[seldom_used]

Hey, after reading a lot on head-fi I gave my Karma a present - a Cmoy built as per Tangent's tutorial, used an OPA2132P and a gain of 6. It sounds great except there is a faint but noticeable hum in my Ety ER-6i whenever the amp is on. It doesn't change with volume, at sane levels at least.
Same hum with a single 9V battery, two in series, and a 24V wallwart.
Is it a 'feature' or can I fix it somehow?
Perhaps a lower gain will help?

Alex.

 

[balou]

hi,
the er-6i have an extremely high sensitivity - they will pick up even the slightest noise.

the best fix is to add an output resistor. this artificially lowers the sensitivity of the IEM.

See http://tangentsoft.net/audio/cmoy-tu...tweaks.html#r5 for details

 

[xmokshax]

i had a problem similar to yours - i built a CMoy for use with my iPod 5.5g and Westone UM1, and there was a noticeable hiss (i'm not sure i'd call it a hum) in the headphones when using the UM1 that wasn't present when the amp was used with my Senn HD525. the hiss was pretty constant in volume over the lower range of iPod volumes, but got noticeably louder if i turned the iPod volume all the way up. i added resistors at the R5 position, which had previously been shorted, but this did little to solve my problems.

i started a thread about my problem, and tangent noted that iPods are one of the louder portable sources, and said that lowering the gain was the correct solution to my problem. i gradually lowered it from 11 down to 3, and this did help the hissing problem. in conjunction with this, i took another head-fi'er's advice and started listening with my iPod volume all the way up, controlling the volume solely with the amp's potentiometer. this (somewhat counterintuitively) eliminated the hiss entirely.

i'm not sure if your issue is the same as the one i was experiencing, but it's certainly worth giving this solution a try. let me know what happens.

 

[seldom_used]

Alright, I lowered gain from 6 to 3 and poof! most of the hum has disappeared. I didn't try the R5 fix yet because a) this is quite satisfactory and b) don't have anything in the 10-100 ohm range.
I did lower it further to 2, but there was no improvement. It's at a level where sitting in a quiet room, I can just hear the amp is on. Definitely nothing to worry about (yet).

FWIW there is no hum in other headphones, Senh HD470 or Shure E3. The Ety's must be more sensitive.
All in all a nice improvement over Karma's amp. The Cmoy is not as dry, with better-defined bass and wider staging, at lower frequencies especially.

Thanks for the pointers,
Alex.