At right the CRO display showing the two sine waves 180 degrees
of phase difference. The top waveform is hot (in phase), the bottom
180 degs reversed (cold). When the two signals are received by a
balanced input, the opposite process occurs, where the cold signal
is inverted back in phase with the hot signal so that the 180 degrees
of phase is cancelled and the two signals can be summed. When this
occurs it produces twice the voltage swing which = 3dB. The great
advantage of the balanced line is that any hum or noise introduced
into the cables will be in the same phase, not + and - like the
audio signals. When fed into the phase reversing input of a balanced
system, any hum and noise in the lines is put 180 degrees out of
phase effectively cancelling them out. It's a clever system.
The DI described here has unity gain, and clips at 4.6 -> 5
VRMS (~ 13.5V pk-pk) between hot & cold, so has enough head
room for any audio application. It is all DC coupled apart from
the input and output, and utilises about 28dB of negative feedback
to control gain and noise. The frequency response is limited to
about 50Khz by the little 27pF capacitor supplying negative feedback
from the collector to the base on transistor Q4. It has an essentially
flat response from 10Hz to 25Khz within + & - < 0.5dB. It
features a hi-'Z' input which is good for guitar and bass, but is
about +5dB noisier than a typical op-amp converter with a loaded
input. In fact whereas an op-amp circuit increases in hiss when
the input is loaded, this discreeet circuit decreases in noise significantly
with a loaded input. |