Running a 5V AC circuit with a 5V DC supply - why does this work?

[timd]

So I know the perils of running a 9V DC stompbox with a 9V AC supply. This is a well documented disaster. Presently, I am forging a lo fi looper with a digital answering machine which runs on 5V AC - no power supply with unit. For testing purposes, I used a 5.4 VDC  500 MA power supply and it ran fine for the 5 minutes I was messing with it. Can this be run this way with DC for an extended period, or am I flirting with disaster?

[R.G.]

Pretty much all analog-audio and all digital electronics work on DC. If a unit is powered by and AC power source, the first thing inside the power jack is a rectifier/filter that makes DC out of it, and then may process it further.

This unit gets 5.4Vac, which is alternating half-sine waves with an RMS value of 5.4V and a peak value of about +/-7.5V. When you stick a 5Vdc supply in there, it does the same thing - it "rectifies" the DC, which only makes it come out the correct polarity no matter which way the DC is plumbed in. The internal filters "filter" it some more, then the internal regulators, etc. use it just like they'd have used the rectified and filtered AC.

The 5Vce supply is a couple of volts lower than the "5.4Vac" supply would make after rectification, so the insides are running on about 2/3 of the DC voltage they normally get, at least before any internal power supply regulation circuits. It is impossible to say with any accuracy whether this is a problem internally or not if we don't know the circuit details.

I personally would take it as a happy accident and get the right power supply for it as soon as possible. It might be fine. But it might now. Can't say.

[timd]

Thanks RG!