I'm wondering if anyone knows what source impedance the ER-4P/4S were designed to be driven from, and how significantly their sound may vary "from advertised" at different source impedances? I'm asking because output (source) impedance varies among amplifiers, from near zero ohms (many dedicated...

Here's an interesting article about a headphone amplifier based on the Texas Instruments THS6012 op amp (originally designed as an ADSL line driver): http://sound.westhost.com/highspeed.htm To quote the author: "As a headphone amplifier, these opamps are superb, with not a single negative...