The reason there is pretty simple.......
We designed the headphone output on the BasX A-100 to act pretty much the way the headphone outputs on most vintage receivers and integrated amplifiers act so we could get a similar sonic signature. A series resistor is the simplest circuit to use, and also the one most commonly seen in vintage equipment. With this circuit topology, the output impedance of the headphone output is equal to the series resistance, which is 220 Ohms. As a result, low impedance headphones, which are often the most sensitive, also see a lower output level - which works out well. But, more importantly, the interaction causes variations in the impedance of your headphones to cause variations in their frequency response. To phrase that differently, it encourages the output of the amplifier to interact with your headphones, rather than the opposite.
Many modern audiophiles would suggest that this sort of interaction is to be avoided, which is why many modern headphone amplifiers have a very low output impedance. However, even more than with loudspeakers, what a headphone should sound like tends to be a very subjective experience, and a significant number of headphone aficionados feel that the headphone outputs on vintage equipment sound better... and this higher output impedance is the major cause of that difference in sound. Because of that resistor, the output of the BasX A-100 tends to sound different with different headphones, and many headphones will sound different when connected to it than when connected to a headphone amplifier that utilizes a modern design...
And, because the amplifier circuitry in the A-100 is very quiet, we've eliminated one of the biggest complaints people used to have against the vintage designs.
(Actually, while we've reduced that limitation relative to vintage designs, we couldn't entirely eliminate it.... which is why some people still notice the noise floor on the A-100.)
With the resistors in circuit, the headphone output on the BasX A-100 has an output impedance of 220 Ohms. As a result:
1) the output level (gain) works reasonably well with a wide variety of different headphones
2) the frequency response of the output tends to interact with the individual headphones you connect to it (giving each a bit of its own "personality")
3) the damping factor applied to the headphones is relatively low (which also affects the way they sound)
4) because of 2) and 3), the headphone output of the A-100 tends to mimic the sound of the headphone outputs on vintage equipment
With the resistors bypassed, the output impedance is very low ( a tiny fraction of an Ohm). As a result:
1) the output level (
VOLTAGE) is essentially load independent
2) the damping factor applied to the headphones is very high
3) the frequency response of the output does NOT interact with the individual headphones you connect to it (this is the way most modern headphone amplifiers act)
4) the noise floor is slightly higher (this is a slight drawback due to some of the other optimizations)
5) the A-100 is capable of delivering dangerously high output levels to low-impedance headphones (which is why all the warnings)
6) the A-100 is capable of delivering the very high output levels some early low-efficiency planar headphones required
In comparison to the approach we chose, using an L-pad will yield a significantly lower output impedance.
With a simple series resistor, and an amplfier with a near-zero internal impedance, the output impedance simply equals the series resistance = 220 Ohms.
With a simple L-pad, and the same amplifier, the output impedance is equal to the PARALLEL COMBINATION of the two resistors in the L-pad.
So, for example, if you use an L-pad with a 33 Ohm resistor and a 3.3 Ohm resistor, the output impedance works out to slightly below 3 Ohms.
This is very similar to the output impedance of many modern headphone amps.
And, with the resistors bypassed, and no L-pad or other attenuator, the output impedance will be super low (and the damping super high).
Also note, when designing attenuators and L-pads, that the maximum output on the BasX A-100 is 25 volts RMS... which is significant.
Therefore, with certain resistor values, and depending on the levels you typically listen at, your resistors may need to handle a LOT of power.
For example, the BasX A-100 is rated to deliver about 80 watts into 8 Ohms.
This means that, when playing a continuous tone, it CAN deliver as much as 18 watts into a 33 Ohm L-pad.....
And as much as about 35 watts into an 18 Ohm L-pad.....
And most of that power is going to heat the "top" resistor in that L-pad.....
Obviously this isn't going to be an issue at lower power levels, and, since music is dynamic, sustained very high levels are quite rare...but you should be aware of the possibility.
(If you make an 18 Ohm L-pad, and inadvertently turn the amp all the way up one day, your L-pad may actually catch fire.
More importantly, power resistors can get very hot - hot enough to melt plastic and burn your hands - even when operating within safe ratings.)
Does anyone know why Emotiva put in a straight resistor rather than a proper voltage divider for the headphone out? 2 extra resistors is not a big cost difference, so that likely isn't it...
I had one problem though. I missed my Darkvoice 336se Tube sound that I have become addicted to. I decided to try something a little different. I used my Darkvoice as a Pre-Amp to my A-100, it does have pre-outs after all. All I can say is OMG what an amazing upgrade! I have kept all my Tube Warmth, Wide Soundstage, and overall Smoothness that I love soooo much about Tubes, but the Bass has at least tripled on my M1060's. This A-100 has so much dynamic range and punch that the only word that comes to mind is Effortless. If there are any Darkvoice owners out there that are thinking of getting this A-100 as a solid state for your Planars give this a try. I could not be more pleased with what this has done for my listening pleasure. Well guess I'm back to re-rolling all my tubes again to see how they sound with this set up lol.
