[sdiy] Electroluminiscence compressors (LA2A, LA3A)

[Peter Cornell]

Hi Jurgen

The LA2 has been finely disected over at the Lab.

http://www.prodigy-pro.com/forum/index.php

All the refugees from Techtalk have ended up over there.

Chris Jenrick has analysed the T4 gain cell & found suitable replacements
for the electro panel & the LDR's. There has even been a full colour layout
for the LA2. One of the members is also putting kits together for those who
dont want to build from scratch. Have a look at the "Black Market".

http://www.vacuumbrain.com/

I think you'll enjoy it over there, lots of very cool projects.......

Peter


----- Original Message -----
From: <jhaible at debitel.net>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, June 23, 2004 12:46 PM
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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