ART Pro Audio HeadAmp6 - 6 Ch. Headphone Amp - Manual (Page 7)

FIGURE 3 - Rear Panel Headphone Outputs
Stereo Headphone Outputs
These stereo TRS jacks are the main outputs for each channel of the headphone amplifier. They are wired in
parallel with the corresponding output jacks on the front panel. Either front panel, rear panel, or both front and
rear panel outputs can be used simultaneously to drive headphones or they can be used as feeds to additional
headphone amplifiers in a distributed audio network.
NOTE: The HeadAmp6 is optimized to drive typical headphone load impedances of 32 to 600 Ohms. It is not
recommended to drive total impedances lower than 16 Ohms however it can be done without damage and will
result in limited maximum output power, and possible clipping distortion depending on the output level and
load. When driving multiple headphones from an individual output channel of the HeadAmp6 (paralleling), the
available output power is split among the various headphones, and due to the combined load impedances the
output may also become limited by premature clipping of the output signal. Most modern headphones are
medium to high impedance and require only milliwatts to achieve full acoustic output so this should rarely be a
problem. If it is a problem simply lower the level or lower the total count of headphones on a particular channel
in order to increase the total load impedance as seen by the channel output.
There is no direct relationship between headphone load impedance and SPL output. The relevant
specification that determines acoustic output is the sensitivity spec of the headphone, i.e. how much SPL it will
put out for a given level of input signal, usually rated at 1 mW. Sensitivity is determined by the overall design
and construction of the transducer. Typically the power required is about 1/1000th of the equivalent amplifier
power required to drive a speaker. Therefore typical headphone amplifiers provide power levels in the 10mW
to 20 mW ranges in order to achieve a very reasonable SPL output. Even an 8 Ohm headphone (if you can
find one since they are rare) can be driven to full SPL output by the HeadAmp6 if it has a high enough
sensitivity, regardless of the fact that the output voltage swing will be limited due to the overload protection
circuitry.
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