Originally Posted by odigg
As a general rule of thumb the ideal headphone amp will have an output impedance that is close to 0.
Many solid state headphone amps probably (I have not measured them personally) have output impedances close to zero. The Presnous HP4 is solid state, uses an Opamp, and still has a hi-z out. I looked inside and I think they are using a resistor and capacitor at the output to prevent DC current at the output jack.
There is one "advantage" to a hi-z output, but it's "advantage" come in a counterintuitive(assuming neutrality is your goal) way. A hi-z output may adjust the frequency response of the sound. You may like this adjustment, and I suspect this recently occurred with one person who purchased a flagship headphone and found he (she?) did not like it until they plugged it into a hi-z (120 ohms?) output. My guess is that there was a noticeable increase in bass.
There is also the question of Dampning Factor
. The general rule of thumb is that a higher dampning factor is better, but I have never found a proper answer to what dampning factor is "good enough" in the sense that human being will not be able to discern an increase in dampning factor after that point.
A zero output impedance is not necessarily the ideal for a headphone amp. It depends on how a particular headphone is designed. There is an international standard IEC 61938 which specifies 5V with 120 ohm output impedance for headphones of the range 8 - 600 ohm. From my experience, many of the higher end European headphones are designed and tuned to mate with this specification. When I listen to the 250 and 600 ohm beyerdynamics, 300 ohm Sennheisers, and higher impedance AKGs, they all sound better to me when fed from 120 ohm output impedance. I don't believe this is coincidental.
At the same time, some headphones are designed to be fed from a low output impedance. From my experience these are usually lower impedance headphones, but not exclusive.
Damping factor isn't a hard and fast rule. For example: If a headphone were designed to be powered per the IEC spec, it would be intentionally overdamped when tested via voltage driven, so that when fed from the 120 ohm output impedance, the damping would be reduced to the amount which the headphone designer intended. This is like current driving (instead of voltage driving) your headphones. The same is true with single driver, wide range, high efficiency loudspeakers from Lowther and Fostex. They have very low moving mass, very high BL, and very low Qts (meaning they are significantly over damped). When driven with a voltage source (amplifier with near zero output impedance), they have very little bass. Power them instead with a current source amplifier (like the Pass Labs First Watt) and now you get a significant increase in bass making it similar level to the midrange and treble. The only downside to high output impedance is power wasted in the amplifier. Fortunately since headphones need such little power, the energy dissipated though the amplifier's internal impedance is low, so you won't need to stress about your electricity bill
I see some posts where people measure the frequency response on the headphone amplifier output while powering different headphones. If the amp has 0 output impedance, the frequency response should be flat with any headphone connected. Amps that have a higher output impedance will have a contoured frequency response across the headphone which depends on the impedance characteristics of the headphone. Many people quickly view this is as bad. This is not necessarily so: If the headphone were designed to be powered from a higher output impedance, say the 120 ohm IEC standard, then this is expected and part of the design towards the total sound quality and tonal balance produced by the headphone. If this same headphone were powered by a 0 ohm output impedance amplifier, this condition would in fact be producing coloration because it is deviating from the intended contour
See my post:http://www.head-fi.org/forums/f4/wha...ml#post5749154
As with most things in life, there are no absolute rules and multiple paths to a goal. When powering headphones, it's all about system synergy. I first like to know and listen to them powered with the boundary conditions (low or high output impedance) as their designers intended. However in the end, it all comes down to personal preference and why I like to see amplifiers with selectable, specified output impedance. I commend Meier Audio for including dual outputs on some of their models.