[Mike Rivers]

Danny T wrote:

The one manual I had sitting around reads about the same as I remember
the others to and it does say not to fall below 100 ohms. So if the
Sony's are only 63, that would mean I could not use them according to
their specs, right?

Well, what are you afraid of if you plug them in? That the
amplifier will fill the room with smoke? That you'll get
cancer of the earlobes? Or the most serious of all - that
you'll void the warranty? Naw, it only means that you'll
get distortion if you turn it up all the way. If you can
stand to listen to it, you can.

I never really looked at headphone impedance before but on the casual
accidental noticing of it found that the impedance of must of them are
rather high.

Neither did anyone else when a headphone jack was connected
to the output of a power amplifier (15 to 250 watts)
isolated with a couple of resistors, and you adjusted the
amplifier's volume control for a comfortable (or
uncomfortable) listening level. It was only when low
voltage/power devices came into our lives that headphone
drive current became a real consideration.

I understand ohm's law but I guess I'm confused about the way they
have the limits based for the amps.

There are hard limits, and there are recommendations.
There's also the consideration of liability if someone
damages his hearing by using headphones that are too loud
when connected to a particular device. Amplifiers generally
are current-limited because sometimes output cables fail and
short-circuit the output. The worst that can happen is that
at one end of the impedance scale, the headphones won't get
very loud at all, and at the other end of the scale, they'll
distort before they get loud enough. So it's more a matter
of customer perception and satisfaction rather than
electrical hazard.

If you need at least 100 ohms -
and for instance, the Behringer powerplay has room for three
headphones per each of the amps in the rack, you'd need at least 300
ohms on each headset or you'd drop to low and start burning the thing.

If it were really designed like that, it would be a bad
design. When you have multiple output jacks from the same
amplifer, there's enough resistance in series between the
amplifier output and the jack so that the amplifier never
sees a lower impedance than that series resistor. This both
protects the amplifier from too much load on its output and
also means that plugging in a second or third set of
headphones doesn't significantly affect the listening volume
of the other headphones.

--
"Today's production equipment is IT based and cannot be
operated without a passing knowledge of computing, although
it seems that it can be operated without a passing knowledge
of audio." - John Watkinson