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Originally Posted by Covenant /img/forum/go_quote.gif
I'm getting some conflicting information here. If damping factor correlates to the control the amp has over the driver, how could being "overly damped" be a bad thing?
Uncle Erik seems to be saying that an amp with a lower output impedance than the headphone being used is the right choice, wheras Doland seems to be taking the stance that the two must be matched and equal. Or am I taking too simplistic a view?
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Yes, there are some differences between what Erik and I say
First, I am not saying the amplifier output impedance and headphone impedance should be identical. I am suggesting the headphone should be powered by an amplifier with output impedance which the headphone's designer intended and used to tune them.
My background is transducer and loudspeaker design. In loudspeakers there is a dimensionless figure of merit called Q which pertains to the speaker's damping of its fundamental resonance frequency. The Q is a function of the speaker's mass, suspension compliance and resistive losses, strength of the motor (product of the voice coil and magnet structure), and electrical impedance. This Q parameter is often used also in defining the tuning of transducer in a sealed enclosure. A Q of 0.5 is defined as critically damped as the diaphragm has minimal overshoot and returns to its zero rest position as quickly as possible. A Q less than 0.5 is considered over damped, while a Q greater than 1 is usually considered under damped.
If you take any transducer and add a series resistor to it, you increase its Q and decrease its damping.
With loudspeakers, they're normally designed to see an amplifier with very low output impedance, less than 0.5 ohm. This is how 99% of the hi-fi amplifiers in the world market are. An amplifier with higher output impedance reduces the speaker's low frequency damping and modifies the speaker's frequency response. Hi-fi tube amplifiers typically have an output impedance of 1-3 ohms depending on topology and if they use feedback or not. Do note: You can design a loudspeaker to sound "right" when feed from a higher output impedance. This is what I do with my high efficiency loudspeakers designed for use with low wattage SET tube amplifiers.
Now let's look at headphones: A headphone transducer might be designed to have the right (according to its designer) damping Q when fed from a low impedance amplifier, meaning for proper sound the designer does not want the headphone to see much amplifier series output impedance to decrease the damping. Other headphone transducers are intentionally designed over damped with low Q, knowing that they'll be used with amplifiers with higher output impedance (perhaps the IEC standard of 120 ohms) which subsequently decreases the headphone's damping and brings them "in tune".
Ultimately there is no absolute right or wrong. What matters is what sounds best to you.