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Originally Posted by adanac061 /img/forum/go_quote.gif
What I don't understand is that an electrical engineer once told me that with some amps, the higher the ohms the easier they are to drive???
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Oh geeze. One of these days we need a sticky thread on ohm's law, efficiency, sensitivity, and what that means to you.
Lets start with the basic presumption of Ohm's Law: You Get Nothing For Free.
When you create an electrical potential across a load, the impedance of that load - how much the load impedes the flow of electricity - defines how much electricity is drawn from the source.
Thus, a low-impedance load draws more current from the amplifier than a high-impedance load. This is unavoidable, force-of-nature, 'canna break the laws of physics cap'n' stuff.
If the source doesn't have a lot of current sourcing capability, this can have a number of ill effects.
Large trasients - like a bass note - can fall flat before they reach their peak if the source runs out of current before it can finish servicing the pulse.
Increased current draw on some kinds of amplifiers can slow down their slew rate, causing the amplifier to slur the sound slightly.
The upside is that you don't need many volts to make the air move around the driver.
A high impedance load presents a very easy load to the source, but because less current is being drawn across it, if we presume that all other properties of the driver are the same, it requires more volts to move the air around the driver.
Your home stereo system has volts to spare. If anything the headphone jack was an exercise in limiting the volts on the output. It's not uncommon for a home stereo type system to be able to swing about 80 volts.
Your computer's sound card can probably only swing about 5 volts. Often as little as three.
Portable devices can generally swing three or less.
So, if you plug 600 ohm DT-990's into your iPod, it's not going to make much noise. If you were going to get an amp, you'd want one that can swing plenty of volts (say, two 9v batteries) but it doesn't need much current - a simple cmoy will do just fine.
If you plug your 32-ohm grados into your ipod, they'll get plenty loud, but they may sound slurred and distorted at high volume due to the difficult load they present to the little headphone amp.
Curiously enough, the ipod shuffle will handle it better than a full size ipod, since it appears to have a switch-mode headphone amp rather than a linear amp. If you were to look at the output on an oscilliscope, the full-size ipod will start out to service a large transient nice and clear, and then just give up. The shuffle makes it all the way across, but there's little squigglies on the top of the wave 'cause the output transistors are switching on and off at frequencies much higher than you can hear.
