The ACA is an unregulated 9V or 12V adapter. For a long time both the ACA and the regulated PSA adapter was available but in the later part of the 90s the ACA adapter was discontinued as all Boss pedals then was designed to run with the PSA adapter.
When a power supply is unregulated, it means that the voltage level will drop as the load is increased. The ACA adapter may give out a full 12 volt when it is powering one or two pedals but if it is hooked up to a long row of pedals the voltage will drop. The ACA adapter is capable of supplying as much current as 250mA but problems keeping the voltage up may occur before the load reaches that level.
The early compact pedals was designed to run on either a 9V DC battery or 12V DC adapter. Because of this the ACA adapter was a 12V adapter. The voltage was reduced to 9V internally by using a 470 Ohm resistor and 1S2473 diode between the minus input on the power jack and ground. The resistor diode pair was later removed and at the same time the ACA adpater was redesigned to output 9V instead.
Powering ACA pedals with a PSA power supply
Powering the older pedals designed for 12V DC input with either a newer ACA or PSA adapter will not work very well. The voltage drop over the resistor and diode will prevent the pedal from getting enough power and its LED will usually only glow faintly. The solution is to use a daisy chain and plug in another pedal designed for the newer ACA or PSA adapter. The lead between the two pedals will short the resistor diode pair and the pedal will receive full power.
Depending on the mains voltage there are 4 were versions of the ACA adapter. ACA-100, ACA-120, ACA-220 and ACA-240. The number denotes the mains voltage that the adpapter should be plugged into. The 9V version of the ACA adapter has a G appended at the end of its name. Pictured is the ACA-120G which is the version sold in the USA.Thanks to Stinkfoot for most of the information above.