The following is unduly long -- especially as I want a short answer -- and
is partly an excuse for bragging about an expensive luxury I own. But the
question is serious, and the answer likely useful to many in this group.

The most-common external power supply (wall wart, if you like) ever made is
likely the Sony AC-E455D. This little black cube pumped out 4.5V at 500mA,
to power hundreds of thousands of Discmans and other portable devices.
They're so common that you can easily find them on eBay for a few bucks
each.

Jump to STAX. A few years back STAX introduced the SR-001 electrostatic
earphone (not headphone) system. These little gems are driven by a tiny
battery-operated amplifier/energizer that puts out 580V DC polarizing
voltage (!!!) and about 300V of AC drive signal. They aren't cheap, but the
sound justifies the price -- if you want really good sound when listening to
portable equipment.

http://www.audioreview.com/mfr/stax/...3_2750crx.aspx

Now, the amplifier/energizer is powered by two AA cells. But there's a jack
on the back for 4.5V DC -- whose diameter and polarity just happen to match
the AC-E455D's. Though STAX sells its own terribly overpriced power supply,
one can't help but think that STAX was deliberately making the SR-001
compatible with a very common AC adapter.

Hang on, we're getting there...

Sony also produced a switching version of the AC-E455D, which operates from
100V to 240V, and puts out 1000mA. Two years ago I bought a dozen of these
on eBay for a buck apiece. As they were thinner and lighter than the
"linear" version, they became the power supplies I carried with my Discman
and STAX SR-001 system.

The interesting thing (finally!) is that most of the switching supplies
won't power the STAX energizer. The energizer's red "low voltage" light
comes on, the energizer tries to turn on (briefly flashing the green "good
voltage" light), but then drops back, and the cycle repeats, producing a
"thump-thump-thump" in the earphones.

Only one of the switching supplies I have will power the earphones -- and it
often has to struggle a bit to get going.

It's not an issue of voltage or current capacity, per se. What, exactly, is
going on?


--
"We already know the answers -- we just haven't asked the right
questions." -- Edwin Land