I sent a guy a pedal recently (MXR D+ with a couple of mods) and first he had some humm because he used an unregulated supply.  So he bought a regulated supply and now he's getting a whistle.  

Humm from unregulated I could understand, but why would a whistle appear with a regulated supply??  Maybe he blew something with the unregulated supply?  I can't remember whether I used a 16V or 25V electro cap across the power supply.  Would an 800mA unregulated 9V supply run a high enough voltage to blow a 16V cap with the low (10mA) load of the pedal?

Any suggestions are welcome.

He's probably using one of the switching power supplies like the One Spot, which will induce a whistle in some effects.  A switcher that I have will whistle with my Boss DS-1s, but not with my SD-1s.  

He should get a Danelectro Zero Hum (DA-1) or the Boss PSA-120, which are good old fashioned (heavy) regulated supplies.  Someone posted a while back with a way to get rid of the whistle.  Something like a 100 ohm resistor in series with the power supply before the pedal, and a 100uF capacitor in parallel with the power supply.

Phillip

My guess is that the supply he picked up was based on a DC/DC converter which is like a stripped down switching supply primary and a linear supply secondary.

They're now cheaper to make than good old linear supply, but they sure are noisy. Typical switchers run at ultrasonic frequencies, but these cheaper units don't.

Maybe that's it?

Take care,
-Peter