ULN-8 Users Guide
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range of matching impedances. The microphone inputs have an impedance of 3.3k Ω, and the line inputs
are 10k Ω. With that in mind, there are a number of aspects of the design that you should take into account
when interfacing with the ULN-8. All multichannel audio connections are made using industry standard
DB25 connectors, which are pin- compatible with cables using the Tascam/Digidesign standard.
Wiring details for this connector can be found in the
appendix.
Whenever possible, use balanced connections with the ULN-8. The performance of balanced interconnects
is much higher and much more resistant to noise interference and electrical (power) wiring problems.
The expense of balanced interconnects is not substantially higher than unbalanced connections, so if the
gear that you are interfacing with supports balanced connection — use it. If you cannot utilize balanced
interconnects, there are connection schemes that you can use that will maximize performance.
On input, at line level, it is sufficient to simply use standard unbalanced (TS) connections. If you are inter-
facing with the Mobile I/O XLR inputs, you will need to ensure that pin 3 is grounded in the unbalanced
adapter cable. More information about adjusting the input level can be found in the MIOConsole3d soft-
ware chapter.
ULN-8 DB25 cables should be wired pin 2 or Tip hot. The front and rear 1/4” connectors are wired Tip hot.
The headphone connector is wired Tip/Left, Ring/Right, Sleeve/Ground.
Connecting to the DIs
The DI inputs provide an excellent high impedance input for directly connecting instruments to the ULN-8.
These inputs are unity-gain summed with the corresponding line input from the back panel. This means
that if you have signal connected to both the DI and line input for a given channel the ADC will see a mix
of those two inputs. If you do not wish to have signal from one of the connectors you need to physically
disconnect that input or otherwise ensure that no signal is present at the unused input.
You may notice that the MIOConsole3d has an input selection for “Inst”. This can be used as a visual
reminder that you are plugged into the DI, but has no operational effect on the input. Selecting “Inst”
on any channel of the ULN-8 sets the input to Line +4; the only DI input is available from the front panel
jacks for inputs 1 and 2.
Each DI input supports balanced connections via TRS cable. You can connect a source to the DI via a bal-
anced interconnect (if the source is actually a balanced output), an unbalanced interconnect (if the source
is unbalanced like most instruments), or a telescoping shielded cable (which is an alternate unbalanced
connection that provides additional shielding).
Since the DI inputs provide an ultra-high input impedance, if you connect an unbalanced source to the DI
via a balanced cable you will float the negative input and will effectively inject a large DC offset into the
output of the DI which will cause the associated ADC to mute. As a result you must use either a TS cable
or a telescoping shield cable with unbalanced sources.
When you remove the connector from the DI jack, the DI connects its inputs to ground internally to remove
any possible residual DC offset or pickup noise. If you disconnect the cable from the source but leave it
connected to the DI input, the cable will function as an antenna and will inject noise into your inputs and
may cause enough of a DC offset to mute the ADC. As a result you should always disconnect the cable
from the DI rather than the instrument.
Balanced Sends and Mic S/R Mode
The ULN-8 has eight balanced sends; these mirror the analog input you have selected to feed that channel’s
A/D converter. For example, if you select the Mic input on analog channel 1, the output of that mic preamp
will be available at send 1. If you select the Line input on analog channel 1, the Line in signal will be
available at send 1. This allows you to use the sends as a splitter to feed a mixing console, recorder or
other equipment.
