Yes, we're talking about a lot of power, and you do need to handle it carefully.
However, for people considering doing this with other headphones, a lot depends on the particular headphones you're using - and their impedance. Always remember that most amplifiers, including the A-100, are voltage source devices - which means that the amplifier itself puts out a certain voltage based on the amplitude of the input signal and where you set the Volume control. Therefore, while the direct connection will give you massive damping, and super-tight control of the drivers in your headphones, the power will be very different depending on the impedance of your particular headphones.
In the context of this discussion:
1) If you connect headphones with relatively low impedance, but that require a lot of power (like some Audeze models), you won't find the controls to be especially touchy at all - because those phones actually want several watts to drive them well. (And those headphones definitely wouldn't be getting enough power with the resistors in-circuit.)
2) If you use high impedance phones, like 300 Ohm or 600 Ohm Beyerdynamics, you also won't find the controls to be especially touchy either. Power = voltage squared divided by resistance. So, with the same input signal level, and the same setting on the Volume control, a 300 Ohm driver draws about one fortieth as much power as an 8 Ohm driver. (So, for a given setting, even though those high impedance cans are a lot more efficient, they'll also be drawing a lot less power.) In fact, with high impedance phones, the 220 Ohm series resistors don't make all that much difference in the output power - so you'll only be getting a little bit more power (2x - 5x or so) with them out of circuit (but the damping will be substantially higher).
3) If you use headphones that are
BOTH HIGH EFFICIENCY AND LOW IMPEDANCE, then you're going to have to be very careful. The A-100 was designed to be super-quiet, so there shouldn't be too much noise, and the damping will be stellar, but you will get a lot of power with a very slight turn on that Volume control when the resistors are jumpered. (We've done it, and it works great, but you do have to be really careful.)
One thing that would give you more control would be to use a separate preamp before the input of the A-100. Note that you could also use a passive volume control, like the Emotiva Control Freak or the Schiit Audio Sys. Yet another option would be to put a pair of fixed attenuators on the input - something around 24 dB should cut the gain down enough to allow you to use a lot more of the range of the volume control (HLabs makes cute little gold in-line ones for about $30 a pair). Finally, you could use an external cable, connected to the speaker terminals, with different value resistors in it. (You
COULD also make a little gizmo to plug into the jumper block, with a resistor on it, for each channel. Note that, if you do, this will be in parallel with the 220 Ohm resistors that are already there - the wire jumper simply shorts them out of the circuit. Also note that there isn't much clearance in there, so be very careful if you go adding stuff.)
Quote:
Just be careful when you jump that resistor, that is a **** load of power. When your looking for a DAC, try to find one that you can use as a preamp. it'll give you a lot more control over the power output.