Studio Technologies 42A 2008 - Manual (Page 21)

Model 42A User Guide Issue 2, February 2008
Studio Technologies, Inc. Page 21
installation and personal technical philoso-
phy. Two unshielded twisted pairs can be
effectively used. The first pair would carry
the DC and channel 1 audio signal and
common. The second pair would carry the
channel 2 audio signal, again along with
common. There will still be some capaci-
tance between the conductors carrying
the two audio channels but it should be
significantly less. Two shielded pairs can
also be used as was discussed in the In-
stallation section of this user guide.
Superior Power Delivery and
Audio Quality
As previously discussed, one of the Model
42A’s strengths is its ability to very effec-
tively deliver energy to the connected IFB
user devices. This allows more devices
to be supported over longer cable runs.
How does the Model 42A accomplish
this? Simply by having circuitry that
is superior to that used in most of the
“industry-standard” equipment. In most
IFB interface devices, an adjustable volt-
age regulator integrated circuit is used
as a combination of audio modulator and
current limiter. While this is a simple and
inexpensive solution, it’s not without signif-
icant limitations. The major problem with
this method is the type of voltage-current
“knee” that is created. As the load cur-
rent increases past about 50% of the rated
maximum the output voltage begins to de-
crease. This means that the usable power
delivered to the connected device(s) will
start to drop well before the rated output
is reached. This limitation will become
significant in applications that use long
cable runs. As the IFB circuit voltage
begins to drop problems with user device
performance can occur. Contrast this situ-
ation with the performance provided by
the Model 42A. The DC voltage supplied
by its IFB circuits won’t “poop out” when
loaded over its 0 to 200 milliamperes
range. This will allow IFB belt pack and
announcer’s console devices to work
correctly in many more applications.
Figure 11 shows the IFB circuit voltage-
current curves for the RTS 4000-series
and the Model 42A Interface. The perfor-
mance differences are quite interesting.
It’s interesting to note the reason why
typical IFB circuit audio quality is less
than pristine. It’s not hard to notice the
background “hiss” that is always present
on pin 2 (DC with channel 1 audio) of the
interface connector. Technically, it’s white
noise that comes from the adjustable
voltage regulator being used as an “AM”
modulator and current limiter. The noise
is an artifact of the design topology and
simply can’t be overcome. How does
Studio Technologies know this? Because
our first “breadboard” designs used this
method and achieved the same poor re-
sults! Only after the problem came to light
did work on an improved circuit begin.
The results were worth the effort.