Issue 6, September 2004
Model 50/51 User Guide
Page 32
Studio Technologies, Inc.
To remedy this problem simply use the
Model 51 Control Console to configure
the input for +4dBu operation. The distor-
tion will go away and the gain structure of
the StudioComm system will be correctly
established.
Technical Notes
Talent Amplifier Cable Length
There are no hard and fast rules defining
the maximum cable length when connect-
ing Model 35 or Model 38 Talent Amplifiers
to the Model 50 Central Controller. The
maximum cable length is directly related
to the amount of resistance in the con-
necting cable; the lower the resistance per
foot (or meter), the longer the cable can
be. (Although cable capacitance affects
high frequency performance, resistance
is the limiting factor is this case.)
To lay out the facts in grammar-school
story problem format: for correct opera-
tion, a Model 35 or Model 38 needs to
see at least +20Vdc between pins 1 and
2 of their input connector. The Model 50’s
talent amplifier output voltage across
pins 1 and 2 is +23Vdc, with a maximum
current draw of 0.2A (200mA). This differ-
ence between the voltage supplied and
the voltage required results in a maximum
voltage drop of 3V over the interconnect-
ing cables. Since cable is rated in ohms
per 1000 feet (or ohms per 1000 meters),
you need to know what the maximum
cable resistance is. This can be easily
calculated by dividing the maximum volt-
age drop by the maximum current flow: 3V
divided by 0.2A = 15 ohms. For example,
a standard 20 AWG microphone cable is
Belden 8412, which has 10.9 ohms resis-
tance per conductor per 1000 feet. Since
we’re using two conductors to carry the
signal (pins 1 and 2) you’d get 21.8 ohms
per 1000 feet of microphone cable. With
our 15 ohm maximum resistance you’d be
able to use 688 feet (210m) of this cable.
By using the numbers provided you can
select a cable, and its maximum length,
for your application.
Model 50 to Model 51 Cable Length
The Model 51 Control Console generates
system-exclusive MIDI messages which
are sent to the Model 50 Central Controller
using a 5-conductor MIDI-style intercon-
necting cable. The MIDI signal is carried
on two of the five conductors. The three
remaining conductors are for common/
shield, DC power, and communications
audio. The limiting factor in the intercon-
necting cable’s length is the transmis-
sion of the MIDI data, which has a rate of
31,250 bits-per-second. The interconduc-
tor capacitance of the cable attenuates the
data, and as the cable length increases
the data becomes unusable; the cable
serves as a low-pass filter. The MIDI speci-
fication calls for a maximum cable length
of 50 feet (15.3m), which will work fine
connecting the Model 51 to the Model 50.
There is really no reason why this length
can’t be increased, as long as good low-
capacitance, shielded cable is utilized.
Runs of 100 to 250 feet (30.5 to 76.2m)
are possible using the latest sophisticated
cable. Test to ensure that the StudioComm
system works correctly with the desired
cable length. There are too many variables
to give a simple formula—good luck!
Definition of Level—dBu and dBV
Whenever possible, Studio Technologies
has opted to use the dBu designation as
it seems to be quite rational. Using dBm
was fine when all audio line outputs were
