OPERATION NOTES
The Manley ELOP
®
follows certain traits and traditions established by the UREI LA-2 and
similar leveling 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 di-
vider to attenuate the signal. The tube line amplifier only functions to provide extra gain to make up for
attenuation losses and then acts as a fine cable driver. Simple, elegant and minimal. Operation of this
type of design is also simple, elegant and minimal. There are usually only “threshold” and “gain” con-
trols. Most have no user adjustment of “attack”, “release”, “ratio” or functions for de-essing or external
sidechains. To put it one way, the user is “stuck” with fixed time constants and a feature list that seems
utterly 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 circuit 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 our Mono and Dual Mono Micpres and Enhanced Pultec Equaliz-
ers (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 choic-
es of Vactrols. In the VOXBOX
®
we spend a lot of effort to get attack and release controls and it required
a radical departure from conventional approaches. There is a major advantage to fewer controls and a rea-
son 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 ap-
proximation 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.
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