The Pendulum OCL-2
Tube Compressor/Limiter
 by Mark Frink - MIX November 1999

   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