Manley Langevin Dual Vocal Combo 1999 - 4/2001 LDVC000 - LDVC178 - Manual (Page 10)

OPERATION NOTES
The Langevin Electo-Optical Limiter follows certain traits and traditions established by
the UREI LA-3A and similar levelling amplifiers. These traits can be divided into two aspects
- electronic and operation. The electronic concept is simple and rather clean. Use the audio to
light up LEDs which shine onto photo-resistors. These photo-resistors in combination with
a fixed resistor simply act as a voltage divider to attenuate the signal. The line amplifier only
functions to provide extra gain to make up for attenuation losses and then act as a fine cable
driver. Simple, elegant and minimal. Operation aspects are also simple, elegant and minimal.
There are usually only a “threshold” and “gain” control. Most have no user adjustment of “at-
tack”, “release”, “ratio” or functions for de-essing or external sidechains. The user is “stuck”
with fixed time constants and a feature list that seems anemic compared to dynamic processors
costing far less.
So why are “LA” style opto based limiters so popular ? Several reasons. To paraphrase
Letterman “The number one reason why “LA” style limiters are favorites is because.... they
work right on vocals”. This “rightness” has a few aspects. The first is that “LA” style limiters
don’t leave much trace of limiting as they work. This is partly due to tubes, partly to the sim-
plicity of the opto circuit and partly because the user can’t alter the attack and release. Almost
every VCA based design seems to leave electronic personality on that critical vocal track. This
is usually undesirable. Our Opto circuits has no active limiting in the signal path. Tube circuits
have the potential to be musically more transparent than transistors because tubes are generally
more linear devices. However, there are many poor examples of tube circuits in use, and many
ways to butcher the quality. We chose to use our Langevin line amplifier circuit which we also
use in the Langevin Enhanced Pultec Equalizers (rather than copy UREI designs) because
frankly our circuit sounds better and cleaner.
Back to this matter with fixed time constants. We get requests to modify our “ELOP” for
more controls but we get even more people raving about how great and useful the “ELOP” is
now. The attack, release, knee and ratio (curve) are a function of the Vactrol Cell we chose to
use. The choice was based on the attack and release characteristics. Changing the time values
in this circuit involves different choices of Vactrols. Not practical. Not in a “LA” style limiter.
There is a major advantage to fewer controls. You simply adjust the Threshold for the desired
limiting amount and adjust the Gain for the desired level to tape - then record. The limiter does
what its supposed to do - nothing more - nothing less. Kinda like automatically right, strangely
quick and easy, and pretty much non-distracting. We use the phrase “Set it and forget it”. This
is a very important feature that would be lost with a variety of controls. A good engineer wants
to be ready to record “now” and does not want to be fussing with controls while a lead vocal is
going to tape. Unfortunately most compressors drag the engineer’s attention away (and often
the singer’s and producer’s attention away as well).
The time and slope characteristics of Opto elements are not easy to describe and probably
even more difficult to simulate. The attack is fast, not super fast “brick wall”, but fast enough to
“catch” consonants. It is also a function of level. At lower reduction levels and lower peaks the
Vactrol is slower. It becomes faster with sharp peaks and heavier levels of reduction. Release
is similar but 10 to 20 times slower. Quick peaks are handled with quick release and as gain
reduction nears zero the Vactrol gets slower like gentle braking to a stop. It is possible to”trick
out” an opto circuit for conventional operation but generally the results have been not well
liked.
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