Is there any connection between hiss and output and load impedance?  Or is it just based on headphone sensitivity?  Some phones that have high sensitivity, I hear hiss in certain amps, and another totally black.  What is in the design that would output hiss on one and not on another?  Gain?

It's just electric noise being converted to acoustic noise by the headphones.
 
Higher sensitivity (higher SPL out per mW power in) means you get a louder sound, be it what you want or hiss. Also, if a headphone has a FR relatively tilted towards emphasizing those low-mid treble frequencies the ear is most sensitive to, then it will be easier to hear hiss. Keep in mind that sensitivity is usually given for 1 kHz input. 
 
Lower impedance means that at a given (voltage) level of noise from the electronics, more noise power is delivered to the headphones (P = V^2 / R). Well, if there's some significant source output impedance, some of the noise will be lost in the amp rather than going across the headphones, reducing the level.

So if you use iems that are very sensitive, would you have poor signal to noise out of them than cans with low sensitivity since you can stretch the volume and noise will be small relative to the high voltage level for the cans?
 
I can hear my transformer humming on my Beta 22 amp with my iems, and it's quit with my orthos.

best signal to noise ratio is usually at max volume, so using a sensitive headphone forces you to lower the volume when the noise floor stays the same.
 
now what you're saying sounds wrong, because with the exact same specs(real, not just @1khz) IEM or fullsize headphones would behave the same and ask for the same voltage to get equal loudness. so they would suffer from the same noise.

SilverEars said:

."So if you use iems that are very sensitive, would you have poor signal to noise out of them than cans with low sensitivity since you can stretch the volume and noise will be small relative to the high voltage level for the cans?

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No. The signal-to-noise ratio is a feature of what is going into the 'phones. Sensitive 'phones play the noise loud, but the signal louder. Insensitive phones play the signal quiet, but the noise even quieter. The sensitivity of the 'phones has no impact whatsoever on the actual or perceived SNR.

w

Headphone sensitivity can affect the SNR in practice at matched SPL, because some of the noise is added to the signal after the volume control (in fact, with digital volume control, the noise voltage is basically constant). In other words, at very low volume settings (as required by the most sensitive headphones and IEMs), the amplifier has worse signal to noise ratio.

stv014 said:

Headphone sensitivity can affect the SNR in practice at matched SPL, because some of the noise is added to the signal after the volume control (in fact, with digital volume control, the noise voltage is basically constant). In other words, at very low volume settings (as required by the most sensitive headphones and IEMs), the amplifier has worse signal to noise ratio.

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This is a feature of the amplifier, not the headphones. You are running the risk of confusing the issue, when the guy clearly needs his understanding of the basic issue improved, not to be led into a minefield of secondary issues. A volume control divides noise equally with signal. The added noise after the volume control is trivial, recall Friis.

w

wakibaki said:

This is a feature of the amplifier, not the headphones. You are running the risk of confusing the issue, when the guy clearly needs his understanding of the basic issue improved

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Well, then understanding the basic issue that noise can be added before and after the volume control (and to some extent even by a volume potentiometer itself), and that the noise SPL resulting from the latter depends on the sensitivity of the headphones (as well as voltage division due to the output impedance if it is high enough), could be useful. The OP asked why the same amplifier is noisy with IEMs, but not with orthodynamic headphones, and I explained it. Although I forgot about not bothering to argue on Head-Fi, so never mind.

thebrunx said:

  i found a way to reduce background hiss on any headphone jack without degrading sound quality, search for a simple audio cable with analog volume control (5$-10$), reduce the volume on the cables volume control until the hiss is not audible, then add volume from the amp volume control to compensate... works flawlessly no need to buy anything else or rather no need to throw away your hissing equipments

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it is indeed a solution for most hiss cases. but there are 2 drawbacks to this:
1/ the source needs to have enough gain for the headphone+ the resistor(as the volume remote is just that, a variable resistor). so depending on the source and headphone, max power could become an issue.
 
2/ impedance!!!! some IEMs/headphones will be affected by the analog volume control. that can lead to significant change in signature with most multidriver IEMs for starters. and the changes are only rarelly positive one.
 
so it's not something I would recommend in general,(be it resistor in series, or voltage divider)  but if you happen to have little audible change, or even better, if you enjoy the new sound, then all is good and you have solved your problem ^_^.

Since this thread is titled hiss, I'll ask here in case others come with the same questions. 
 
I have an ATH-IM02. It is 36 ohms and has a sensitivity of 113 dB/mW. But I'm getting a ton of hiss from Sansa Clip+ and from my Fiio E07K. As far as I know, hiss is not a characteristic of either, normally. The E07K is praise for its lack of hiss with iems. On the other hand, my Magni 2 had no hiss at all. Of course, it was way too powerful for this iem. 
 
Is there something in the specs of the ATH-IM02 that should have alerted me to the hiss. Or is there some spec I should be looking for when purchasing a portable dac/amp that will ensure the iem be without hiss. I'd like to be able to make a more informed choice in the future.  
 
Thanks all! 

imackler said:

  Since this thread is titled hiss, I'll ask here in case others come with the same questions. 
 
I have an ATH-IM02. It is 36 ohms and has a sensitivity of 113 dB/mW. But I'm getting a ton of hiss from Sansa Clip+ and from my Fiio E07K. As far as I know, hiss is not a characteristic of either, normally. The E07K is praise for its lack of hiss with iems. On the other hand, my Magni 2 had no hiss at all. Of course, it was way too powerful for this iem. 
 
Is there something in the specs of the ATH-IM02 that should have alerted me to the hiss. Or is there some spec I should be looking for when purchasing a portable dac/amp that will ensure the iem be without hiss. I'd like to be able to make a more informed choice in the future.  
 
Thanks all! 

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http://www.head-fi.org/t/777273/whats-that-idiot-castleofargh-always-talking-about-a-dap-hissing-i-cant-hear-a-thing
 
 the hiss is an electrical noise coming from the amp(exactly like music), it reaches a certain voltage that in turn translates into a certain loudness into the IEM. http://www.innerfidelity.com/images/AudioTechnicaATHIM02.pdf  you can see at the bottom right that it was measured as needing 0.016v to be 90db loud(that's very sensitive!!!).  so let's pretend the sansa clip outputs a noise of 0.0001v(random value I don't know what the noise floor of the clip is) then into the IM02 it would result in a sound close to 46db loud.
 
now into the rockets(random IEM ) http://www.innerfidelity.com/images/AurisonicRockets.pdf  even though they show 18ohm of impedance, they needed 0.04v to be 90db loud. so the same hypothetical noise of 0.0001v from the sansa would now be 38db loud into those IEMs.
 
knowing that 10db louder feels about twice as loud, it becomes obvious that you will get more hiss from the IM02 than from that other IEM or any less sensitive ones.
 
as to why a source hisses and not another one, it just means that the Magni makes noises of lower voltage or at less annoying frequencies than the sansa or the E07K.  because manufacturers often don't give enough information or don't use a clear enough standard for noise, it's pretty hard to anticipate the audible noise of a source from the very basic specs.
I believe the safest way to pick your source is to find a few guys like myself who hate hiss, are very sensitive to it, and own a few very sensitive IEMs. and ask us if we have tried the source you want to buy. reviews from a guy like @shigzeo (here or on ohm-image or cymbacavum) will never forget to mention when a source's hiss is noticeable. so if a guy like him writes that a source has a clean background, you can fully trust the statement. sorry that's not super scientific but it works, I've been a huge fan of this guy for years for that very reason ^_^.

castleofargh said:

  http://www.head-fi.org/t/777273/whats-that-idiot-castleofargh-always-talking-about-a-dap-hissing-i-cant-hear-a-thing
 
 the hiss is an electrical noise coming from the amp(exactly like music), it reaches a certain voltage that in turn translates into a certain loudness into the IEM. http://www.innerfidelity.com/images/AudioTechnicaATHIM02.pdf  you can see at the bottom right that it was measured as needing 0.016v to be 90db loud(that's very sensitive!!!).  so let's pretend the sansa clip outputs a noise of 0.0001v(random value I don't know what the noise floor of the clip is) then into the IM02 it would result in a sound close to 46db loud.
 
now into the rockets(random IEM ) http://www.innerfidelity.com/images/AurisonicRockets.pdf  even though they show 18ohm of impedance, they needed 0.04v to be 90db loud. so the same hypothetical noise of 0.0001v from the sansa would now be 38db loud into those IEMs.
 
knowing that 10db louder feels about twice as loud, it becomes obvious that you will get more hiss from the IM02 than from that other IEM or any less sensitive ones.
 
as to why a source hisses and not another one, it just means that the Magni makes noises of lower voltage or at less annoying frequencies than the sansa or the E07K.  because manufacturers often don't give enough information or don't use a clear enough standard for noise, it's pretty hard to anticipate the audible noise of a source from the very basic specs.
I believe the safest way to pick your source is to find a few guys like myself who hate hiss, are very sensitive to it, and own a few very sensitive IEMs. and ask us if we have tried the source you want to buy. reviews from a guy like @shigzeo (here or on ohm-image or cymbacavum) will never forget to mention when a source's hiss is noticeable. so if a guy like him writes that a source has a clean background, you can fully trust the statement. sorry that's not super scientific but it works, I've been a huge fan of this guy for years for that very reason ^_^.

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Thank you so much for taking so much time in your response! This is tremendously helpful. So basically, I can avoid these problems by getting iems that need more energy to get to a certain volume. OR be as vigorous as I can in checking before buying a source/amp. And honestly, the more I look, the more I see that this could be a problem for many iems... The ATH-IM02 is ridiculously sensitive, but actually less unlike others than I thought.  
 
I've been disappointed by the E07K. I read reviews that promised virtually no hiss, both from Clieos and Headfonia, but in my estimation, I've got to be able to find better. Do you have any recommendation for either a portable dac/amp or an amp (and I could keep the E07K) as a source? I'd be fine with a DAP as well. Maybe something under $300. I'd like the freedom to get whatever iems I wanted without worrying about hiss...ever again. 
 
Thanks again for your response! I was getting nowhere looking at signal to noise ratio, etc. I didn't know where to look. 

Tyll's measurement on my IM02 puts it as a very sensitive IEM, and thus very sensitive to hiss and noise. BA superior sensitivity makes them more prone to hiss in general. Impedance and power at one point doesn't tell you very much, since the crossover turns the impedance curve into a roller coaster.
The noise would hopefully be given by SNR/Dynamic range compared to output value, explicitly stating the actual noise level as a voltage makes it even more clear.
 
Castleofargh's random guess isn't too far from the -86.5 dBV (~0.000047 Vrms) noise level on the Clip+, as measured by QuantAsylum.
E07K noise seems to depend on how you use it, by measurement. I'm sure the Magni 2 noise is being rated quite conservatively (better than -102 dBV), but ideally the Fiio is supposed be around -100 dBV by spec and measurement.

imackler said:

  Thank you so much for taking so much time in your response! This is tremendously helpful. So basically, I can avoid these problems by getting iems that need more energy to get to a certain volume. OR be as vigorous as I can in checking before buying a source/amp. And honestly, the more I look, the more I see that this could be a problem for many iems... The ATH-IM02 is ridiculously sensitive, but actually less unlike others than I thought.  

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that's exactly it.  and it's a big problem as many high end IEMs are just too sensitive, while portable sources aren't historically the very best of what audio gear can do. it's getting better but many sources still have a crappy amp section.
 
super captain obvious pro tip!!!!!!!!!!!!!! double check the units given for sensitivity by the manufacturer. some give db@1mw some db@1V some db @1mV, as if those were the same thing.
for that reason going to see Tyll measurement to get 90db isn't a bad idea when he has the IEM you're looking for. it avoids making some boring calculus, and it also shows if the manufacturer tried to push his values in a slightly optimistic way .
all those values still only give 1khz information, but usually it's enough as it's a frequency where we're very sensitive. still, if you see the IEM's frequency response dropping 25db @1khz compared to the rest, you might want to avoid it if the 1khz sensitivity was already pretty high.
 
 
 

imackler said:

  I've been disappointed by the E07K. I read reviews that promised virtually no hiss, both from Clieos and Headfonia, but in my estimation, I've got to be able to find better. Do you have any recommendation for either a portable dac/amp or an amp (and I could keep the E07K) as a source? I'd be fine with a DAP as well. Maybe something under $300. I'd like the freedom to get whatever iems I wanted without worrying about hiss...ever again. 
 
Thanks again for your response! I was getting nowhere looking at signal to noise ratio, etc. I didn't know where to look. 

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I don't know the E07K, in fact I don't know many amps. of those I have tried, most had some hissing problems, at least enough hiss for some very sensitive IEM and quiet room to reveal it.
I've settled for an odac/o2 at home, it's in effect one of the quietest source I have tried. using gain1X, hiss really is something of the past. the problem can be channel imbalance, the ODAC gives 2V, so even in 1X you need to lower the volume/voltage level to get to listening levels with sensitive IEMs, and I can get up to 0.5db imbalance between left and right when the knob is in the lowest part. it's not necessarily audible, but hey we're hifi dudes, we expect better than that.
and outside I go with less sensitive IEMs most of the time for convenience, even though my favorite ones are always the ultra sensitive IEMs. or if I feel strong and it's cold enough to carry a jacket(pocket provider^_^) I carry my leckerton UHA760 that is also super clean and very much built with sensitive IEMs in mind. it's over your budget, but you can very confidently go for a UHA 6S MKII. you can read a lot about it from pros and forum members, we all agree it's a clean neutral amp. the JDS C5 also seems to be loved, but I haven't tried it myself so you'll need to ask others.
 
I'm sure there is a long list of AMPs that would do a proper job, but I have tried maybe 10 portables amps in my life and half of them it was in a noisy meeting, so I learned nothing. I believe looking for low gain value is a good start if you only ever plan to drive IEMs. many highly regarded amps are praised for how much power/max voltage they can deliver into fullsize cans, for our overly sensitive IEMs those amps make no sens. so sticking to something with low gain is probably not a bad starting point.

castleofargh said:

I carry my leckerton UHA760 that is also super clean and very much built with sensitive IEMs in mind. it's over your budget, but you can very confidently go for a UHA 6S MKII. you can read a lot about it from pros and forum members, we all agree it's a clean neutral amp. the JDS C5 also seems to be loved, but I haven't tried it myself so you'll need to ask others.

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6S-II hisses with sensitive IEMs unless you have Nick lower the gain.