Designing a 500-series Optical Compressor pt. 2 December 20, 2017 18:07

Hey there, folks! It’s time to update everyone on the second collaboration between JC-diy and DIYRE, the OLA5 optical leveling amplifier for the 500-series modular format. After an initial post back in March we received a lot of useful feedback (thank you!) which we have taken into consideration as we worked toward a final product. - Joel Cameron, JC-DIY

As discussed in the first post, the objective of the OLA5 was to reproduce the unique, musical dynamic character of the classic “LA”-style optical leveling amplifiers we all know and love in a compact, DIY package, while updating its usefulness for the modern studio environment.

I started with the sidechain of the LA-4, but used an entirely updated, IC-based signal path, which allows folks who want to to ‘roll’ their own ICs for the sound they prefer. And like the EQP5 Passive Equalizer kit before it, the OLA5 is compatible with the optional Vintage Output kit, which includes a discrete opamp, a steel core output transformer and a NOS tantalum capacitor to add a bit of vintage flair to your leveling amp!


The most recent OLA5 front panel design

To Blend or not to blend?

This was the big question from my first post: were we going to give the OLA5 parallel functionality or not? There were differing and passionate opinions, though overwhelmingly people voted "yes"!

Nevertheless, I do understand many folks' hesitation to do so with such a classic sidechain. Arguably, parallel processing is more commonly used with heavy handed devices that crush and sometimes outright distort signals for blending with the unprocessed versions. As such, one might question the usefulness of this functionality on an “LA” style optical limiter, none of which are known for heavy-handed treatment of source material.

But the usefulness of this function is immediately apparent once you begin to put it to use. Hitting the limiter solidly and blending with the unprocessed signal can really stick a track where you want it dynamically, giving it more authority and simplifying mixing without evoking the feeling of having been processed at all. Perfect for uber-dynamic singers or for smoothing out an unevenly strummed acoustic guitar track. Beautiful.

But fear not, if you were one who preferred keeping the OLA5 a purely serial processor we have not abandoned you—it can be a purely serial processor as well. How? Its parallel functionality (with its corresponding summing stage) only comes into play when the “Mix” button is depressed (at which time the “Mix” knob becomes active). With this button switched out, the summing portion of the circuit is entirely removed from the signal path, leaving it a purely serial processor, just like the classics!

Let’s be Discrete: Rolling our own opto cells

The OLA5 differs not only from other DIY optical kits, but also from most every currently manufactured optical limiter in that it uses discrete optical gain cells. That is, optos made up of a discrete light source coupled with a separate light dependent resistor (LDR). Most every optical design currently available uses off-the-shelf optocouplers (which contain these two elements in a small, premanufactured, hermetically-sealed package with wire leads for soldering) made by companies like Vactrol and Luna (formerly Silonex).

The extremely fast response times of these opto-couplers are ideal for use with modern sidechains that provide control for time constants, and they can yield very musical, if rather transparent dynamic control. The classic LA-series of optical levelers, however, relied upon the time constants inherent to the LDR, providing engaging dynamic processing marked by a distinct, and beloved non-linear release characteristic. Try as I might I could not get any modern sidechain design I tinkered with to properly mimic this classic behavior in a convincing way. Ultimately, I decided that if it wasn’t broken I wasn’t gonna try to fix it—a discrete opto it must be!

Since the gain element is light-sensitive these optos need a dark environment to do their thing, so our concern became how best to provide light-tight enclosures for discrete optos on an otherwise ‘open’ 500-series pcb. Well, Peterson devised a devilishly simple and effective way to enclose each of the two opto cells, keeping them happily in the dark.

A cross section of our discrete opto design

A red LED (the light source) solders directly to the motherboard via a round spacer/riser with a diameter the same as the leading edge of the LDR, while the LDR solders to a small daughterboard that is then mounted (using screws) directly facing the LED. Acting as spacer between these two components is a black plastic tube which fits snugly around both the rim of the LDR and the spacer at the base of the LED, effectively cutting both devices off from the light of the outside world. This design is simple to assemble and works perfectly in a brightly lit room, so you can be certain it will work superbly when mounted within the interior darkness of your 500-series rack.

Compress or Limit?

In keeping with the spirit of the classic leveling amps the OLA5 offers a simple choice of either ‘compress’ or ‘limit’ functionality. The compress selection provides a roughly 2:1 ratio with a low threshold, great for general leveling. The ‘limit’ selection is user definable (via jumper on the main pcb) from two choices, both with a higher threshold setting. Use the pcb jumper to select your ideal ‘limit’ ratio, and then use the front panel pushbutton to select between compress and limit function in use. The build manual will provide resistor values for limit ratios of 4:1, 8:1, 12:1 and 20:1, so users will have choices of which two to include in their build.

More Features: Accurate metering, stereo link, and HPF

Metering of gain reduction on the OLA5 is provided via a 10-segement LED display with a -26dB range using TI’s LM3916 dot/bar driver IC. Unlike an analogue VU meter, this display is extremely fast, so reduction on attack transients (the attack is faster than you might think!) is accurately displayed as is the initial release followed by the slower, non-linear release. Ultimately, one chooses settings for a device like this by ear, but it’s still nice to see what’s really going on inside the thing!

Two OLA5s can be strapped for stereo use using the ‘link’ connectors provided on the back of many 500 racks. If your host rack doesn’t have link connectors you can make this connection directly between your two OLA5s with a single wire before mounting them in the rack. When you want the pair to track in stereo simply push the ‘link’ buttons on both units, while making sure to match other settings as well. With the ‘link’ switches out they are separate, mono processors.

And lastly, we have further expanded the usefulness of the OLA5 by giving its sidechain a simple and effective hi-pass filter. Simply engage this filter when you want to keep strong LF content from kicking up the leveling too much, yielding a beefier tonality to processed material. Great with parallel function for mixes and subgroups!

Onto Calibration – no jig required!

This is a another bit of cool that I’m really pleased with on the OLA5. Proper function requires calibration after the unit is built (metering and stereo tracking). One of my early concerns for doing a project like this was how to make it so people can calibrate it if they don’t have a test jig (a nifty little rig that plugs into the host rack and extends connections via wires/ribbon cable). I hated the idea of making people buy a test jig for the one, ever, time they calibrated their OLA5—what a waste of money. Once again Peterson, always the clever fellow, came to the rescue.

His solution was to mount the necessary trimpots 90-degrees off the pcbs, facing the front, sandwiched between the front panel controls and recessed below the plane of the faceplate. This way users can build the unit (sans faceplate), mount it in their 500 rack and have front access to the trimpots needed for calibration. After they finish calibrating, simply power down, pull the module, and attach the faceplate and knobs to officially complete the build. Done! (No special jig required! :)

And the calibration itself requires no special gear either. All you need is a flat-head precision screwdriver and a DAW with basic I/O and a signal generator plug-in capable of producting a 1kHz sine wave (and if, for whatever reason your DAW cannot produce this test tone a link to a downloadable WAV file of it will be included with your OLA5 kit).

The most recent OLA5 prototype

See You Next Year

So as you can see, we're very close to putting the finishing touches on this thing. All of the major circuit points have been ironed out and stress-tested and our prototype front panel came in looking great. All that's left is what we like to call "the final 99%": ordering parts, laying out the kit, making the assembly guide, etc.

If all goes well, we hope to announce a launch date and pricing in January. Thank you so much for following along and providing feedback during our design journey. We are so excited to get this kit into your studio!