Optoma HA200 - Manual (Page 4)

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NuForce HA-200 Headphone Amplifier
Balanced Headphone Mode
Unique to the HA-200 is its ability to quadruple output power by simply adding a second unit and operating them
together in “Balanced Headphone Mode.” For this configuration, all that’s needed is to use the XLR inputs and
outputs on each HA-200 along with a Balanced headphone cable available from your local or online Headphone
Specialty Stores for connecting the headphones.
In Balanced Mode, four completely separate amps (2 per unit) are utilized to drive the four phases of the music
signal (right normal, right inverted, left normal, left inverted). Please keep in mind though that this is NOT the
same technique as mentioned above in the “push-pull” Class-B scenario. In Balanced Mode, all transistors are
biased in Class-A so as to conduct current at all times – only the phase is inverted 180 degrees in the case of the
left & right inverting amplifiers.
The primary benefit of such a design is that the available output voltage is doubled, so the actual output power is
quadrupled. It also doubles the effective slew rate (the maximum change in voltage per second that an amplifier
can achieve), as both amplifiers are operating in opposite phase to increase the dynamic realism of the music.
While “technically” not a pure SE design, since Balanced Mode remains in full Class-A operation at all times it can
be thought of as a “dual SE” architecture that retains the best of SE performance with the added benefits of higher
power and better dynamic control.
Sonically, the resulting sound is completely “grain-free”, the soundstage is wide and deep while instruments and
voices are reproduced with extreme clarity and purity of tone. The incredible power and dynamic control over the
music simply must be experienced to be appreciated.
Class-A’s Modest Disadvantage
The one slight disadvantage that Class-A amplifiers have is that transistor bias current must be set such that the
output transistor continually operates in the center of its “linear region”, which simply speaking is the midpoint
between its fully-on and fully-off limits. Therefore, regardless of the actual audio power being used Class-A
amplifiers continuously consume their full-rated output power and dissipate that power in the form of waste heat.
Now in some cases this might be a serious efficiency issue, but mostly in those applications involving loudspeaker
amplifiers capable of delivering several hundred watts of power. With headphone amplifiers, this is less of a
concern due to their need for significantly less output capability. Nevertheless, while in operation the HA-200
will continuously consume 24W of power. In fact, that value actually exceeds the idle power consumption of a
NuForce Reference 18 amplifier! The upshot is that in order to benefit from the superlative performance that the
HA-200 has to offer, one must be willing to accept its somewhat lower power efficiency. In our opinion, that’s a
relatively small price to pay for such a great reward, and we believe that many will agree.
Constant Current Output
In addition to the Class-A, Single-Ended and Balanced Mode advantages, the HS-200’s Constant Current output
feature allows it to drive difficult headphone loads and/or headphones exhibiting erratic electrical impedance
across the frequency spectrum. The Constant Current topology has the ability to constantly adjust output
voltage as the headphone impedance changes with frequency – which is more common than not with most all
headphones to some degree. Constant Current operation avoids the extreme performance and available volume
variations that often plague other amplifier topologies, and ensures consistently good performance regardless of
headphone type and/or impedance. Listeners that use differing types of headphones will greatly appreciate this
feature.