Extensive compressor reviews and FAQ

Lightning Boy Opti-Mu Prime: This pedal uses two 12AU7 tubes; the first one drives an optical comp element, which in turn controls the gain of the second tube. Together they form a "variable-mu" compression system ("mu" is the voltage gain of small AC signals). I believe this model was discontinued in early 2016; see my more recent review of the Op-2 which is the current model.
With most tube comps, and tube pedals/preamps in general, the tubes are put there to promise you a "fat, warm" tone. Just read the ads to see how much they gush about their vintage fatness and warmth. In many cases though, the tube does very little at all, and does not keep that promise. But the Opti-Mu Prime delivers the goods! It adds so much warm furry thickness that each note sounds like fluffy mashed potatoes with butter, or a long-haired cat on a pillow.
On the other hand, if what you want is a crisp bright snappy sound, you won't find that here. The highs get rolled off a bit, darkening the tone. There is no loss of low end, in fact the lows sound HUGE.
It has a noticeable noise floor, not terrible, but a little more than I'd like. It's a steady low hiss that does not change at any knob setting. I used the power supply they recommend, a Visual Sound 1Spot. As with most other tube devices, any noise may be a by-product of the wiring and EMF in the room, and my work room is electrically noisy.
The only controls are Comp, Level, and a switch that toggles between hard and soft "knee". Knee describes the slope of how strongly the compression engages. With a hard knee the full strength (highest ratio) of the compressor is applied as soon as your signal crosses the threshold; while with a soft knee the effect ramps up across a range of your signal strength, so that smaller peaks get a lower ratio and taller peaks get a higher ratio. The Comp knob mainly controls the maximum ratio, but turning it up also lowers the threshold a bit. In practical use though, different Comp and Knee settings just provide subtle variations in the intensity of the effect.
The action is smooth, slow, even, and not exaggerated in its envelope. It is not really suited to peak limiting or other "corrective" compression, it's more of a smoothing and thickening effect. There is no LED to indicate your signal crossing the threshold. You have to turn the Level knob most of the way up to get unity gain, where the output is the same overall volume as the input, so it doesn't offer a lot of extra boost. The Level pot is also an on/off switch for the pedal.
It runs on 9V DC, with a standard Boss-type plug, however it needs at least 600 mA current to perform properly. So it works great with high-current daisy chain supplies like the 1Spot or Powerall, but will not work with a Boss supply or most Voodoo Labs or other isolated bricks with lower current per output. Using a higher-voltage supply can damage the pedal. Mike at L.Boy explained to me that he chose 9V operation so that guitarists would not have to think about a non-standard power supply. He says "I spent a few years coming up with a starved plate design that doesn't suck."
The tubes stick out the top, with roll bars to protect them. It's easy to swap in different tubes if you want. The housing is the medium Barber/Diamond size, bare metal, with hand-engraved text. The construction quality is, well, clearly handmade. The wiring is "point-to-point", which means each component is soldered directly to each other, with no board or frame for structure; it's a jumble all held in place by solder and hot glue. Those of you who swear by PTP wired amps will find this very exciting; others may think it looks like a mess. You can select either a buffered or "true bypass" footswitch at the time you order.
If what you want is thick fat dark warmth; if you are tired of waiting for a Retrospec Squeezebox or an FEA Opti-FET; if you tried the EHX Black Finger or the DHA comp and came away disappointed; then you definitely should give the Opti-Mu Prime a shot.

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