Boss ACA & PSA – what gives?

Most of the Boss pedals require the normal PSA adapter, while some has a sticker that says ”ACA” on them. What’s the difference, and how can I make them work in my setup? To answer these questions and a few more, let’s start at the beginning.

History

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 about 12 volts DC from the adapter, and then used 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 even though some seem to have been labeled 9vDC, they did put out roughly 12 volts DC at a normal 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 unregulated ACA and regulated 9vDC PSA adapters – and the pedals they powered – lived side by side. As the older (ACA) designs were discontinued, the newer replacements were of course PSA powered. But some of the old designs refused to die, and in august 1997 Boss made all the pedals compatible with the PSA adapter. In order to make the ACA pedals that were 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?

So, all 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, Boss still sells the ACA adapter, labelled 9vDC (rather than the 12vDC stated on at least some of the older ones). But that doesn’t mean it puts out 9vDC. This is due to how unregulated power supplies work (they start high and drop the voltage as the load increases) and some list the voltage at max load, and others at a nominal load. Anyway, at the low loads generated by these pedals, the voltage will be a healthy 12 or so volts.

However, if you want to avoid having to buy a separate adapter (who doesn’t?), you can modify your pedal the same way Boss did – that’s what I do, and here’s how you can do it too. But there is another way to make it work with a 9vDC supply:

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 it says ”PSA” on the bottom (or on the sticker next to the adapter jack), 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, Joyo JP-02, 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.
  • Unless the pedal is super rare and to be kept in collector’s condition, I always recommend modifying it to PSA specs. Heck, I do that even if it’s a rare pedal, as long as it’s something I want to actually use. You can always save the components and put them back if need be. :)

Easy, eh? :)