[sdiy] Electroluminiscence compressors (LA2A, LA3A)

[Thomas Holley]

There has been a lot of discussion about DIY of the LA2A and variants at
http://www.prodigy-pro.com/forum/index.php

There seem to be some pretty knowledgeable folks on the LAB forum WRT the
electroluminescent properties of this compressor.

Thomas
----- Original Message ----- 
From: <jhaible at debitel.net>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, June 23, 2004 6:46 AM
Subject: [sdiy] Electroluminiscence compressors (LA2A, LA3A)


> I finally found a little time to experiment with a "Night Light",
> whose electroluminiscent element will be abused for an opto-
> electronic compressor inspired by the LA2A and LA3A.
>
> Before I go on making anything practical, here are some findings
> and also some questions.
>
> (1) These foils have some threshold below which the light
> intensity will drop rapidly. So you have the limiter threshold
> built-in. Together with the fact that these are AC driven
> (no rectifier needed!), these should make a very *easy*
> compressor circuit. Let the LDR handle the time constants,
> and just vary the gain of the side chain for setting the
> compression level (and compression rate ?).
>
> (2) These things need high voltage. So tubes would be best.
> Solid state version (LA3A) has a step-up transformer to get
> the required voltage.
>
> (3) These things are *capacitive*. I measured 7.5nF for
> the Night Light, and in my first experimental setup this was
> responsible for a considerable HF loss. Not good! This
> means higher frequencies will have a much higher compression
> threshold! Seems the original compressors have rather low
> driving impedance. (10k anode resistor on LA2A, hefty driver
> circuit on LA3A - does anybody know the data for that
> step up transformer? I've made experiments with a 1W 100V
> ELA speaker transformer, but this is too "weak" to drive
> the electroluminiscent foil at higher audio frequencies!
> I start to believe that going tubes will be easier than finding
> the right transformer here ...)
>
> (4) I've noticed that the originals have some HF boost in the driving
> circuit. This is certainly to compensate some (or all?) of the foil's
> HF loss. Will make the driving circuit saturate earlier for HF;
> may or may not result in altered compression behavior at HF.
> LA2A / LA3A's are famous for their colouration of sound - could
> *that* effect be a major part of it?
>
> (5) Do these foils have a maximum voltage rating?
> I saw thatthe solid state / transformer version has one LDR wired
> in parallel to the foil. Is this some protective limiting?
> Or part of the audio compression function? The tube version
> does not have this ...
>
> Comments welcome, more questions to follow.
>
> JH.
>
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