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I guess my question is how does the amplification that an active pre-amp provides to a heaphone differ from that of an integrated amplifier, assuming both have a headphone jack that is fed by each unit's gain stage (I hope I used that term correctly)? Is there a difference in the way each amplifies a signal, other than how much? |
both are really no more than impedance converters/matching devices with one
adding a power stage to the gain stage to get the drive requirements and the other
dumping power from the driving section.
where it gets a bit snarly is the actual interface between the output drive section and the low impedance (relatively speaking using a line iput as reference) headphone.
If you tried to drive an average headphone load directly from a preamplifier output it needs to satisfy two requirements
1-needs to have enough current reserves to handle the additional load.When you increase the load on an output stage you ease the drive requirements of that stage while at the same time reducing its current delivery or power (just like when you use a 16 ohm speaker on a 100wpc/8 ohms amp you now have a 50wpc amp).
Reduce the output load impedance and you increase the demands not just on the circuit itself but the power suppy feeding the citcuit and again using a speaker amp as an example why many can do 8 or 16 ohms (even 32 grado fans
) they totally fll apart and have LESS power into at 4 ohm load.Voltage delivery into a low impedance load means more current must be delivered and this being a stressful state,one that many designs are not up to for reasons of cost,the power supply sags,the output section clips and the sound sucks.
so if you want a line stage to drive a headphone it needs help in the output section and why you see buffers or discrete stages added for more drive.
The reverse is a typical headphone amp can not only drive headphones but a true 600 ohm input after a very long interconnect.the classic "line driver" is really the same as a headphone amp.
2-The second part of the equation is low end rolloff.ALL audio gear has a bass cut or it would be flat to DC and DC being a voltage as well as a frequency means it will be passed along to every stage following and if thise stages also pass DC the final result is blown speakers or blown headphones ! They HATE DC and were not designed to withstand any but a miniscule amount.
So knowing there needs to be a method to cast this off or block it most preamps have a DC blocking capacitor and depending on the quality is not detrimental to the overall sound (BTW-DC servos eliminate the cp but have their own "sound" and DC blance controls drift so choose your poison).
Where this can be a problem is in what was the assumed load impedance of the next stage when the designer chose the part ?
for example :If the data sheet for the preamp says "20-20,000hz +/- 0.5dB into a 10K load or above" and you load the output with a 5K load you just shifted the -5dB bass rolloff point from 20hz to 40hz ! 2.5k and it becomes 80hz and worse,100 ohms (some headpones ) and it is 200hz !!!!!!
No way anyone settles for that and calls it music so the capacitor needs to be taken into consideration if one is present and willbe the over riding factor in ultimate sonic quality.If none is present on the output the above part above output section drivew holds precedence.
Power amplifers:
Whole different beast and already designed to play into a low impedance load,even lower than headphones,and with serious damping and serious VA capabilities so why is this a bad thing ?
If your headphones were designed to play off a power amp (K1000,AKG "M" series,some KOSS.MAYBE the Sony V-6) they will be voice matched to play from the superior damping and power available,will in fact sound
exactly as they were designed to sound on the test bench but use a headphone meant to be driven from a less muscular output stage,one with less damping and you could lose all by from overdamping the headphones !
The fix is to play around with matching resistor values to get the matchups "just so" but otherwise you took the bass away and will proclaim the experiement a failure.
Botom line is ALL power amps can drive EVERY headphone while SOME preamplifiers can drive SELECTIVE headphones as long as you pay attention to the specs and do your matchups accordingly
hope this rambling response helps a bit