Manual

6
OPERATION NOTES
TheManley Electo-Optical Limiter follows certain traits and traditions established by the UREI
LA-2 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 or lamps
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 tube 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 "attack", "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 simplicity 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 favorite simple tube line
amplifier circuit which we also use in the 40 dB Mic Pre and 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. In the VOXBOX we spend a lot of effort to get attack and release controls and it
required a radical deparure from conventional approaches. There is a major advantage to fewer controls
and a reason for the coolness of LA type limiters. 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. While normal cheapo VCA limiters are much simpler the best
approximation is 10 ms attack and 500ms release. We spec 2.5 sec for release which accounts for that
slow down near zero. The attack spec number is similarly an approximation. Who cares - it works.