A/D converters in a recording situation are usually set to have a +22dBu maximum input
level to achieve full scale. This allows for occasional loud hits to not clip the converter.
D/As for this system would typically be aligned to have unity gain with the A/Ds. This is
done by running a -18dBfs signal at 1 kHz with the digital oscillator to the individual
D/A outputs one channel at a time. Turn the 2Bus LT output gain knob all the way up and
measure the voltage across pins 2 and 3 of the 2Bus LT output connector. If there is a
device in the studio that has a calibrated VU meter (tape machine, compressor,
Dangerous MQ) patching the 2Bus LT into it makes for convenient adjustment. Set the
D/A converter gain so that the volume indicator reads 0 VU or 1.23 volts AC on the
voltmeter.
After the D/As are aligned, the A/Ds are aligned by using the oscillator and one of
the D/As as a source. Feed that signal into the A/Ds one at a time and meter it in the
DAW. Adjust the A/D for -18dB full scale. Once all the converters are aligned, the
system becomes easier to handle because there is a wonderful consistency from channel
to channel. This makes cross patching and bussing assignment seamless.
Mastering systems are typically calibrated at -14dB from full scale for a
maximum analog signal level of +18dBu. The reason for this is because mastered signals
tend to be compressed and bright. There is a lot of rapid change (slew-rate) in the signal
that overloads processing gear and converters before the amplitude gets up to +22dBu.
Some savvy 2Bus LT users set their D/A converters as if they were mastering to
be able to take full advantage of the dynamic range such a calibrated system has to offer.
In other words, A/Ds are set to avoid clipping for recording and D/As are set to avoid
overloading the mixdown converter. This is done by aligning one D/A at -18dBfs and
using it to calibrate the A/Ds as above. Then set the all the D/As so that -14dB full scale
is 0VU by running the digital oscillator at -14dB and adjusting the D/A output levels to
read 1.23 volts ac or 0VU. This allows one to run the stems at hot levels without
overloading the mixdown A/D converter.
Balanced and Unbalanced connections
The beauty of balanced connections is that they promote the idea that current should be
prevented from flowing down cable shields while letting the audio pass. Pins 2 and 3
carry a signal across them (transverse mode) and any interference that gets through the
shield is picked up equally by the wires (common mode). The common mode noise is
canceled by the differential action of the instrumentation amplifier in the first stage of the
2Bus LT. Signal gets through and the grounds stay put inside their respective pieces of
gear.
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