I have an aged pair of IEMs from around 18 years ago - Shure e5s. I've barely used them as I found the coupling to my ears to be insanely sensitive to getting a tight seal - even a slight gap results in almost no bass response. They also have cables that wrap over the tops of the ears which make them more than averagely fiddly to fit. The rather unusual characteristic they have is that their sensitivity is extreme at 122dBSPL/mW - at their impedance of 110ohms this puts the 0dB point (threshold of hearing) at 0.26uV. From browsing Amir's reviews of headphone amps over on ASR I found precisely none to be low enough in noise so that its residual hiss is certain to be inaudible through the e5s. Which would mean a noise at output of below 260nV. The nearest seems to be the Topping A90 for which Amir records a 50mV referenced SNR of 93dB. Meaning its output noise is 1.1uV. That is at least 12dB too noisy.
Looking into more modern IEM designs I see the trend is towards lower impedances which make the HPs even more sensitive in voltage terms. I don't know if any reach over 130dB/V (the math gives me nearly 132dB for the e5s) but there are some mentioned here that go over 120dB/V so they too are too sensitive for the lowest noise recorded in ASR for a HP amp. What's a sensitive IEM user to do when no extant amps are going to deliver him/her a noise-free listening experience?
Clearly an output attenuator is called for, in order though to avoid adding more noise a resistive one would be less preferable than a transformer. With a big enough step-down ratio in hand I could drive the Shures directly from my Dark LED I/V, bypassing the need of an amp altogether. 10:1 step down would mean the output noise could be up to 2.6uV and as far as I recall, the Dark LED's output noise is lower than this when fed current from PCM58s. The PCM58 being about the lowest noise R2R DAC chip going. The Dark's maximum output level is 2VRMS which, through the trafo would send the e5s 200mV hence a peak SPL of 118dB. Should be sufficient.
Looking into more modern IEM designs I see the trend is towards lower impedances which make the HPs even more sensitive in voltage terms. I don't know if any reach over 130dB/V (the math gives me nearly 132dB for the e5s) but there are some mentioned here that go over 120dB/V so they too are too sensitive for the lowest noise recorded in ASR for a HP amp. What's a sensitive IEM user to do when no extant amps are going to deliver him/her a noise-free listening experience?
Clearly an output attenuator is called for, in order though to avoid adding more noise a resistive one would be less preferable than a transformer. With a big enough step-down ratio in hand I could drive the Shures directly from my Dark LED I/V, bypassing the need of an amp altogether. 10:1 step down would mean the output noise could be up to 2.6uV and as far as I recall, the Dark LED's output noise is lower than this when fed current from PCM58s. The PCM58 being about the lowest noise R2R DAC chip going. The Dark's maximum output level is 2VRMS which, through the trafo would send the e5s 200mV hence a peak SPL of 118dB. Should be sufficient.
