How is hissed caused on IEMs?  

First of all, the IEMs by themselves do not hiss. Any audible noise thus has to come from whatever you plug them into. (Seems trivial but does not seem to be common knowledge.)

The level of hiss observed thus is governed by:

• level and spectrum of source noise
• combined frequency response of source output impedance + cans
• sensitivity of cans

I'd explain your impression that the triple.fis "don't hiss" as follows:

1. Your sources/amps with very low output impedance have low noise levels, and
2. Your noisier sources/amps have higher output impedance.

If you look at the UEs' impedance response, you'll find that it makes a first-rate rollercoaster ride, with a maximum of about 65 ohms around 1 kHz and a minimum of about 7 ohms around 10 kHz. It takes very little output resistance to warp frequency response significantly (as output resistance and driver impedance form a complex voltage divider), in fact for a deviation of less than 1 dB from an ideal zero ohm output you want less than 1 ohm of output resistance. These are fussy beasts. Here's what happens on your D10, for example. (Using my lil' spreadsheet, this works out to a guesstimated 5.6 ohm series resistor.)

In practice this means that a few ohms of output resistance are quite sufficient to make for fairly dark to muffled sound. Moreover, impedance (and thus frequency response) minimum occurs right where hiss tends to be the most objectionable.

Noise spectra on sources/amps tend to be largely white, with a significant 1/f (excess noise) component only on CMOS ICs (where it can extend up to about 1 kHz). My old Cowon G3 had quite a bunch of plainly audible excess noise on SE420s.

So what's the part you don't understand?

Headphone efficiency itself is governed by two basic influences:

1. Driver efficiency. Here the same basic principles as for any other such transducer apply.
2. Coupling. As a rule of thumb, the less the enclosed volume, the louder the same driver will play. IEM > supraaural > circumaural >> speakers.

Multi-driver setups tend to lose a bit of efficiency in the crossovers.

As you can see, there's good reason for BA IEMs being very sensitive - balanced armature drivers are fairly efficient affairs by themselves (if with a somewhat restricted frequency range), and stuff that sits right at the entrance of your ear canal will play very loud anyway. Now you can probably imagine that it is almost impossible to get very high resistance on a tiny voice coil even when using hair-thin wire, so resulting impedance won't be too high either. So your high efficiency also translates into high sensitivity. Hence a good match with things of limited output amplitude and power.

Your question wasn't very clear, but I think you are asking what hiss is?  IEMs don't have hiss, they're simply more sensitive due to their higher efficiency.  The level of noise produced by electronics can depend on the load, but to a first order, the main sources are detailed quite clearly here:

http://en.wikipedia.org/wiki/Electronic_noise

I don't know your specific IEMs, but in general we are more sensitive to noise in the midrange and up.  An IEM that sounds "dark" or rolled-off will have less perceived noise than one that is hot because it will tend to filter the perceived level of high-frequency noise.

Quote:

Originally Posted by High_Q 

IIf you can't explain what circuit topology prevent hiss in sensitive iems, you don't understand hiss.

If you understand it so well then you obviously don't have a question, and should be explaining.  This is a waste of time.

Well he kinda does have a question but doesn't seem to be interested in the answer (?), heh.

Explain what.

Look at the specs of the components, the gain structure of the amp, quality of the parts/pcb/shielding etc.

They all contribute to the final noise floor at the output, see mrspeaker's response above for types of noise and their cause.

Quote:

Originally Posted by High_Q 

I'm sure some one understands why hiss is caused on amps for sensitive IEMs can explain it.  Below is not an explaination.  I don't think you really know.  If you knew, you can explain it the say this is how is caused, this part has significance over other, circuit configurations, etc.  I'm thinking more significan't contributions are from impedance and sensitivity of the phones.  But, don't know exactly how it works. 

LOL quote. Waste of time indeed!

Quote:

Originally Posted by High_Q 

^If you can't explain, please don't waste my thread space. What you typed above is pretty much waste time and space, and gets in the way of answering my question.  Please do not post response to my thread anymore. 

LOL x2, or what?