[sdiy] Electroluminiscence compressors (LA2A, LA3A)

[jhaible at debitel.net]

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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