Violectric HPA V280 User Manual page 21

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The lower the output impedance of an amp, the lower its influence on
the transmission quality
·
Using "normal" headphones with phone jacks will lead to relatively big
faults caused by the common ground cable and the less than perfect
connectivity from phone jack to phone socket because of much too big
contact surfaces.
·
The ground potential is not static but suffers from voltage drops caused
by the internal impedances and the influence of the common ground
cable from the "normal" headphone.
The ground potential is polluted by a summed left + right signal, a
mono signal (!!), which can be measured and heard as crosstalk and
intermodulation.
A balanced amp will show up like this:
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