my car stereo, I would try to find a car lighter plug that already did some voltage regulation for me, rather than a dumb cord with just a plug and maybe a fuse. In fact I would probably buy a cheap cell phone charger that can put out an amp at 5 volts, and go from there.
A wall adapter
Wall adapters (or wall warts as they are sometimes called) plug into the wall and output a DC voltage. Actually, some of them put out an AC voltage, but we don't need to concern ourselves with those. I'm assuming you've got a wall adapter and you are wondering, is it safe to use this to power my Stella amp? The answer is, it depends on the wall adapter. First, read it and see if it outputs an AC voltage instead of a DC one. If the output is AC the wall adapter is the wrong kind.
After you know you have a DC wall adapter, you need to figure out if you have one based on a transformer, or a switching regulator.
A wall adapter with a transformer
Transformer-based wall adapters use a big, heavy transformer with a minimum of support components, probably just a bridge rectifier and a capacitor or two to tame the ripple a little. The important thing to note is that these wall adapters have no voltage regulation in them. There are three dangers with using this kind of wall adapter straight up with no voltage regulation.
First, although it may say that the output voltage is 9 volts, it is actually a guarantee that the adapter will provide 9 volts at whatever the amperage rating is. So if your 9 volt adapter is rated for 1 amp, it's actually guaranteeing 9 volts at 1 amp. Of course, as you load an unregulated voltage source, the voltage drops. As you unload it, the voltage increases. The upside is that a wall adapter labeled 9 volts at 1 amp might actually put out MUCH more voltage when it is not drawing the full 1 amp.
Second, let's say you measure the unloaded voltage of the adapter. It comes to 11 volts. Sweet, that's well within the 12 volt maximum of the Stella amp, and when the voltage source is under load it will be even less than that. You hook it up, everything sounds great. You take it over to your buddy's house and he is is so impressed he decides to buy a kit for himself and pay you to make it for him. Yay!
In a couple months you go on a road trip to your cousin's house in another state. You plug in your Stella amp to show him and it works great for about 10 minutes but then POOF out comes the magic smoke. What happened?
A transformer is not a smart device. It takes in a voltage and spits out a voltage on the other side, which is reduced by the built in ratio of the transformer. At your house, unbeknownst to you, your AC voltage is 120 volts RMS. When you measured your unloaded transformer you got 11 volts. But at your cousins house his AC voltage is 138 volts RMS. If you do the math your transformer is now putting out 12.65 volts unloaded, which is over the absolute maximum the op amp can take. Ooops, you fried your op amp.
I know you're thinking "138 volts? Seriously? But I thought AC voltage was 110/115/120/125 volts!?" Let me disabuse you of that notion. I used to live in a house where our AC power was at 138 volts (you thought I made that number up, didn't you?) and it was fun times. At 138 volts AC, light bulbs burn brighter, and burn out quicker. The oven runs hotter. Computer power supplies age more rapidly. So this is something you have to keep in mind.
But third, and even more importantly, with a wall wart like this you're not protected against sudden voltage spikes. I have a friend who lives on the highest inhabited hill in his state. Every few years the amount of money he has to spend to replace his electronic equipment due to lightning strikes is staggering. Yes he uses surge suppressors, but many surge suppressors only suppress the first surge. The second or third one comes through and BAM the computer is a goner.
You may not be like my friend living on a high hill in thunderstorm country. But nonetheless you get a fair share of voltage spikes coming in on your AC line. It's a hazard of AC power. You might think that's what you have a surge suppressor for, but the vast majority of voltage spikes are not going to be high enough to be curbed by a standard surge suppressor. These spikes are very brief, maybe not even enough to cause the lights to flicker. But because there is no voltage regulator between your wall adapter and your Stella amp, you're leaving yourself wide open for damage to take place. Enough of these little spikes can cause the whole circuit to become non-functional.
If I sound a little strident about this, it's because I destroyed a fair amount of electronics projects early in my making career before my uncle took me aside and explained why everything I did (eventually) went up in smoke. Learn from my mistakes!
So why would you want to use one of these wall adapters anyway? Well, they're cheap and easy to find. You probably have a box full somewhere in your house. And all you have to do to use them properly is make a voltage regulator circuit and put it between the Stella amp and the adapter. Ladyada has a great tutorial on how to build a simple adjustable regulator with an LM317. You can get all of the parts for this at Radio Shack. Of course instead of powering it with a 9 volt like she has there, you would power it with your wall adapter. You can get also get power jacks in common sizes at Radio Shack as well, to fit your choice of transformer.
Another reason to put a regulator on your wall adapter is hum. Sometimes in a fit of cost cutting, a company will get the absolute cheapest wall adapter possible. Many of these super cheap adapters have inadequate filter capacitors (sometimes they don't have ANY filter capacitors). The product engineers will then design additional power filtering and smoothing circuitry in the product itself, to deal with the crummy "DC" power.
The problem is if you get one of these duds of a wall adapter you'll be coupling 60 (or 120) Hz hum directly into your amp. If you build a proper power supply you can avoid that.
A wall adapter without a transformer.
Some wall adapters have what is called a switching power supply instead of a transformer. Switching power supplies are probably going to be newer, light weight, and smaller than a transformer based wall adapter. They also deliver regulated power, so if the wall adapter says 9 volts on it, then the output is going to be 9 volts. These adapters are great, I love them.
A USB port on my computer (be careful!)
The USB specification has specs for how much capacitive load you can put on the USB line. Sadly, the Stella amp is a much bigger capacitive load than USB can safely handle. If you try it, it will stress out your USB ports on your computer or powered hub! DON'T DO IT. Dealing with a fried USB port on a computer is no fun at all! You can destroy the entire USB system on your motherboard!
If you do want to power your amp from your USB ports, there is a better way. Get this USB lithium-ion battery charger from Adafruit, and one of their rechargable lithium-ion batteries. Now, when you play your amp you are actually powering it from the battery. The charger takes care of drawing power from the USB port in a safe way.
This makes a great portable vacation amp! You don't even need a computer, you can use a USB wall charger to charge your amp! No hunting for batteries and no more lugging around a great big amp and having to search for a power outlet. (Of course it won't be very loud. At 4.2 volts the Stella amp won't have enough headroom to overpower a crowd or anything like that. But when I'm on vacation I rarely play anything for a ton of people.)