Aphex
Thermionics
- Division of Aphex Systems Ltd.
Model 1100 MkII Mic Preamp - Owner’s Manual
proprietary limiting module acting directly upon
the microphone signal limits the peak level reach-
ing the preamplifier input to completely prevent
any clipping caused by excessive input levels up
to approximately 20dB over the normal clipping
level. When used mainly for protection against
unexpected overload levels during live shows
and recording, it is amazingly transparent. Use it
moderately to help you record or reproduce con-
sistent full maximum peak levels without fear of
distortion. Read more about this powerful feature
in section 5, “Understanding & Using MicLim”.
1.7 Headroom Meter
20 LED segments indicate the output signal’s
peak level below the point of clipping, thus it
shows you how much headroom is left at all
times. In the case of digital audio, the scale also
represents the digital dynamic range of the inter-
nal A/D converter, i.e., the meter is the same as
a digital peak level meter referenced to 0dB(fs)
3
.
This meter is useful in establishing the operating
gain to attain consistently high but safe recording
levels. You will quickly appreciate how it helps
you to fully use this preamplifier’s extraordinary
dynamic range.
1.8 Line Output Calibration
This recessed control attenuates the analog line
output stage gain from 0 to -12dB, allowing you
to match up the dynamic range of the preamp to
your connected equipment. Use it to match the
-20dB test tone to your -20dB reference level. In
many cases, this will be done simply by getting
a 0VU tone indication on your analog recorder
or console with the record level or fader set to
a nominal position. Once calibrated, the Model
1100’s headroom meter will closely match the
headroom of your connected equipment and
you can proceed to operate the preamp with
complete confidence that you’re getting the best
possible noise performance and dynamic range.
1.9 Clock Source
One of the two digital audio controls, this button
selects the A/D converter’s clock source. When
“internal” is selected, an internal low-jitter crys-
tal oscillator supplies the clock reference. When
“external” is selected, then word clock is taken
from the rear panel BNC clock input jack. The
1100 MkII will go into a hunt mode until it finds
the word clock parameters and locks on. The
frequency indicator lights DO NOT come on to
tell you the locked frequency. These lights are
reserved to indicate clock frequencies during
internal sync only. If external clock is not pres-
ent, then the 1100 MkII will continue hunting
for a lock indefinitely and the “locked” light will
never come on. The digital audio output will be
squelched, i.e., there will be no AES/EBU output
at all. If the Clock Reference is present and valid,
then the “locked” light will come on and the
AES/EBU output will be present and synchro-
nized to the clock source. The “locked” light will
always come on whenever the “internal” mode
is selected.
1.10 Internal Rate
The second of two digital audio controls, this
push-button selects the A/D converter’s sample
rate when the clock source is internal. The
choices are 44.1kHz, 48KHz, 88.2kHz, 96kHz,
and 192kHz. It has no function when the Clock
Source is set for “external”.
1.11 A/D Converter Indicators
There are three lights related to the A/D converter
in each of the channel windows. In the channel
1 window, you will see the lights relating to the
Clock Source status and the “locked” status. In
the channel 2 window you will see the internal
rate indicators. The internal rate lights will all
be dark when external clock is selected, but will
be lighted in internal clock mode to indicate the
chosen sample rate.
1.12 Gain
This switch sets the preamp gain in precise 4dB
steps from 21dB to 65dB. Use it to optimize the
operating level of the preamplifier. While observ-
ing the peak headroom meter, keep the meter
peaking as high as possible without frequently
hitting 0dB.
1.13 Low Cut (LoCaf
TM
)
This rotary switch sets the low cut (high pass)
corner frequency to one of 11 selected values as
illustrated below. In the Off position, the low cut
filter is tuned very low, around 1 Hertz, taking it
completely out of the way. It comprises a unique
servo cancellation circuit with a second order
modified Butterworth response to optimize the
sonic effectiveness of the filter. The servo cancel-
lation method is so unique we have named it
LoCaf.
Page 1-2
Front Panel Features
