
Aphex
Thermionics
- Division of Aphex Systems Ltd.
Can you imagine if you could have a preamp that wouldn’t be overloaded by an unpredictable live performer? You could freely run
higher gain to pick up the whispers, then sit back and relax while the orator rings out in mellifluous splendor. Ordinarily you’d have
to ride preamp gain to avoid clipping during those loudest moments, but
MicLim
does it for you instantly. Maybe you don’t deal with
live show performers, but maybe you have dreamed of being able to record whatever you want at hotter levels without any fear of
overload. Learn how this exclusive feature works and how you can use it effectively for your own application.
With MicLim comes a new kind of freedom.
Page 3-1
3
3.1 How MicLim Works, Basically
Microphone limiters traditionally consist of a mic
preamp followed by a limiter circuit. Until MicLim
was devised, that was the only practical way to do
it. Everybody is familiar with the clipped overload
that’s heard when a performer suddenly gets on
the mic too loudly. The compressor/limiter held
down the P. A. level, but the preamp was clip-
ping horribly. Until MicLim, there was no other
choice except to constantly keep a hand on the
preamp’s gain knob.
The difference with MicLim
is that the limiter
works with the microphone itself, before the
preamplifier has any chance to clip at all. Patent
pending technology had to be devised to make
that possible since just about anything inserted
between the mic and preamp will add noise or
distortion. With MicLim now available you will
seldom find it possible to overload the Model
1100 MkII mic preamp, even under severe con-
ditions. That’s because there is typically at least
16dB of limiting headroom afforded to any
microphone of 150 ohms or higher. Effectively
that means that, with MicLim, you get 16dB
more preamp headroom before distortion.
3.2 How MicLim Works in Detail
This discussion is especially for the technically
astute reader. The rest of you can skip this until
later (or forever) if you’re not really all that inter-
ested in the small technical details of how this
works.
The “optically coupled attenuator” of figure 3-1
comprises a specially developed optically coupled
load resistor that is loaded across the mic input
line to react against the microphone’s self imped-
ance, causing a variable loss of level depending
upon the optocoupler drive current. Drive cur-
rent is generated by a very fast peak detector.
The maximum amount of attenuation available
depends upon the microphone’s self impedance,
but typically at least 16dB is achieved with a stan-
dard 150 ohm microphone. Increased attenuation
is available for microphones with higher imped-
ances. That means that MicLim can cleanly limit
microphone levels that are up to an incredible
16dB or more over the normal clipping point of
the preamplifier!
The peak detector operates in a feedback arrange-
ment sensing the preamp’s output level to antici-
pate accurately when any signal is approaching
the clipping point. A progressive current is sent
to the optocoupler as soon as the peak threshold
is exceeded. The attack time of this limiter loop is
designed to be fast enough to avoid any discern-
ible clipping. Considerable inventiveness was
Pre
Amp
Peak
Limiter
Still
Heavily
Clipped
Conventional Mic Limiter
Pre
Amp
Optically
Coupled
Attenuator
Peak
Detector
No
Clipping
Aphex MicLim Mic Limiter
Figure 3-1 Comparing MicLim
Understanding & Using MicLim
TM