I knew from the outset of the LA-2A project that a good quality optical compressor unit would be one of the more difficult parts to come across. And as I described a couple of days ago, this is the heart and soul of the compressor, so finding the right unit is a big deal.
JBL apparently had a big "yard sale" several years ago now and sold off all their remaining stock of the original units. So no more NOS units, except when you can sometimes find one on eBay for huge $$. Universal Audio is making them again, to sell with their reissues. However, they aren't selling them separately -- I supoose they're perfectly aware that lots of people have the know-how to build their own if they can get the right compressors. But in any case, the word on the street is that their new ones just don't sound as musical as the old ones.
In that vacuum, DIY folks have come up with all sorts of alternatives... one involves using vactrols. These optocouplers are essentially either a neon bulb, an incandescent light, or (most commonly now) an LED packaged up in a case with a light-dependent resistor. When the LED glows brighter, the resistance decreases... very much like the electroluminescent panel and cadmium-sulfide photocells in the original T4B. The only problem is, the LEDs and other options just don't behave the same way as the old EL panels... their response is different. The speed of glow onset when voltage is applied, the amount of glow you ultimately get, the rate of decay of that glow when the voltage is lowered, and "memory" of recent exposure to light are all factors. The compression curves you get with alternatives are also therefore different, and therefore lend a different character to the compressed sound. Not that it's necessarily bad, but it won't be quite like an EL panel.
Moreover, as explained here, these panels are actually mostly used for lighting aircraft avionics, and the manufacturers have, over the years, "improved" the product for that purpose. So a new panel made with the new process will sound different from the old ones. I'd guess this probably explains why the reissue ones don't behave or sound like the old ones.
Thankfully, there are people that are obsessive about this stuff, and it appears that "Joe Electro", the proprietor of Kenetek is just such an individual. He seems to be quite the authority on these units, and if his website is to be believed -- and I don't see any reason to doubt it -- he's tracked down some EL material that behaves in a very similar manner to the good old stuff. He's built a pretty cool looking test rig to prove it too.
Anyway, it's clear that he's thought about this stuff. A lot. So I plunked down my $100+shipping for one of his reproduction units, and we'll give it a whirl. The unit arrived today, serial #147, with a label indicating date of manufacture of Feb. 25 of this year. Note that the unit in the picture above from his page is only dated Feb. 8, looks like serial #138. So I'm guessing he hasn't been selling them for very long. Does it sound like a T4B? I dunno, I guess we'll find out. Because these units just plug straight into an octal socket, it only takes about 2 seconds to swap 'em in and out. Perhaps we'll be able to find someone in the local area that will loan us an authentic one for a head-to-head listening comparison. I have a good vibe about this though, as this dude has been building and working on LA-2As for over twenty years...
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