Why the
world needs another optical compressor remained a mystery until
I tried Pendulum's OCL-2. I used the two-channel OCL-2 as a vocal
compressor on several gigs that I mixed, including John Prine
and several dozen bands at the Portland Blues Festival the following
week.
Especially
common to live sound, VCA compressors achieve accurate compression
ratios and well-defined thresholds. A chart of a VCA's response
shows a 45-degree slope, a bend at the threshold of compression
and a second, flatter straight line whose slope is determined
by the compression ratio. Many VCA-based compressors have enjoyed
success with soft-knee features that allow a smoother transition
into compression, illustrated by a rounding of the intersection
of these two slopes.
An optical
compressor uses a photo-attenuator, whose resistance changes
with the light intensity, but cannot respond as quickly as the
light. Vintage models like the LA-2A use an electro-luminescent
panel shining on a light-sensitive semiconductor. The classic
soft-knee response is a result of level detection from this circuit's
output (feedback detection), instead of the input (feed-forward)
detection used in VCA compressors. Unlike a VCA's clinical compression
slope with a fixed ratio, the response of an optical compressor
moves through a soft-knee at threshold, reaches a plateau of
compression at the target ratio, and gradually makes a transition
to higher ratios of compression at its extremes. As a signal
leans into it harder, these increasing ratios add a 'density'
to the sound that is pleasing to the ear.
The
Pendulum OCL-2 uses a proprietary method for enhancing the response
of a modernized version of this mechanism, providing faster attack
and release times than are available from traditional optical
compressors, and offering unrivaled transparency in its fastest
settings.
I usually find optical compressors a bit dull and woolly for
live sound, where clarity and presence are paramount for lead
vocals to stand out in the mix, but the
OCL-2's performance on vocals is bound to earn it a place in
the outboard rack of top tours. On its first outing, the Pendulum's
open, clear sound earned many compliments, with an almost embarrassing
series of audience members lining up to compliment the sound
at the end of the show.
In the Fast mode it performs a peak averaging
of the signal, offering a very open sound. This is the
setting for live work, and is even faster than the unit's fastest
manual settings. Six other presets, generally destined for studio
applications, are similar to those found on the original Fairchild
compressors. Four presets have increasingly slower attack and
longer release times, with each letting a little more dynamic
range punch through than the previous. Preset 5 has a program-dependent
release time emulating the LA-2A's two-stage release. Preset
6 is also program-dependent, but with three release stages. Both
of these presets are useful for full-program mastering or vocal
tracking, where the engineer doesn't want the compressor to "suck
back to zero" immediately to reduce pumping or breathing.
The optical
compressor section is followed by a class A, transformerless
tube amplifier. Class A circuitry, of course, eliminates the
cross-over distortion found in push-pull output stages, and the
omission of a transformer offers an uncolored signal. The OCL-2
can be optionally ordered with Jensens for applications where
galvanic isolation is desired. The transformerless OCL-2 reviewed
was unbalanced, with both quarter-inch and XLR connections, but
it worked problem-free with standard balanced insert cables.
The OCL-2
also has side-chain access on a TRS quarter-inch jack, for inserting
an EQ to allow frequency-tailored compression. Pendulum also
supplied 6 dB/oct. high-pass filters cleverly built into the
back-shells of TRS jacks to take advantage this "open architecture,"
reducing the proximity-effect dominance of the compressor's action
in close-mic'ed live vocals, and opening up the compressor's
sound further.
With
large VU meters, this blue-faced two-space unit has the classic
look of it's genre. These hand-built gems employ poly caps, metal-film
resistors and single-sided audio PC boards. When turned
on, the tube's power supplies gently ramp up to extend their
life, and after taking a couple of minutes to warm up, relays
un-mute the outputs.
Though the
name Pendulum remains a closely-held secret,
the exquisite craftsmanship in the
OCL-2 is reminiscent of their audiophile SPS-1 acoustic instrument
pre-amps, which have found their way into the racks of
top players like James Taylor, Leo Kottke and David Wilcox for
years. Two other offerings were also introduced at the last AES
show: the MDP-1 Tube Mic/DI Preamp and the 6386 Variable Mu Tube
Limiter. This year designer Greg Gualtieri can be found at AES
booth number 1345 in New York. - Mark Frink
