Aphex
Thermionics
- Division of Aphex Systems Ltd.
required to devise a stable MicLim control loop
with such a fast and accurate response. Previously
there have been no successful fast
attack optocoupler feedback lim-
iters because the inherent latency
and memory of Cadmium Sulfide
(CdS) photocells causes oscillatory
reactions. Only feed forward optocoupler limit-
ers have been produced. For MicLim to work in
this application, however, it has to operate in the
feedback configuration.
In addition to inventing a unique new way to
control an optocoupler element, it was necessary
to devise a new kind of CdS optocoupler that can
be driven to reach a very low “on” resistance, typ-
ically 10 to 100 times lower than previously avail-
able units. Considerable research and cost was
expended to develop that element. Finally, after
nearly a year of research, we found success.
Cadmium Sulfide photocells have been used
in popular “optical” studio compressors for
decades. Their complex time constants and opti-
cal “memory” are known to provide unusually
smooth compression under all types of program
material. Although our new technology photo-
cell achieves a much greater dynamic range than
ever before, it shares in the timing complexity of
its predecessors. That’s one reason why MicLim so
gracefully handles both transient and sustained
limiting.
3.3 How To Use MicLim
Within each channel’s control window there is
a push-button for turning on and off the MicLim
feature. All you have to do is turn MicLim on
and it will instantly become armed. Nothing will
happen, however, until an audio peak approaches
clipping: then the limiter will act on the micro-
phone signal to prevent the peak from reaching
the clip point. Whenever the limiter hits a peak,
the red Clip/Limit light in the channel control
window will flash.
There are no special rules about making MicLim
work for you. The concept is simple: run the
preamp gain at a point where the peaks indicated
on the Headroom Meter only reach the MicLim
threshold infrequently or unexpectedly.
It may be evident to the astute that if one delib-
erately increased the preamp gain until peaks
frequently limit, one could obtain a very dense
output level by bringing the average signal so
much closer to the peak ceiling. Although it is
certainly possible to operate MicLim in that fash-
ion, and often the effect can be desirable, we rec-
ommend moderation. MicLim was designed to be
fast and unobtrusive for protection against acci-
dental overloading, especially with voices. You
may find the fast speed of this limiter too aggres-
sive for general purpose limiting. Your favorite
studio compressor/limiters can still be put to good
use. That’s why we include an insert path in the
product.
3.4 MicLim and the Microphone
MicLim works on all microphones. The use of
phantom power in no way inhibits the perfor-
mance of MicLim.
Using The Z-Comp Switch
The 1100 MkII has an
impedance compensa-
tion (Z-Comp)
switch near each mic input jack
to compensate for extremely low low mic imped-
ances. and extend MicLim’s effective range. If
you are using powered mics that typically have
50 ohms or less output impedance, you will need
to turn this switch on. There will not be any com-
promise in sound quality. However, the potential
noise floor may rise by up to a scant 2dB for
preamp gains over 40dB.
When using dynamic mics with impedances typi-
cally over 120 ohms, MicLim does not need the
use of the Z-Comp switch. There will be no ill
effect of having it on, however, other than the
slightly increased noise floor as stated above.
3.5 Whenever in Doubt, Use It!
Not sure about the MicLim? Just use it. If you
never need it then it won’t have any effect at all.
But when that big ass peak comes along, Bam!
You’re saved.
Photocell
Page 3-2
More Gain - Less Pain
