ART Pro Audio DIO - A/D Converter - Manual (Page 6)

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TUBE WARMTH CONTROL
This control allows you to vary the tube circuits warmth contribution to
the audio signal chain. At the Clean end, there is essentially no tube
processing and the yellow Warm LED should not light.
As you turn up the control, the drive to the tube is increased while its
output is adjusted, to maintain a fairly constant output signal to the
A/D. At higher signal levels you will see the yellow (Warm) LED start
to come on, indicating that the tube circuit is doing some of the
processing. The audible effect is subtle at lower settings.
At the full Warm position, you will see the Warm LED on more of the
time, even at lower signal levels. Audibly, at this setting, the tube adds
low-order harmonics that tend to musically “thicken” the signal. Signal
peaks are also softened, rather than hard-clipped. Both of these effects
are what give the DI/O that “Analog Tape Saturation” sound at higher
levels with higher settings of the Warmth control.
POWER LED INDICATOR
The green Power LED lights solid to indicate that the DI/O is receiving
adequate power from its external power source. If it ever flickers, check
the connections, power adapter, or line voltage.