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Issue 1, September 2007 Model 233 User Guide
Page 36 Studio Technologies, Inc.
was very good, limiting the maximum
level to at most 10 dB above the nominal.
The nominal audio level associated with
a Clear-Com system was harder to char-
acterize. It appeared to be a few dB less
than –10 dBu, but the dynamic range was
much larger. Level peaks of 10 to 20 dB
over nominal were easy to produce.
This objective data led to the following
Model 233 design decisions: When audio
from intercom channels 1 and 2 was used
as headphone cue sources level sensitiv-
ity selection switches or trim pots were not
required. The level range available on the
Model 233’s front-panel controls proved
to be sufficient for the user to be able to
establish the desired listening level.
When talkback audio was routed to the
intercom channels a single audio level
proved to work well with both RTS and
Clear-Com systems. This was mainly
possible due to the excellent dynamic-
range-control provided by the compressor
circuit. Its threshold (2 dB above Model
233 internal nominal level) and compres-
sion ratio (5:1) resulted in excellent talk-
back-to-intercom audio. So in the end,
no level or compatibility switches of any
kind were required to achieve the desired
“listen” and “talk” performance.
The above paragraphs may elicit howls
of protests from a host of engineers and
intercom system experts. But for years
we’ve heard differing reports as to the
actual nominal audio levels for RTS and
Clear-Com systems. The “in-the-know
cats” agreed that RTS TW intercom (and
IFB) was –10 dBu, a value that we con-
firmed in our tests. But the nominal level
for Clear-Com was variously reported as
–20, –15, –12, –10, and “you know, the
Clear-Com level!” It’s most likely that early
Clear-Com systems did use a nominal
level of approximately –20 dB. But after
making controlled tests, the contemporary
Clear-Com equipment seemed to be much
closer to –10 dBu. And with the limited
dynamic-range control that we experi-
enced, the actual level during operation
may vary widely. That’s why intercom
interface sensitivity or compatibility switch-
es, or rotary controls were not included
in the Model 233. But just in case a
change is ever required intercom-interface
gain levels are set using several ¼-watt
1%-tolerance resistors. If necessary, these
can be changed by a qualified technician.
In conclusion, the engineers at Studio
Technologies are always open to learn-
ing more. Additional information from the
field concerning such topics as intercom
system levels, impedance matching, and
DC power sourcing would be welcomed.
Stopping by our offices for an in-person
chat would be also great. Bringing along
a bit of road “swag” would be appreciat-
ed. Just park the production trailers in the
alley behind our warehouse—plenty
of power is available!
Connecting an IFB Circuit
A broadcast-standard “wet” (DC with au-
dio) IFB circuit can be directly connected
to the Model 233’s intercom input. Origi-
nated by sources such as the RTS 4000-
series IFB system or IFB interface devices
from Studio Technologies, the connected
IFB circuit can provide two channels of
cue audio as well as DC power to oper-
ate the Model 233. No Model 233 talkback
audio can be effectively sent to the IFB
circuit due to its one-way nature. It’s a low-
source-impedance/high-input-impedance
circuit arrangement.