#1
Recently I've discovered something that baffles me. At my Church building, we have two wall ports for monitors. Connected to that, we have a 2 output power amp that drives the passive monitors connected to those ports. We have one of the wall ports connected to a daisy chain of passive monitors, like loudspeakers. Connected to the other port, though, is a set of headphones for the drummer. And it's been working like this for some time.

The drummer's headphones haven't ever busted from too hot a signal, and they are always at reasonable listening volume (as long as the board mix is good, of course). Shouldn't such a hot signal that is able to move loudspeaker monitors tear through headphones? Wouldn't the power amp be too strained? What ever happened to ohms?

I haven't been able to check any of the ratings on the power amp unit. But I would think the headphones would have been fried by now. The same port that is connected to the headphones is also connected to an active monitor, chained somehow. I don't get it. I know solid state amps can handle ohm mismatches up, but I don't know about wattages. Tube amps hate ohm mismatches, and a 20w speaker taking 100w is no fun either. Do solid state amps only send enough power to what the speaker load requires?
Last edited by Will Lane at Jun 25, 2015,
#2
Not enough data, sorry.

A couple of things though. As you've noted, it's been working fine for a long time, so it clearly CAN work. Just because it appears to be contrary to what you understand doesn't make it so. I expect there's more at work here than you see. I suspect, based on what you've told us, that one output is powered and one is line level or thereabouts. What make and model is the amp in question?
“Ignorance more frequently begets confidence than does knowledge.”
Charles Darwin
#3
Solid state power amps produce a fraction of their rate power based on the ratio between the minimum load impedance stated on the amp and the input impedance on your device.

Ideally, if your power amp is rated at 100w @ 2ohm, it will produce 50w when presented with a load of 4ohm and 25w when presented with a load of 8ohm.

Some headphones have quite high input impedances - akg k240 (first gen) have a 600ohm input impedance, which would mean around 1/3rd of a watt, at full power.

I'm guessing these headphones simply have a pretty high input impedance, and the power amp is veeery unlikely to employ tubes.
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