Steve Deckert
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The resistor value depends on a few different things:
A) how much power you have.
B) how much noise you have.
C) how efficient the headphones are.
D) the impedance of the headphones, which basically dictate how much power the amp will put out, so it effects C) efficiency.
In an amplifier with low noise and just a few watts, a value from 150ohm to as high as 1Kohm can be used depending on the headphone. Amps with more power tend to also have more noise. Both have to be reduced to be used with headphones, so you can expect the values to be higher.
If you ask me to do it, and I don't have a pair of headphones in my collection that match yours, I will have you send the headphones to me so I can play the resistor swap game until I find the perfect value. I basically listen for the lowest value that has no hum and start there. Raising the value will reduce volume and dynamics.
Most headphones will get far louder than speakers and are dangerous when connected to speaker amplifiers unless "tuned" with a series resistor of the correct value which can only be found by ear.
Yes, no math can predict it because it overlooks the listener. I set up all of our headphone rigs by ear based on my comments above...
I sent a headphone amplifier to a big shot reviewer on one of the headphone forums years ago and he called in a panic that the amp was distorting badly. It turned out that when it arrived back here it was perfectly fine and the reviewer was actually hard of hearing. By over 6dB. I took the 6dB reducing resistor out and ran the amp wide open and sent it back to him. It was WAY TO LOUD and you could start to hear hum. He called elated that it was perfect and proceeded to give it rave reviews.
-Steve
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