Originally Posted by bigandtall
I didn't find a 50k Alps on their site. I tried, though :)
50K Alps RK27's are the optimum for headphone amps - 100K introduces noise; 10K has issues with many sources because it doesn't offer enough input impedance to ensure full frequency response, etc.
However - 50K are special order items from Alps themselves ... and it costs a great deal because they charge you a minimum lot to make a special manufacturing run (several hundred). That's why AMB and I (and formerly Tangent) are among the only places they can be found.