[katfisch]

My Etmotics ER-4p canalphones hum when connected to the RA-1, but the ER-4s version (higher impendance) is completely DEAD SILENT. Cranking the RA-1 volume knob all the way up doesn't increase the hum at all, by the way. Thus, it's the phones that are humming, not the amp.
I know almost nothing about electronics, but I can do empirical trials to beat the band.

 

[sanukmac]

Dead silent for me.

 

[Reticuli2]

Just means there's a base hum that doesn't change with volume.  That's still the amp's fault.  The ER4S can be quiet on a great many noisy amps due to their sensitivity and impedance.  They're not as insensitive as some beyers and senns, but they are more so than the P model.
 
Quote:

My Etmotics ER-4p canalphones hum when connected to the RA-1, but the ER-4s version (higher impendance) is completely DEAD SILENT. Cranking the RA-1 volume knob all the way up doesn't increase the hum at all, by the way. Thus, it's the phones that are humming, not the amp.
I know almost nothing about electronics, but I can do empirical trials to beat the band.

Output impedance?  Max input?