Studio Technologies 40 - Manual (Page 15)

Issue 1, July 1994 Model 40 User Guide
Page 16 Studio Technologies, Inc.
1 and 2 is +23Vdc, with a maximum cur-
rent draw of 0.2A (200mA). This difference
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, along with
the resistance of your specific cable, you
can select a cable, and its maximum
length, for your application.
¼-Inch Plugs versus EIA RS-453
An incompatibility problem lurks between
some ¼-inch 2-conductor and 3-conductor
phone plugs and the jacks found on pro-
fessional audio equipment. While all the
plugs seem to “look” the same, some do
not comply with the industry standard,
called EIA RS-453. This standard defines
the physical dimensions, including the
shape of the plug’s tip. It seems that some
plug manufacturers don’t bother to make
the tip comply with the standard. Why is
this relevant to you? Because the phone
jacks used on the Model 40 Central Con-
troller, Model 35 Talent Amplifier, and
Model 38 Talent Amplifier do comply with
the standard. They expect to be mated
with plugs that also meet the specification.
When interfacing your line inputs or head-
phones be careful with the plugs you
utilize. Should a connection appear “flaky,”
sound noisy, or make intermittent contact,
the most likely problem is a non-standard
phone plug. Replace the plug if this is the
case. You should find that all plugs from
Switchcraft or Neutrik will work correctly,
specifically Switchcraft No. 297 or Neutrik
NP3C. In our experience, headphones are
the most likely place to find non-standard
plugs—we even found them on some
“world-class” headphones that we use in
our lab!
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
terminated with 600 ohm loads. In this way
it was easy to say that 0dBm is 1 milliwatt
dissipated in the known load (i.e., 0dBm
across 600 ohms will measure 0.7747V).
In contemporary situations an output is
rarely terminated with 600 ohms; generally
10k ohms or higher. The dBu designation
is better because it refers to dB referenced
to 0.7747V, with no reference to load
impedance. This takes into account
today’s audio scene where signals have
a low source impedance and a high input
impedance. The dBu designation is be-
coming the standard for the professional
audio industry.
The Model 40 is designed to interface with
audio signals that have nominal signal
levels of –10dBV to +10dBu. You might
wonder why dBV came into the picture.
Most people don’t realize that equipment
that utilizes “–10” levels usually mean
–10dBV—substantially different from
–10dBu (–10dBV = –7.78dBu). The dBV