The Sub-$200 Portable Amps Shootout – 12 (+10) amps compared - Page 7

50 ohm isn't exactly low impedance, but it is very high either. Unless this headphone is ultra-sensitive, I doubt it will be affected by hiss much.

Basically it goes like this: The higher the impedance of a headphone goes, the more voltage it needs; the lower the impedance of a headphone goes, the more current it needs. Yes, they are opposite of each other. So if everything else is equal, an 8 ohm headphone needs a lot more current than a 80 ohm headphone, while the 80 ohm headphone needs a lot more voltage than an 8 ohm headphone. A good amp should be able to supply both adequately.

For all intended purpose, gain = voltage. Actual the full term should be 'voltage gain'. On most solid state amp, how much current is generally limited by the opamp internal design. Take O2 for an example, the output stage has two opamp (in parallel) to literally double its current output. Sometime, you will see a buffer stage as well (no voltage gain, purely for current output). Some opamp (say AD8397) has a buffer stage built into the chip to output higher current. The typical case is, as long as the load doesn't need more current that what the amp can supply, the effect of current is much less audible to the ears. Voltage on the other hand has much more obvious 'side effect' when you set the gain too high. In fact, most opamp perform best when there is no gain (gain factor of 1). Once you increase the gain, you are actually amplifying the input signal and thus causing noise/distortion (because there is no such thing as a 'perfect amplification'). The higher the gain, the worst it goes. At some point, you will go outside of the chip's spec and noise becomes a problem. For most sensitive IEM, actually an 0~3dB gain is more than enough when you are dealing with iPod's level line-out signal (which is only 0.5Vrms, far shorter from the standard 2Vrms).

Unlike current, you can control the voltage using the volume knob. With low impedance headphone, the problems are two folds: first, if the gain on the amp is too high, a slight turn on volume knob can make the headphone too loud. Since many volume knob also have imbalance issue on very low position, you will also get imbalance issue as well. That's the first sign your amp's gain is too high for your headphone. Often, it is also a sign your headphone doesn't need any voltage gain at all. In cases like this, digital volume control (which doesn't suffer from imbalanced) and zero voltage gain are preferred, because it is really the current you are interested at this point. The second issue is, high gain brings higher noise level, which comes in the form of hiss. That can be easily picked up by a low impedance, high sensitivity headphone, like custom IEM.
Edited by ClieOS - 9/9/12 at 9:49am

By amp section alone, I won't say E11 is better than E17. It is more like 'different'.