stollesen skrev:kan man gør nå ved det, hvis der er det?
songmic fra head-fi.org skrev:I'll add one more important factor regarding impedance that I haven't mentioned before. I have stated that any output impedance of a headphone amp is tolerable as long as the distortion caused by the difference between the maximum and minimum resistances in the frequency-impedance curve is less than 1 dB, by the use of a mathematical formula.
However, this alone does not explain why an amp with high output impedance, such as many OTL amps, doesn't work so well with low-impedance orthodynamic headphones like Audezes or Hifimans, which have a VERY flat impedance curve. And in fact, these ortho headphones still have higher impedance than 25-ohm Denon or Fostex headphones. The distortion would be much, much lower for these headphones, yet OTL's are bad for them. Why?
This has something to do with the voltage distribution between headphone and amp output, and another value known as efficiency/sensitivity (which is measured as dB/mW).
When a headphone is connected to an output, the headphone and the output section of the amp forms a "series connection." In a series connection, the amount of voltage distributed to each part is determined by the ratio of their resistance/impedance ratio. The lower the headphone impedance, or the higher the output impedance of the amp, less voltage is distributed to the headphone. If the output impedance of an amp is very small (less than 0.1 ohm), or if the headphone has very high impedance (300-600 ohms), nearly 100% of the voltage will be distributed to the headphone. This explains high voltage swing is more important than current for high-impedance headphones like Sennheisers. Conversely, if the headphone impedance is low and the output impedance of the amp is high, less voltage will be distributed to the headphone. Imagine an OTL amp with 75-ohm output impedance driving a 25-ohm D7000. In this case, the ratio of voltage distribution of D7000 and amp output will be 1:3. Only 1/4 of the entire voltage in the circuit is distributed to the headphone.
Because voltage is relatively small, current plays a more important role for driving low impedance cans. (Power = Current x Voltage) The good news is, 25-ohm Denon and Fostex headphones have very high efficiency/sensitivity at 100 dB/mW, meaning it needs relatively less power to drive these cans well. On the other hand, the sensivity of orthos such as Audezes, Hifimans, is somewhere in the 80's and 90's dB/mW. Since we're dealing with dB which is in logarithm, 80 dB/mW and 90 dB/mW means that these headphones are 1/100 and 1/10 less sensitive than 100 dB/mW, or that they would need 100 and 10 more times the power to be driven properly, respectively. That's why high output impedance amps are a no-no for orthos; not because of distortion, but because of lack of power. But the Denons and Fostex have high sensitivity, so we need not worry about lacking power; we should worry more about whether the distortion is greater than 1 dB, the smallest difference in volume that the human ear can discern.
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