I've got my cobwebbed soldering iron out, and am poking around this amp I have which requires me to solder different tracks to cater for different headphone output impedance. Currently it's set to >250ohms.
As I'm getting rid of the HD600, the rest of the phones I have will be 48 ohms at the most. I can set this. But my question is how important is impedance matching, and how will it affect the sound? I am hopeless at detailed comparisons and I'm not going to resolder the tracks to the various settings, listen to them and resolder again!
Anyone help? What would a setting for a lower headphone impedance sound like on a low impedance phone, and what would a high headphone impedance sound like on a low impedance phone?
Bangraman, my view on this can be found in the "Don't Knock Portable MD Amps" thread, along with a link to a relevant translation contributed by Halcyon.
In a nutshell, I think high impedance outputs are evil when it comes to dynamic headphones. You can expect inferior damping of the driver, resulting in distortion which depends on the impedance curve of the headphone, a reduction in accuracy and a corresponding change in tonal balance.
It could be that the lower the impedance of the headphone, the more important it is to have a low impedance amplifier to drive it (to maintain an equivalent damping factor). What surprised me is that for a given impedance curve, it is the high impedance points or peaks where you can expect trouble if you use a high impedance amplifier. This makes sense though, when you consider that high impedance points represent natural resonances in the driver design. Those are the exact frequencies where you want the amplifer to provide some electrical damping.
I would suggest you use the minimum possible output impedance setting. Even better is to put any necessary current-limiting resistors inside the feedback loop and get a "virtual" zero impedance output from your amp.
On the other hand, Etys and some Beyers seem to be intended to be driven with impedances around 100-120 ohms. This is something beyond my comprehension. If the objective is to attain a certain frequency equalisation (such as a bass-treble balance) I think there are better ways to do this, but then the product is no longer a simple headphone but a combination of 'phones and dedicated amplifier/equaliser.