Owners Manual

LIMITERS
The first compressors we know of used special tubes designed for
radio and gain control. These are the "remote cut-off" type like the
6386 as used in the Fairchild 670. Manley has been making the
"Variable MU" compressor/limiter for many years based on the
same principle.
Opto limiters probably began in the early 60's with the UREI LA2A
which used a small electro luminescent panel driven by audio
directly from a tube circuit. This panel was in a light shielded box
with a photo-resistor so that when the panel lit, the light shone on the
photo-resistor which in turn dropped in resistance and shunted audio
to ground, and reduced its level. Very simple but effective technique
which has stood the test of time. Part of the reason is the simplicity
of the 2 knob approach and part is the inherant attack and release
times both the electroluminescent panel and photo resistor have and
part is the sound of the tube/transformer circuits.
There weren't many companies building pro audio gear in the 60s
and 70s, but we had FET limiters, discrete transistor voltage controlled
amplifier (VCA) limiters, biased diode limiters and, in general, all
had plenty of color and distortion. One of the best known FET based
limiters is the UREI 1176 which brought more control to attack and
release times and had ratio switches. The first few generations of IC
based VCAs were also less than perfect, and VCAs got a bad name
but slowly improved over the years. With cheap easy to use op-amps
and VCAs, gear prices dropping, music business growing, we began
to see more gear but somehow the antique 670s and LA2As and
1176s were still in use and preferred.
In the early 90's a few maverick audio manufacturers including us
responded to that knowledge and developed new-old technologies.
Manley, for example began the ELOP using a LED/photo resistor
component called a Vactrol and combined it with ICs to drive it and
tube circuits for the audio. Over the years, that opto circuit was
revisited in a discrete transistor Langevin ELOP, variations on the
theme used in the VOXBOX and once again here in the SLAM!.
In developing the SLAM!, we began with the idea that probably we
could find alternate Vactrols that could be used to give some variety
to the opto-limiting. In the end, after trying every one out there, we
decided the one we had always used, was our favorite and the others
had more drawbacks than advantages. We did improve the drive and
metering electronics, and added a HP filter in the side-chain and
added the jacks to allow a user to insert their own EQ into the side-
chain. In most aspects, the opto-limiter circuit is similar to the one
used in our previous Elop's and uses audio to drive the LEDs. This
means that we can't possibly adjust the attack and decay characteristics
significantly without changing Vactrols. On the other hand, this
mode of operation, seems to act more like an RMS responding
circuit and reacts to many sounds in a way that we prefer over opto's
with the conventional attack, release, etc controls and all the
complications that a gain control element with its own timing
characteristics adds to that recipe. To make a long story short, we
like what happens with that simple old-school approach. The
difference in driver circuits plus the side-chain filter does seem to
make the opto a lot more useful on mixes and drums than our
previous units. We also allow some fast LED metering of the opto
in addition to the regular VUs which helps show how fast it tends to
react and gives a more complete picture for critical applications.
"The FET Limiter"
Because we couldn't improve much on our old opto circuit we
decided to add a second 'type' of limiter with its own characteristics
and its own historical roots. Some early limiters like the 1176 used
FETs for the gain reduction element which offered much faster
attack times and controllable releases.
The problem with FETs when used as a gain control element is that
they can add unpleasant distortion unless the signal is very low (like
-30 to -40 dB) and we also wanted a few gain controls which also eat
signal unless they are cranked. Throwing op-amps around to get
gain where needed is easy, but not our style. Keeping to an all-tube
Class-A concept requires different approaches.
We took a novel approach and used a transformer as part of the shunt
circuit, which not only reduced the signal to a nice level for the FET
but allowed us to use a pair of FETs in counter-phase to reduce
distortion. An expensive approach but worth it.
Our main goal of the FET limiter was to achieve the fastest release
that we could cleanly. This is the quality that causes the gain to return
as fast as possible, which is what gives us our perception of
'loudness' .The goal here was a great 'go-louder' box. Fast attacks,
and ∞ : 1 brickwall limiting is important to prevent 'overs' but not for
increasing the average level. A very fast transient that gets through
will clip but as long as the duration is short enough it will not be
perceived as distortion. Very fast release times, unfortunately,
usually imply modulation distortion where the limiter traces the
waveform at low frequencies rather than the volume envelope. This
is inevitable, but we made it possible to achieve faster and cleaner
releases than usual. It can still get crunchy so be careful.
This FET sidechain uses several techniques to get those fast releases.
The usual full-wave rectifier was replaced with a quadrature rectifier
that uses 4 phases to determine peaks and allows twice as fast
smoothing. Then we combine multiple side-chains and the typical
exponential capacitor release was modified for linear decay rates.
All of this resulted in faster clean releases, thus more loudness. Still,
at the fastest release times it is quite possible to get modulation
distortion, which is sometimes a useful color and often a problem
with wide spectrum sounds like mixes or instruments with lots of
lows. Listen for a growly sounding distortion on faster release times.
The multiple side-chains also gave us the possibility of introducing
an attack switch, which is usually not found on a limiter (compressors,
yes). The attack switch works on the slowest side-chain, which gives
much of the audible familiarity of the control while the other side-
chains are still biting the fastest transients. Like most attack controls,
as you go from fastest to slowest positions, you tend to lose some
threshold or limiting, so adjusting the FET LIMITER threshold will
probably be required, conversly more clockwise for VF settings.
A CLIP setting is on the release switch, that introduces a very
rounded clipping with a variable threshold. This type of distortion
is reminiscent of speaker distortion and tends to be mentally associated
with 'loud'. Of course, more conventional clipping is possible and
by turning up the INPUT, and turning down the OUTPUT, or if one
wants 'drastic' switching to MIC or INSTRUMENT will do that of
course. And, no, you won't hurt anything as long as, phantom is off,
and you prudently turned the OUTPUT down first. It wasn't designed
to simulate a guitar amp but intended to 'assist' an already overdriven
tone. Mostly CLIP is used to get a few extra dB of 'angry loud'.
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