Read History to learn more:
In 1978, Boss launched the Compact pedal line, and all the early products were small current consumers. The OD-1 draws about 4mA, and the CE-2 (introduced in 1979) draws about 8mA. For some reason Boss decided to run their adapters at 12 volts, and then use a resistor (usually 470 ohm) and a silicon diode inside the pedal to reduce the incoming voltage. The adapter was unregulated (as the current draw grows, the output voltage drops) and put out 12 volts DC at a nominal load.

As the pedal range grew, the current consumption of each new pedal grew as well. The larger current draw of the new pedals didn €™t fit the unregulated adapter system very weel, so Boss decided to start designing the new pedals to accept a regulated power source instead. The trick with regulation is that the voltage stays constant from no load all the way up to the max capacity of the regulator (in the case of the PSA, 200mA).

For years, the 12 volt ACA and 9 volt PSA adapters lived side by side €“ until august 1997, when Boss made all their pedals compatible with the PSA adapter. In order to make the ACA pedals still in production compatible, the resistor and diode were simply removed and replaced with jumpers. These pedals all carry a €PSA € sticker.

Which power supply do I need?
Boss compact pedals made after mid 1997 use the PSA (9vDC, regulated) power supply. But what about the ACA ones made before that (the ones that still have the ACA sticker)? Do I need to buy the ACA adapter for those?

Well, you can €™t €¦ for some inexplicable reason, Boss still sells the ACA adapter, but now it is 9 volts instead of 12 €¦ so we can €™t use it. You can of course modify your pedal the same way Boss did

The daisy chain thing
When powering a Boss ACA (old-style) pedal from a 9v PSA adapter, the LED barely lights up. This is normal and expected behaviour, since the 9 volts fed to the pedal really is too little €“ it wants 12 volts, remember? But when the power is being provided via a daisy-chain, with other PSA pedals in the chain and signal cables connecting them, the LED suddenly lights up as normal €¦ What happens is that all the pedals in the daisy chain share a common ground, which bypasses the voltage limiting components in the ACA pedal(s), causing the pedal to receive the full 9 volts. This trick works with any power supply where the outputs share a common ground €“ daisy chain setups, DC Brick etc.

Obviously, this won €™t work with an isolated power supply like the Voodoo Lab Pedal Power 2, since the isolated outputs prevent common grounds from forming. This is why that power supply has a 12 volt/ACA setting.

Conclusions
If your Padel says €PSA € on the bottom (or on the sticker next to the adapter power jack for round barrel connector), use a 9 volt regulated supply.
If it says €ACA € on the bottom or on the sticker, it will need 12 volts on the adapter jack, rather than 9. Unless you are using a daisy chain or power supply with common ground for all outputs (DC Brick, Cioks, PowerPad etc). In that case, the pedal will work with 9 volts, as long as there €™s a €˜proper €™ 9 volt pedal in the chain with it. And provided that there are signal cables connecting the pedals as well.

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