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mshilarious
Banned
This is a high impedance headphone amp design? 24V is a lot of swing for low-impedance cans, which means potentially lots of power dissipated in the 5532s, gets awfully close to its maximum rating. 5532's current limit is kinda low for low-impedance cans; 1V into 24 ohms is 42mA, 42mW of audio power and ~0.5W lost to the opamp.
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designer.zang
New member
yes,it was designed for studio headphones with high impedance 300~600ohm.
thanks for you refer about the low-impedance cans issue.i add 100ohm resistors at the ne5532 output,so that it is not very hard to drive low Z.
zang
thanks for you refer about the low-impedance cans issue.i add 100ohm resistors at the ne5532 output,so that it is not very hard to drive low Z.
zang
mshilarious
Banned
That doesn't help the current limit, which places a limit on volume. You've just substituted 1V into 24 ohms for ~6V into 124 ohms. That does transfer a good amount to heat loss to the resistor, but you still don't have any more power in the headphones. The 100 ohm trick works for power amps because they can dish out lots and lots of current.
Next, you do have to consider the effect of increasing source impedance on the frequency response of the headphones. That depends on the shape of impedance curve of the headphones in question. Headwise seems to think all low-impedance headphones have flat impedance curves so they don't worry about it, but I'm not sure that is a safe presumption. From the few curves I've see, some are flat but others are not. With the 100 ohm source impedance they would lose bass, get notably bright, and also have a peak at their resonance, generally in the 200Hz range. When designing an amplifier, generally you try to not create changes in frequency response of +/-3dB, which could easily happen here.
All I'm suggesting is since you have an opamp buffer already, why not use an output amp that can supply a reasonable amount of current? I see everybody using small-signal opamps for headphone drivers and it just doesn't make much sense to me unless it's strictly a high-impedance design.
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commanda
New member
Amplifiers also have a characteristic called "Damping Factor". If you Thevenise the amplifier output equivalent circuit, you have a voltage source with a series impedance. The ratio of speaker impedance to amplifier output impedance is the damping factor. Damping stops the speaker cone from waffling. Typically, you want the damping factor as high as possible. Numbers in the 50 to 80 range are typical. So, if your headphone is 8 ohms, a damping factor of 50 would imply an amplifier output impedance of 8/50, or 0.16 ohms. Your 100 ohm series resistor just killed your damping factor. Forget about getting a nice tight bass sound out of it.