Aphex
Thermionics
- Division of Aphex Systems Ltd.
Model 1100 MkII Mic Preamp - Owner’s Manual
The transformerless active balanced line output is
provided through a standard 3-pin XLR jack.
The Model 1100 MkII balanced output is a special
type known as “impedance balanced” that elimi-
nates many of the problems often encountered
when interfacing with a variety of balanced and
unbalanced studio gear.
When wiring the Model 1100 MkII to the bal-
anced input of other equipment, there is noth-
ing special to do; just use standard wiring. Your
equipment will see the Model 1100 MkII’s output
as a perfect balanced source and you will receive
all the hum and noise rejection afforded by a
balanced line transmission. The fact is, how-
ever, there is signal driven only to pin 2. Pin 3
is returned to ground through an impedance
equal to pin 2’s output impedance. How, you
may wonder, can that be equal to a balanced
output circuit? The answer is a bit technical, and
for those who really want to know, you can refer
to the end of this section for a full explanation.
For everyone else, please trust us. It works and it
works very well.
2.5.2 Nominal Output Reference
This switch simply inserts a 12dB pad into the
output line to reduce the line output level into
the proper range for semi-pro gear typically
connecting through RCA jacks. It converts the
professional standard level of +4dBu to the IHF
standard of -10dBV (316mV) . More and more
equipment is moving towards or somewhere
around the IHF operating levels, so be aware
that you may need to set that switch accordingly.
What you want is a good and proper level into
your gear when the Model 1100 MkII’s Head-
room Meter is peaking within the proper range.
To learn about the Headroom Meter, look in sec-
tion 1 of this manual.
2.5.3 Mic Input
This is where you plug in the microphone. Pins 2
and 3 are phantom powered when the phantom
power is turned on by the front panel button.
There is little more to say about this jack. The
input phase is matched to the output phase, i.e.,
pin 2 is positive at both jacks.
2.5.4 Z Compensation Switch
Some microphones have an output impedance
appreciably lower than 150 ohms. The Model
1100 MkII is quite happy with these mics, except
for the fact that MicLim can have only a limited
amount of effect. To extend the limiter effective-
ness, you should switch on the Z Comp to insert
a small amount of resistance into the input cir-
cuit. This does not reduce the output level of the
microphone nor does it affect the tonality. The
only tradeoff is in the achievable noise floor at
high preamp gain (>40dB). With Z Comp on,
the noise floor may rise slightly, but typically less
than 2dB.
While on the subject of noise, one point many
people wonder about is why, when there is no
microphone plugged into a microphone preamp
(any preamp), is the output noise so much
higher? The answer is not that cosmic rays are
getting into the jack. Actually, it has to do with
something called the “noise resistance”. When
the microphone is plugged in, its impedance cre-
ates a low noise resistance about equal to the
mic’s impedance, for example 150 ohms. Low
resistances generate low noise. When the jack is
open, the many times higher input circuit resis-
Figure 2-4 Daisy Chaining the Local Master Clock
Internal
Clock
Unit 1
Unit 2
Unit 3
Unit 4
In
Out
In
Out
In
Out
In
Out
Terminator
External
Clock
External
Clock
External
Clock
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