Hey guys, I have a big problem! I got a great bargain I hope. An Alesis Microverb 3 for € 10. But without a power supply. It takes 9V AC. I still have a 15V AC wall wart. How do I get this to 9V AC now? With DC I would just get a 7809 and be done, but I don’t know anything about AC!
You really just should get a 9VAC wall wart for another 10 euros.
But if you insist on risking starting a fire and/or blowing up you new toy, you could put a resistor in series with your 15VAC wall wart.
You have to determine (guess?) how much current your Microverb needs and then use a resistor that will drop 6V at that current and be able to dissipate that power in heat.
To keep it simple lets assume for example the Microverb need a current of I = 1 amp, the resistor would need to be R = V/I = 6 / 1 = 6 ohms and be able to dissipate the power P = VI = 6 * 1 = 6 watts, that is a big small resistor.
If the actual current pulled by the Microverb is lower than what you estimated it to be, you could fry the Microverb because then the voltage dropped by the resistor would be too low and thus the voltage at the Microverb would still be too high.
If the actual current pulled by the Microverb is higher than what you estimated it to be, you could fry the external resistor because it would exceed the wattage we calculated before, or it could just overheat and set a fire and/or, depending on the design of the voltage regulator inside the Microverb, you could also fry it because the external resistor would drop the voltage too low and the regulator could try to pump it up by drawing more current (yes some switching regulators can overheat if insufficient voltage is provided, unlikely for a wall wart powered gadget, but not impossible).
So, are you feeling lucky?
If so, just throw a 10 ohm, 10 watt resistor (which you probably don’t have) in series with your 15VAC wall wart and hope you didn’t just lose 10 euros, by trying to save 10 euros.
Alternatively, if you have the schematic of the Microverb you might be able to determine if it is OK with 15VAC or a even a DC input or you could determine how to modify it.
In conclusion, as I said in the introduction, you should really just get a 9 VAC wall wart.
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You’ll laugh, I actually have some high load resistors. I bought it when I converted my PC power supply to a laboratory power supply.
But you convinced me. Then I prefer to buy a new one. Then the 15V wall wart is left for the Yusynth modules 
THX a lot !!!
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