11-May-2020, 11:20
The HD650 specifications say it has 300 ohm impedance and a sensitivity of 103 dB for 1 mW input at 1 kHz.
The headphone power calculator at http://www.digizoid.com/power.php gives you a convenient way to work out how much power (and hence voltage and current) the amplifier needs to provide to reach a given sound pressure level.
For example to achieve "very loud" output (115 dB SPL) on the HD650 it says you'll need about 16 mW power which in turn requires the amplifier to deliver 2.18 V rms. Therefore I'd suggest setting the maximum output level from the stereo pre-amp outputs to somewhere in the range 2 to 2.5 V rms; at least that might be a good starting point for experimentation. If you cranked the output level up to the maximum 4 V rms, you'd be able to deliver up to about 53 mW into the headphones which is still well within their 500 mW maximum rating so you won't damage the headphones (although you might damage your hearing!)
However, be aware that this assumes the pre-amp outputs have a low enough output impedance to drive the headphones properly. In other words, to reach the 16 mW power output (for example) the pre-amp outputs also need to be able to deliver at least the corresponding output current which is about 7 mA rms.
Ideally the pre-amp outputs should have an output impedance at least ten times lower than the impedance of the headphones you're driving, which would be less than 30 ohms in your case.
I don't know what the output impedance of the Devialet's pre-amp outputs actually is, but I guess it's higher than 30 ohms - remember, it's mainly intended to drive line inputs of other audio equipment which usually present a much "easier" (higher impedance, typically 47 kohm) load than headphones, so it's probably not optimised for delivering current into a low-impedance load.
The headphone power calculator at http://www.digizoid.com/power.php gives you a convenient way to work out how much power (and hence voltage and current) the amplifier needs to provide to reach a given sound pressure level.
For example to achieve "very loud" output (115 dB SPL) on the HD650 it says you'll need about 16 mW power which in turn requires the amplifier to deliver 2.18 V rms. Therefore I'd suggest setting the maximum output level from the stereo pre-amp outputs to somewhere in the range 2 to 2.5 V rms; at least that might be a good starting point for experimentation. If you cranked the output level up to the maximum 4 V rms, you'd be able to deliver up to about 53 mW into the headphones which is still well within their 500 mW maximum rating so you won't damage the headphones (although you might damage your hearing!)
However, be aware that this assumes the pre-amp outputs have a low enough output impedance to drive the headphones properly. In other words, to reach the 16 mW power output (for example) the pre-amp outputs also need to be able to deliver at least the corresponding output current which is about 7 mA rms.
Ideally the pre-amp outputs should have an output impedance at least ten times lower than the impedance of the headphones you're driving, which would be less than 30 ohms in your case.
I don't know what the output impedance of the Devialet's pre-amp outputs actually is, but I guess it's higher than 30 ohms - remember, it's mainly intended to drive line inputs of other audio equipment which usually present a much "easier" (higher impedance, typically 47 kohm) load than headphones, so it's probably not optimised for delivering current into a low-impedance load.
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