What causes hiss in IEMs?

[doctorjuggles]

I have been wondering about the cause of hiss in sensitive IEMs when plugged into certain DAPs/amps etc.
I have only experienced it rarely, but several of my friends cite hiss as a constant issue for them and it got me wondering - is it a design flaw in certain sources? Certain IEMs? Or is it just something that can’t be avoided due to wildly varying circumstances and technical variations/mismatches that occurs when so many combinations in the audio chain can be made?

 

[RayDunzl]

I have been wondering about the cause of hiss in sensitive IEMs when plugged into certain DAPs/amps etc.

The noise floor of the amplifier, and/or the noise produced by the source it is amplifying..

The more sensitive the transducer, the more likely whatever noise the amplifier outputs will be audible.

 

[JohnYang1997]

Get an L30. Problem solved.
But seriously, the noise is mainly from the amplifier which is the last component before the IEM. Everything before that can be attenuated by the potentiometer, both music and noise. The output stage has a noise that's independent to the volume control. IEMs are usually much more sensitive to noise because for the same level of noise voltage, you get louder noise due to higher sensitivity. Getting a clean amp will solve the issue.

 

[doctorjuggles]

The noise floor of the amplifier.

The more sensitive the transducer, the more likely whatever noise the amplifier outputs will be audible.

So you’re really looking for an amp that can deal with low impedances? Assuming you have that, then even the most sensitive of IEMs shouldn’t hiss, is that correct?

 

[JohnYang1997]

So you’re really looking for an amp that can deal with low impedances? Assuming you have that, then even the most sensitive of IEMs shouldn’t hiss, is that correct?

It's the noise of the output stage instead of being able to deal with low impedances.

 

[RayDunzl]

So you’re really looking for an amp that can deal with low impedances? Assuming you have that, then even the most sensitive of IEMs shouldn’t hiss, is that correct?

You are looking for an amplifier with a very low noise floor.

I don't think the amplifier impedance is directly related in this case.

 

[JohnYang1997]

In case you don't know what measurement can indicate lower noise for IEMs, the 50mV SNR measurement is what you are looking for.

 

[RayDunzl]

Noise = random (more or less) voltage variations at the amplifier output.

With no signal, output should be 0 volts. It isn't, though it can be milli, micro, or even nano volts.

Example: For a 1 volt output, 1 digital bit (of 16) of noise level = 0.0000305 volts, or 30.5 microvolts.

That would be noise at around -90dB relative to the 1 volt max output.

 

[MRC01]

I have been wondering about the cause of hiss in sensitive IEMs when plugged into certain DAPs/amps etc. ... several of my friends cite hiss as a constant issue for them and it got me wondering - is it a design flaw in certain sources? Certain IEMs? ...

As others mentioned, that noise is the IEM responding to noise in the amp's output. You can think of it as a design flaw in the amp. But more accurately, a limitation not a flaw, if the amp was designed for inefficient headphones not for IEMs.

If the amp has a high SNR (more than 100 dB) you might wonder, how is this possible? The amp's SNR ratio (noise level) is usually measured at full volume, which is misleading because nobody actually listens at full volume, because it's way too loud for most headphones or speakers. The 50 mV output is 32-38 dB quieter than full output and tells you how much noise there when the signal is turned down to something close to a reasonable listening level. If you imagine a volume control as turning down the signal, while the noise remains constant, you can see why the SNR gets worse as you turn down the volume. Some amps do better than others when you turn the volume down, depending how they're designed and built.

 

[doctorjuggles]

Thank you for the info everyone

So the benefit in making some high-end IEMs so sensitive (I’m particularly thinking of the release today of Empire Ears’ Odin, rated at 3ohm/108dB) is to make them easy to drive, but surely there’s a point where it’s too sensitive and becomes so fussy about amp pairings as to be more hassle than they’re worth?

 

[JohnYang1997]

Thank you for the info everyone

So the benefit in making some high-end IEMs so sensitive (I’m particularly thinking of the release today of Empire Ears’ Odin, rated at 3ohm/108dB) is to make them easy to drive, but surely there’s a point where it’s too sensitive and becomes so fussy about amp pairings as to be more hassle than they’re worth?

It's not TO make them easy to drive. Many BA drivers have nomial impedance of around 8ohm. If you add more drivers(in parallel) and some other crossover components it's very natural to get sub 16ohm impedance. Less than 5ohm seems to be a bit too low but not uncommon.

 

[danadam]

but surely there’s a point where it’s too sensitive and becomes so fussy about amp pairings as to be more hassle than they’re worth?

Yes, there is, and in my opinion most IEMs are past that point

Long time ago, my first IEM was Shure E4c with sensitivity ~125 dB SPL/1Vrms *. Turns out, that for me that is this point. When a cable broke and it was time to buy a new one, I thought why not another Shure and I bought Shure SE535. Those have sensitivity ~133 dB SPL/1Vrms and I started hearing hissing from anything portable. So I did some research and I think the only thing available to me with reasonable sensitivity were Ultrasone IQ Pro, with 121 dB SPL/1Vrms. After a few years the cable broke again and now I'm using Tin T2, with 120 dB SPL/1Vrms.

*) All efficiencies converted to sensitivities using: https://web.archive.org/web/20141115085636/http://www.jensign.com/S4/calc.html

 

[dfuller]

It's just general circuit noise (also called Johnson noise or thermal noise) which comes from the electrons moving around from heat. Good designs will minimize it. IEMs are hard because they're so crazily sensitive.